Several Critical Vulnerabilities on most HP machines running Windows

I always have considered bloatware a unique attack surface. Instead of the vulnerability being introduced by the operating system, it is introduced by the manufacturer that you bought your machine from. More tech-savvy folk might take the initiative and remove the annoying software that came with their machine, but will an average consumer? Pre-installed bloatware is the most interesting, because it provides a new attack surface impacting a significant number of users who leave the software on their machines.

For the past year, I’ve been researching bloatware created by popular computer manufacturers such as Dell and Lenovo. Some of the vulnerabilities I have published include the Remote Code Execution and the Local Privilege Escalation vulnerability I found in Dell’s own pre-installed bloatware. More often then not, I have found that this class of software has little-to-no security oversight, leading to poor code quality and a significant amount of security issues.

In this blog post, we’ll be looking at HP Support Assistant which is “pre-installed on HP computers sold after October 2012, running Windows 7, Windows 8, or Windows 10 operating systems”. We’ll be walking through several vulnerabilities taking a close look at discovering and exploiting them.

Here is a table of contents to help navigate my research. In any case, I strongly suggest reading the General Discovery and General Exploitation sections as they provide significant foundational material important to understanding most of the vulnerabilities.

Table of Contents

  1. General Discovery
  2. General Exploitation
  3. Local Privilege Escalation Vulnerabilities
  4. Arbitrary File Deletion Vulnerabilities
  5. Remote Code Execution Vulnerability
  6. Proof of Concept
  7. “Remediation”
  8. Protecting your machine

General Discovery

Before being able to understand how each vulnerability works, we need to understand how HP Support Assistant works on a conceptual level. This section will document the background knowledge needed to understand every vulnerability in this report except for the Remote Code Execution vulnerability.

Opening a few of the binaries in dnSpy revealed that they were “packed” by SmartAssembly. This is an ineffective attempt at security through obscurity as de4dot quickly deobfuscated the binaries.

On start, HP Support Assistant will begin hosting a “service interface” which exposes over 250 different functions to the client. This contract interface is exposed by a WCF Net Named Pipe for access on the local system. In order for a client to connect to the interface, the client creates a connection to the interface by connecting to the pipe net.pipe://localhost/HPSupportSolutionsFramework/HPSA.

This is not the only pipe created. Before a client can call on any of the useful methods in the contract interface, it must be “validated”. The client does this by calling on the interface method StartClientSession with a random string as the first argument. The service will take this random string and start two new pipes for that client.

The first pipe is called \\.\pipe\Send_HPSA_[random string] and the second is called \\.\pipe\Receive_HPSA_[random string]. As the names imply, HP uses two different pipes for sending and receiving messages.

After creating the pipes, the client will send the string “Validate” to the service. When the service receives any message over the pipe except “close”, it will automatically validate the client – regardless of the message contents (“abcd” will still cause validation).

The service starts by obtaining the process ID of the process it is communicating with using GetNamedPipeClientProcessId on the pipe handle. The service then grabs the full image file name of the process for verification.

The first piece of verification is to ensure that the “main module” property of the C# Process object equals the image name returned by GetProcessImageFileName.

The second verification checks that the process file is not only signed, but the subject of its certificate contains hewlett-packard, hewlett packard, or o=hp inc.

HP IsPathAbsolute method

The next verification checks the process image file name for the following:

  1. The process path is “rooted” (not a relative path).
  2. The path does not start with \.
  3. The path does not contain ...
  4. The path does not contain .\.
  5. The path is not a reparse or junction point.

HP VerifyProcessPath method

The last piece of client verification is checking that each parent process passes the previous verification steps AND starts with:

  1. C:\Program Files[ (x86)]
  2. C:\Program Files[ (x86)]\Hewlett Packard\HP Support Framework\
  3. C:\Program Files[ (x86)]\Hewlett Packard\HP Support Solutions\
  4. or C:\Windows (excluding C:\Windows\Temp)

If you pass all of these checks, then your “client session” is added to a list of valid clients. A 4-byte integer is generated as a “client ID” which is returned to the client. The client can obtain the required token for calling protected methods by calling the interface method GetClientToken and passing the client ID it received earlier.

One extra check done is on the process file name. If the process file name is either:

  1. C:\Program Files[ (x86)]\Hewlett Packard\HP Support Framework\HPSF.exe
  2. C:\Program Files[ (x86)]\Hewlett Packard\HP Support Framework\Resources\ProductConfig.exe
  3. C:\Program Files[ (x86)]\Hewlett Packard\HP Support Framework\Resources\HPSFViewer.exe

Then your token is added to a list of “trusted tokens”. Using this token, the client can now call certain protected methods.

Before we head into the next section, there is another key design concept important to understanding almost all of the vulnerabilities. In most of the “service methods”, the service often accepts several parameters about an “action” to take via a structure called the ActionItemBase. This structure has several pre-defined properties for a variety of methods that is set by the client. Anytime you see a reference to an “action item base property”, understand that this property is under the complete control of an attacker.

In the following sections, we’ll be looking at several vulnerabilities found and the individual discovery/exploitation process for each one.

General Exploitation

Before getting into specific exploits, being able to call protected service methods is our number one priority. Most functions in the WCF Service are “protected”, which means we will need to be able to call these functions to maximize the potential attack surface. Let’s take a look at some of the mitigations discussed above and how we can bypass them.

A real issue for HP is that the product is insecure by design. There are mitigations which I think serve a purpose, such as the integrity checks mentioned in the previous section, but HP is really fighting a battle they’ve already lost. This is because core components, such as the HP Web Product Detection rely on access to the service and run in an unprivileged context. The fact is, the current way the HP Service is designed, the service must be able to receive messages from unprivileged processes. There will always be a way to talk to the service as long as unprivileged processes are able to talk to the service.

The first choice I made is adding the HP.SupportFramework.ServiceManager.dll binary as a reference to my C# proof-of-concept payload because rewriting the entire client portion of the service would be a significant waste of time. There were no real checks in the binary itself and even if there were, it wouldn’t have matter because it was a client-side DLL I controlled. The important checks are on the “server” or service side which handles incoming client connections.

The first important check to bypass is the second one where the service checks that our binary is signed by an HP certificate. This is simple to bypass because we can just fake being an HP binary. There are many ways to do this (i.e Process Doppleganging), but the route I took was starting a signed HP binary suspended and then injecting my malicious C# DLL. This gave me the context of being from an HP Program because I started the signed binary at its real path (in Program Files x86, etc).

To bypass the checks on the parent processes of the connecting client process, I opened a PROCESS_CREATE_PROCESS handle to the Windows Explorer process and used the PROC_THREAD_ATTRIBUTE_PARENT_PROCESS attribute to spoof the parent process.

Feel free to look at the other mitigations, this method bypasses all of them maximizing the attack surface of the service.

Local Privilege Escalation Vulnerabilities

Discovery: Local Privilege Escalation #1 to #3

Before getting into exploitation, we need to review context required to understand each vulnerability.

Let’s start with the protected service method named InstallSoftPaq. I don’t really know what “SoftPaq” stands for, maybe software package? Regardless, this method is used to install HP updates and software.

HP InstallSoftpaqHandler method

The method takes three arguments. First is an ActionItemBase object which specifies several details about what to install. Second is an integer which specifies the timeout for the installer that is going to be launched. Third is a Boolean indicating whether or not to install directly, it has little to no purpose because the service method uses the ActionItemBase to decide if it’s installing directly.

The method begins by checking whether or not the action item base ExecutableName property is empty. If it is, the executable name will be resolved by concatenating both the action item base SPName property and .exe.

If the action item base property SilentInstallString is not empty, it will take a different route for executing the installer. This method is assumed to be the “silent install” method whereas the other one is a direct installer.

Silent Install

HP Silent Install code path

When installing silently, the service will first check if an Extract.exe exists in C:\Windows\TEMP. If the file does exist, it compares the length of the file with a trusted Extract binary inside HP’s installation directory. If the lengths do not match, the service will copy the correct Extract binary to the temporary directory.

HP Extract method integrity checks

The method will then check to make sure that C:\Windows\TEMP + the action item base property ExecutableName exists as a file. If the file does exist, the method will ensure that the file is signed by HP (see General Discovery for a detailed description of the verification).

HP ExtractSoftPaq integrity checks

If the SilentInstallString property of the action item base contains the SPName property + .exe, the current directory for the new process will be the temporary directory. Otherwise, the method will set the current directory for the new process to C:\swsetup\ + the ExecutableName property.

Furthermore, if the latter, the service will start the previously copied Extract.exe binary and attempt to extract the download into current directory + the ExecutableName property after stripping .exe from the string. If the extraction folder already exists, it is deleted before execution. When the Extract.exe binary has finished executing, the sub-method will return true. If the extraction method returns false, typically due to an exception that occurred, the download is stopped in its tracks.

HP ExtractSoftPaq integrity checks #2

After determining the working directory and extracting the download if necessary, the method splits the SilentInstallString property into an array using a double quote as the delimiter.

If the silent install split array has a length less than 2, the installation is stopped in its tracks with an error indicating that the silent install string is invalid. Otherwise, the method will check if the second element (index 1) of the split array contains .exe, .msi, or .cmd. If it does not, the installation is stopped.

The method will then take the second element and concatenate it with the previously resolved current directory + the executable name with .exe stripped + \ + the second element. For example, if the second element was cat.exe and the executable name property was cats, the resolved path would be the current directory + \cats\cat.exe. If this resolved path does not exist, the installation stops.

If you passed the previous checks and the resolved path exists, the binary pointed to by the resolved path is executed. The arguments passed to the binary is the silent install string, however, the resolved path is removed from it. The installation timeout (minutes) is dictated by the second argument passed to InstallSoftPaq.

After the binary has finished executing or been terminated for passing the timeout period, the method returns the status of the execution and ends.

Direct Install

When no silent install string is provided, a method named StartInstallSoftpaqsDirectly is executed.

The method will then check to make sure that the result of the Path.Combine method with C:\Windows\TEMP and the action item base property ExecutableName as arguments exists as a file. If the file does not exist, the method stops and returns with a failure. If the file does exist, the method will ensure that the file is signed by HP (see General Discovery for a detailed description of the verification).

The method will then create a new scheduled task for the current time. The task is set to be executed by the Administrators group and if an Administrator is logged in, the specified program is executed immediately.

Exploitation: Local Privilege Escalation #1 to #3

Local Privilege Escalation #1

The first simple vulnerability causing Local Privilege Escalation is in the silent install route of the exposed service method named InstallSoftPaq. Quoting the discovery portion of this section,

When installing silently, the service will first check if an Extract.exe exists in C:\Windows\TEMP. If the file does exist, it compares the length of the file with a trusted Extract binary inside HP’s installation directory. If the lengths do not match, the service will copy the correct Extract binary to the temporary directory.

This is where the bug lies. Unprivileged users can write to C:\Windows\TEMP, but cannot enumerate the directory (thanks Matt Graeber!). This allows an unprivileged attacker to write a malicious binary named Extract.exe to C:\Windows\TEMP, appending enough NULL bytes to match the size of the actual Extract.exe in HP’s installation directory.

Here are the requirements of the attacker-supplied action item base:

  1. The action item base property SilentInstallString must be defined to something other than an empty string.
  2. The temporary path (C:\Windows\TEMP) + the action item base property ExecutableName must be a real file.
  3. This file must also be signed by HP.
  4. The action item base property SilentInstallString must not contain the value of the property SPName + .exe.

If these requirements are passed, when the method attempts to start the extractor, it will start our malicious binary as NT AUTHORITY\SYSTEM achieving Local Privilege Escalation.

Local Privilege Escalation #2

The second Local Privilege Escalation vulnerability again lies in the silent install route of the exposed service method named InstallSoftPaq. Let’s review what requirements we need to pass in order to have the method start our malicious binary.

  1. The action item base property SilentInstallString must be defined to something other than an empty string.
  2. The temporary path (C:\Windows\TEMP) + the action item base property ExecutableName must be a real file.
  3. This file must also be signed by HP.
  4. If the SPName property + .exe is in the SilentInstallString property, the current directory is C:\Windows\TEMP. Otherwise, the current directory is C:\swsetup\ + the ExecutableName property with .exe stripped. The resulting path does not need to exist.
  5. The action item base property SilentInstallString must have at least one double quote.
  6. The action item base property SilentInstallString must have a valid executable name after the first double quote (i.e a valid value could be something"cat.exe). This executable name is the “binary name”.
  7. The binary name must contain .exe or .msi.
  8. The file at current directory + \ + the binary name must exist.

If all of these requirements are passed, the method will execute the binary at current directory + \ + binary name.

The eye-catching part about these requirements is that we can cause the “current directory”, where the binary gets executed, to be in a directory path that can be created with low privileges. Any user is able to create folders in the C:\ directory, therefore, we can create the path C:\swsetup and have the current directory be that.

We need to make sure not to forget that the executable name has .exe stripped from it and this directory is created within C:\swsetup. For example, if we pass the executable name as dog.exe, we can have the current directory be C:\swsetup\dog. The only requirement if we go this route is that the same executable name exists in C:\Windows\TEMP. Since unprivileged users can write into the C:\Windows\TEMP directory, we simply can write a dog.exe into the directory and pass the first few checks. The best part is, the binary in the temp directory does not need to be the same binary in the C:\swsetup\dog directory. This means we can place a signed HP binary in the temp directory and a malicious binary in our swsetup directory.

A side-effect of having swsetup as our current directory is that the method will first try to “extract” the file in question. In the process of extracting the file, the method first checks if the swsetup directory exists. If it does, it deletes it. This is perfect for us, because we can easily tell when the function is almost at executing allowing us to quickly copy the malicious binary after re-creating the directory.

After extraction is attempted and after we place our malicious binary, the method will grab the filename to execute by grabbing the string after the first double quote character in the SilentInstallString parameter. This means if we set the parameter to be a"BadBinary.exe, the complete path for execution will be the current directory + BadBinary.exe. Since our current directory is C:\swsetup\dog, the complete path to the malicious binary becomes C:\swsetup\dog\BadBinary.exe. The only check on the file specified after the double quote is that it exists, there are no signature checks.

Once the final path is determined, the method starts the binary. For us, this means our malicious binary is executed. To review the example above:

  1. We place a signed HP binary named dog.exe in C:\Windows\TEMP.
  2. We create the directory chain C:\swsetup\dog.
  3. We pass in an action item base with the ExecutableName property set to dog.exe, the SPName property set to anything but BadBinary, and the SilentInstallString property set to a"BadBinary.exe.
  4. We monitor the C:\swsetup\dog directory until it is deleted.
  5. Once we detect that the dog folder is deleted, we immediately recreate it and copy in our BadBinary.exe.
  6. Our BadBinary.exe gets executed with SYSTEM permissions.

Local Privilege Escalation #3

The next Local Privilege Escalation vulnerability is accessible by both the previous protected method InstallSoftpaq and InstallSoftpaqDirectly. This exploit lies in the previously mentioned direct install method.

This method starts off similarly to its silent install counterpart by concatenating C:\Windows\TEMP and the action item base ExecutableName property. Again similarly, the method will verify that the path specified is a binary signed by HP. The mildly interesting difference here is that instead of doing an unsafe concatenation operation by just adding the two strings using the + operator, the method uses Path.Combine, the safe way to concatenate path strings.

Path.Combine is significantly safer than just adding two path strings because it checks for invalid characters and will throw an exception if one is detected, halting most path manipulation attacks.

C# Path.Combine method

What this means for us as an attacker is that our ExecutableName property cannot escape the C:\Windows\TEMP directory, because / is one of the blacklisted characters.

This isn’t a problem, because unprivileged processes CAN write directly to C:\Windows\TEMP. This means we can place a binary to run right into the TEMP directory and have this method eventually execute it. Here’s the small issue with that, the HP Certificate check.

How do we get past that? We can place a legitimate HP binary into the temporary directory for execution, but we need to somehow hijack its context. In the beginning of this report, we outlined how we can enter the context of a signed HP binary by injecting into it, this isn’t exactly possible here because the service is the one creating the process. What we CAN do is some simple DLL Hijacking.

DLL Hijacking means we place a dynamically loaded library that is imported by the binary into the current directory of the signed binary. One of the first locations Windows searches when loading a dynamically linked library is the current directory. When the signed binary attempts to load that library, it will load our malicious DLL and give us the ability to execute in its context.

I picked at random from the abundant supply of signed HP binaries. For this attack, I went with HPWPD.exe which is HP’s “Web Products Detection” binary. A simple way to find a candidate DLL to hijack is by finding the libraries that are missing. How do we find “missing” dynamically loaded libraries? Procmon to the rescue!

Using three simple filters and then running the binary, we’re able to easily determine a missing library.

Procmon filters Procmon results

Now we have a few candidates here. C:\VM is the directory where the binary resides on my test virtual machine. Since the first path is not in the current directory, we can disregard that. From there on, we can see plenty of options. Each one of those CreateFile operations usually mean that the binary was attempting to load a dynamically linked library and was checking the current directory for it. For attacking purposes, I went with RichEd20.dll just because it’s the first one I saw.

To perform this Local Privilege Escalation attack, all we have to do is write a malicious DLL, name it RichEd20.dll, and stick it with the binary in C:\Windows\TEMP. Once we call the direct install method passing the HPWPD.exe binary as the ExecutableName property, the method will start it as the SYSTEM user and then the binary will load our malicious DLL escalating our privileges.

Discovery: Local Privilege Escalation #4

The next protected service method to look at is DownloadSoftPaq.

To start with, this method takes in an action item base, a String parameter called MD5URL, and a Boolean indicating whether or not this is a manual installation. As an attacker, we control all three of these parameters.

HP DownloadSoftPaq method snippet

If we pass in true for the parameter isManualInstall AND the action item base property UrlResultUI is not empty, we’ll use this as the download URL. Otherwise, we’ll use the action item base property UrlResult as the download URL. If the download URL does not start with http, the method will force the download URL to be http://.

HP isValidUrl integrity check

This is an interesting check. The method will make sure that the Host of the specified download URL ends with either .hp.com OR .hpicorp.net. If this Boolean is false, the download will not continue.

Another check is making a HEAD request to the download URL and ensuring the response header Content-Length is greater than 0. If an invalid content length header is returned, the download won’t continue.

The location of the download is the combination of C:\Windows\TEMP and the action item base ExecutableName property. The method does not use safe path concatenation and instead uses raw string concatenation. After verifying that there is internet connection, the content length of the download URL is greater than 0, and the URL is “valid”, the download is initiated.

If the action item base ActionType property is equal to “PrinterDriver”, the method makes sure the downloaded file’s MD5 hash equals that specified in the CheckSum property or the one obtained from HP’s CDN server.

HP Final Verification snippet HP VerifyDownloadSignature snippet

After the file has completed downloading, the final verification step is done. The VerifyDownloadSignature method will check if the downloaded file is signed. If the file is not signed and the shouldDelete argument is true, the method will delete the downloaded file and return that the download failed.

Exploitation: Local Privilege Escalation #4

Let’s say I wanted to abuse DownloadSoftPaq to download my file to anywhere in the system, how could I do this?

Let’s take things one at a time, starting with the first challenge, the download URL. Now the only real check done on this URL is making sure the host of the URL http://[this part]/something ENDS with .hp.com or .hpicorp.net. As seen in the screenshot of this check above, the method takes the safe approach of using C#’s URI class and grabbing the host from there instead of trying to parse it out itself. This means unless we can find a “zero day” in C#’s parsing of host names, we’ll need to approach the problem a different way.

Okay, so our download URL really does need to end with an HP domain. DNS Hijacking could be an option, but since we’re going after a Local Privilege Escalation bug, that would be pretty lame. How about this… C# automatically follows redirects, so what if we found an open redirect bug in any of the million HP websites? Time to use one of my “tricks” of getting a web bug primitive.

Sometimes I need a basic exploit primitive like an open redirect vulnerability or a cross site scripting vulnerability but I’m not really in the mood to pentest, what can I do? Let me introduce you to a best friend for an attacker, “OpenBugBounty”! OpenBugBounty is a site where random researchers can submit web bugs about any website. OpenBugBounty will take these reports and attempt to email several security emails at the vulnerable domain to report the issue. After 90 days from the date of submission, these reports are made public.

So I need an open redirect primitive, how can I use OpenBugBounty to find one? Google it!

OpenBugBounty Google search

Nice, we got a few options, let’s look at the first one.

OpenBugBounty vulnerability

An unpatched bug, just what we wanted. We can test the open redirect vulnerability by going to a URL like https://ers.rssx.hp.com/ers/redirect?targetUrl=https://google.com and landing on Google, thanks TvM and HP negligence (it’s been 5 months since I re-reported this open redirect bug and it’s still unpatched)!

Back to the method, this open redirect bug is on the host ers.rssx.hp.com which does end in .hp.com making it a perfect candidate for an attack. We can redirect to our own web server where the file is stored to make the method download any file we want.

The next barrier is being able to place the file where we want. This is pretty simple, because HP didn’t do safe path string concatenation. We can just set the action item base ExecutableName property to be a relative path such as ../../something allowing us to easily control the download location.

We don’t have to worry about MD5 verification if we just pass something other than “PrinterDriver” for the ActionType property.

The final check is the verification of the downloaded file’s signature. Let’s quickly look review the method.

HP Final Verification snippet HP shouldDelete check

If the file is not signed and the shouldDelete argument is true, the method will delete the downloaded file and return that the download failed.

The shouldDelete argument is the first argument passed to the VerifyDownloadSignature method. I didn’t want to spoil it in discovery, but this is hilarious because as you can see in the picture above, the first argument to VerifyDownloadSignature is always false. This means that even if the downloaded file fails signature verification, since shouldDelete is always false, the file won’t be deleted. I really don’t know how to explain this one… did an HP Developer just have a brain fart? Whatever the reason, it made my life a whole lot easier.

At the end of the day, we can place any file anywhere we want in the context of a SYSTEM process allowing for escalation of privileges.

Discovery: Local Privilege Escalation #5

The next protected method we’ll be looking at is CreateInstantTask. For our purposes, this is a pretty simple function to review.

The method takes in three arguments. The String parameter updatePath which represents the path to the update binary, the name of the update, and the arguments to pass to the update binary.

The first verification step is checking the path of the update binary. HP VerifyProgramPath method

This verification method will first ensure that the update path is an absolute path that does not lead to a junction or reparse point. Good job on HP for checking this.

The next check is that the update path “starts with” a whitelisted base path. For my 64-bit machine, MainAppPath expands to C:\Program Files (x86)\Hewlett Packard\HP Support Framework and FrameworkPath expands to C:\Program Files (x86)\Hewlett Packard\HP Support Solutions.

What this check essentially means is that since our path has to be absolute AND it must start with a path to HP’s Program Files directory. The update path must actually be in their installation folder.

The final verification step is ensuring that the update path points to a binary signed by HP.

If all of these checks pass, the method creates a Task Scheduler task to be executed immediately by an Administrator. If any Administrator is logged on, the binary is immediately executed.

Exploitation: Local Privilege Escalation #5

A tricky one. Our update path must be something in HP’s folders, but what could that be? This stumped me for a while, but I got a spark of an idea while brainstorming.

We have the ability to start ANY program in HP’s installation folder with ANY arguments, interesting. What types of binaries does HP’s installation folders have? Are any of them interesting?

I found a few interesting binaries in the whitelisted directories such as Extract.exe and unzip.exe. On older versions, we could use unzip.exe, but a new update implements a signature check. Extract.exe didn’t even work when I tried to use it normally. I peeked at several binaries in dnSpy and I found an interesting one, HPSF_Utils.exe.

When the binary was started with the argument “encrypt” or “decrypt”, the method below would be executed.

HPSF_Utils.exe snippet

If we passed “encrypt” as the first argument, the method would take the second argument, read it in and encrypt it, then write it to the file specified at the third argument. If we passed “decrypt”, it would do the same thing except read in an encrypted file and write the decrypted version to the third argument path.

If you haven’t caught on to why this is useful, the reason it’s so useful is because we can abuse the decrypt functionality to write our malicious file to anywhere in the system, easily allowing us to escalate privileges. Since this binary is in the whitelisted path, we can execute it and abuse it to gain Administrator because we can write anything to anywhere.

Arbitrary File Deletion Vulnerabilities

Discovery: Arbitrary File Deletion

Let’s start with some arbitrary file deletion vulnerabilities. Although file deletion probably won’t lead to Local Privilege Escalation, I felt that it was serious enough to report because I could seriously mess up a victim’s machine by abusing HP’s software.

The protected method we’ll be looking at is LogSoftPaqResults. This method is very short for our purposes, all we need to read are the first few lines.

HP LogSoftPaqResults method

When the method is called, it will combine the temporary path C:\Windows\TEMP and the ExecutableName from the untrusted action item base using an unsafe string concatenation. If the file at this path exists AND it is not signed by HP, it will go ahead and delete it.

Exploitation: Arbitrary File Deletion #1

This is a pretty simple bug. Since HP uses unsafe string concatenation on path strings, we can just escape out of the C:\Windows\TEMP directory via relative path escapes (i.e ../) to reach any file we want.

If that file exists and isn’t signed by HP, the method which is running in the context of a SYSTEM process will delete that file. This is pretty serious because imagine if I ran this on the entirety of your system32 folder. That would definitely not be good for your computer.

Exploitation: Arbitrary File Deletion #2

This bug was even simpler than the last one, so I decided not to create a dedicated discovery section. Let’s jump into the past and look at the protected method UncompressCabFile. When the method starts, as we reviewed before, the following is checked:

  1. cabFilePath (path to the cabinet file to extract) exists.
  2. cabFilePath is signed by HP.
  3. Our token is “trusted” and the cabFilePath contains a few pre-defined paths.

If these conditions are not met, the method will delete the file specified by cabFilePath.

If we want to delete a file, all we need to do is specify pretty much any path in the cabFilePath argument and let the method delete it while running as the SYSTEM user.

Remote Code Execution Vulnerability

Discovery

For most of this report we’ve reviewed vulnerabilities inside the HP service. In this section, we’ll explore a different HP binary. The methods reviewed in “General Exploitation” will not apply to this bug.

When I was looking for attack surfaces for Remote Code Execution, one of the first things I checked was for registered URL Protocols. Although I could find these manually in regedit, I opted to use the tool URLProtocolView. This neat tool will allow us to easily enumerate the existing registered URL Protocols and what command line they execute.

Scrolling down to “hp” after sorting by name showed quite a few options.

URLProtocolView HP URLs

The first one looked mildly interesting because the product name of the binary was “HP Download and Install Assistant”, could this assist me in achieving Remote Code Execution? Let’s pop it open in dnSpy.

The first thing the program does is throw the passed command line arguments into its custom argument parser. Here are the arguments we can pass into this program:

  1. remotefile = The URL of a remote file to download.
  2. filetitle = The name of the file downloaded.
  3. source = The “launch point” or the source of the launch.
  4. caller = The calling process name.
  5. cc = The country used for localization.
  6. lc = The language used for localization.

After parsing out the arguments, the program starts creating a “download request”. The method will read the download history to see if the file was already downloaded. If it wasn’t downloaded before, the method adds the download request into the download requests XML file.

After creating the request, the program will “process” the XML data from the history file. For every pending download request, the method will create a downloader.

During this creation is when the first integrity checks are done. The constructor for the downloader first extracts the filename specified in the remotefile argument. It parses the name out by finding the last index of the character / in the remotefile argument and then taking the substring starting from that index to the end of the string. For example, if we passed in https://example.com/test.txt for the remote file, the method would parse out text.txt.

If the file name contains a period, verification on the extension begins. First, the method parses the file extension by getting everything after the last period in the name. If the extension is exe, msi, msp, msu, or p2 then the download is marked as an installer. Otherwise, the extension must be one of 70 other whitelisted extensions. If the extension is not one of those, the download is cancelled.

After verifying the file extension, the complete file path is created.

HPDIA CreateLocalFilename method

First, if the lc argument is ar (Arabic), he (Hebrew), ru (Russian), zh (Chinese), ko (Korean), th (Thai), or ja (Japanese), your file name is just the name extracted from the URL. Otherwise, your file name is the filetitle argument, ` – `, and the file name from the URL combined. Finally, the method will replace any invalid characters present in Path.GetInvalidFileNameChars with a space.

This sanitized file name is then combined with the current user’s download folder + HP Downloads.

The next relevant step is for the download to begin, this is where the second integrity check is. First, the remote file URL argument must begin with https. Next, a URI object is created for the remote file URL. The host of this URI must end with .hp.com or end with .hpicorp.net. Only if these checks are passed is the download started.

Once the download has finished, another check is done. Before the download is marked as downloaded, the program verifies the downloaded content. This is done by first checking if the download is an installer, which was set during the file name parsing stage. If the download is an installer then the method will verify the certificate of the downloaded file. If the file is not an installer, no check is done.

HPDIA VerifyHPSignature method

The first step of verifying the certificate of the file is to check that it matches the binary. The method does this by calling WinVerifyTrust on the binary. The method does not care if the certificate is revoked. The next check is extracting the subject of the certificate. This subject field must contain in any case hewlett-packard, hewlett packard, or o=hp inc.

When the client presses the open or install button on the form, this same check is done again, and then the file is started using C#’s Process.Start. There are no arguments passed.

Exploitation

There are several ways to approaching exploiting this program. In the next sections, I’ll be showing different variants of attacking and the pros/cons of each one. Let’s start by bypassing a check we need to get past for any variant, the URL check.

As we just reviewed, the remote URL passed must have a host that ends with .hp.com or .hpicorp.net. This is tricky because the verification method uses the safe and secure way of verifying the URL’s host by first using C#’s URI parser. Our host will really need to contain .hp.com or .hpicorp.net.

A previous vulnerability in this report had the same issue. If you haven’t read that section, I would highly recommend it to understand where we got this magical open redirect vulnerability in one of HP’s websites: https://ers.rssx.hp.com/ers/redirect?targetUrl=https://google.com.

Back to the verification method, this open redirect bug is on the host ers.rssx.hp.com which does end in .hp.com making it a perfect candidate for an attack. We can redirect to our own web server where we can host any file we want.

This sums up the “general” exploitation portion of the Remote Code Execution bugs. Let’s get into the individual ones.

Remote Code Execution Variant #1

The first way I would approach this seeing what type of file we can have our victim download without the HP program thinking it’s an installer. Only if it’s an installer will the program verify its signature.

Looking at the whitelisted file extensions, we don’t have that much to work with, but we have some options. One whitelisted option is zip, this could work. An ideal attack scenario would be a website telling you need a critical update, the downloader suddenly popping up with the HP logo, the victim pressing open on the zip file and executing a binary inside it.

To have a valid remote file URL, we will need to abuse the open redirect bug. I put my bad file on my web server and just put the URL to the zip file at the end of the open redirect URL.

When the victim visits my malicious website, an iframe with the hpdia:// URL Protocol is loaded and the download begins.

This attack is somewhat lame because it requires two clicks to RCE, but I still think it’s more than a valid attack method since the HP downloader looks pretty legit.

Remote Code Execution Variant #2

My next approach is definitely more interesting then the last. In this method, we first make the program download a DLL file which is NOT an installer, then we install a signed HP binary who depends on this DLL.

Similar to our DLL Hijacking attack in the third Local Privilege Escalation vulnerability, we can use any signed binary which imports a DLL that doesn’t exist. By simply naming our malicious DLL to the missing DLL name, the executed signed program will end up loading our malicious DLL giving us Remote Code Execution.

Here’s the catch. We’re going to have to set the language of the downloader to Arabic, Hebrew, Russian, Chinese, Korean, Thai, or Japanese. The reason is for DLL Hijacking, we need to set our malicious DLL name to exactly that of the missing DLL.

HPDIA CreateLocalFilename method

If you recall the CreateLocalFilename method, the actual filename will have ` – ` in it if the language we pass in through arguments is not one of those languages. If we do pass in a language that’s one of the ones checked for in the function above, the filename will be the filename specified in the remote URL.

If your target victim is in a country where one of these languages is spoken commonly, this is in my opinion the best attack route. Otherwise, it may be a bit of an optimistic approach.

Whenever the victim presses on any of the open or install all buttons, the signed binary will start and load our malicious DLL. If your victim is in an English country or only speaks English, this may be a little more difficult.

You could have the website present a critical update guide telling the visitor what button to press to update their system, but I am not sure if a user is more likely to press a button in a foreign language or open a file in a zip they opened.

At the end of the day, this is a one click only attack method, and it would work phenomenal in a country that uses the mentioned languages. This variant can also be used after detecting the victim’s language and seeing if it’s one of the few languages we can use with this variant. You can see the victim’s language in HTTP headers or just the “navigator language” in javascript.

Remote Code Execution Variant #3

The last variant for achieving remote code execution is a bit optimistic in how much resources the attacker has, but I don’t think it a significant barrier for an APT.

To review, if the extension of the file we’re downloading is considered an installer (see installer extensions in discovery), our binary will be checked for HP’s signature. Let’s take a look at the signature check.

HPDIA VerifyHPSignature method

The method takes in a file name and first verifies that it’s a valid certificate. This means a certificate ripped from a legitimate HP binary won’t work because it won’t match the binary. The concerning part is the check for if the certificate is an HP certificate.

To be considered an HP certificate, the subject of the certificate must contain in any case hewlett-packard, hewlett packard, or o=hp inc. This is a pretty inadequate check, especially that lower case conversion. Here are a few example companies I can form that would pass this check:

  1. [something]hewlett PackArd[someting]
  2. HP Inc[something]

I could probably spend days making up company names. All an attacker needs to do is create an organization that contains any of those words and they can get a certificate for that company. Next thing you know, they can have a one click RCE for anyone that has the HP bloatware installed.

Proof of Concept

Local Privilege Escalation Vulnerabilities

Remote Code Execution Vulnerabilities

“Remediation”

HP had their initial patch finished three months after I sent them the report of my findings. When I first heard they were aiming to patch 10 vulnerabilities in such a reasonable time-frame, I was impressed because it appeared like they were being a responsible organization who took security vulnerabilities seriously. This was in contrast to the performance I saw last year with my research into Remote Code Execution in Dell’s bloatware where they took 6 months to deliver a patch for a single vulnerability, an absurd amount of time.

In the following section, I’ll be going over the vulnerabilities HP did not patch or patched inadequately. Here is an overview of what was actually patched:

  1. Local Privilege Escalation #1 – Patched ✔️
  2. Local Privilege Escalation #2 – Unpatched ❌
  3. Local Privilege Escalation #3 – Unpatched ❌
  4. Local Privilege Escalation #4 – “Patched” 😕 (not really)
  5. Local Privilege Escalation #5 – Unpatched ❌
  6. Arbitrary File Deletion #1 – Patched ✔️
  7. Arbitrary File Deletion #2 – Patched ✔️
  8. Remote Code Execution Variant #1 – Patched ✔️
  9. Remote Code Execution Variant #2 – Patched ✔️
  10. Remote Code Execution Variant #3 – Patched ✔️

Re-Discovery

New Patch, New Vulnerability

The largest change observed is the transition from the hardcoded “C:\Windows\TEMP” temporary directory to relying on the action item base supplied by the client to specify the temporary directory.

I am not sure if HP PSIRT did not review the patch or if HP has a lack of code review in general, but this transition actually introduced a new vulnerability. Now, instead of having a trusted hardcoded string be the deciding factor for the temporary directory… HP relies on untrusted client data (the action item base).

As an attacker, my unprivileged process is what decides the value of the temporary directory used by the elevated agent, not the privileged service process, allowing for simple spoofing. This was one of the more shocking components of the patch. HP had not only left some vulnerabilities unpatched but in doing so ended up making some code worse than it was. This change mostly impacted the Local Privilege Escalation vulnerabilities.

Local Privilege Escalation #2

Looking over the new InstallSoftpaqsSilently method, the primary changes are:

  1. The untrusted ActionItemBase specifies the temporary directory.
  2. There is more insecure path concatenation (path1 + path2 versus C#’s Path.Combine).
  3. There is more verification on the path specified by the temporary directory and executable name (i.e checking for relative path escapes, NTFS reparse points, rooted path, etc).

The core problems that are persistent even after patching:

  1. The path that is executed by the method still utilizes unvalidated untrusted client input. Specifically, the method splits the item property SilentInstallString by double quote and utilizes values from that array for the name of the binary. This binary name is not validated besides a simple check to see if the file exists. No guarantees it isn’t an attacker’s binary.
  2. There is continued use of insecure coding practices involving paths. For example, HP consistently concatenates paths by using the string combination operator versus using a secure concatenation method such as Path.Combine.

In order to have a successful call to InstallSoftpaqsSilently, you must now meet these requirements:

  1. The file specified by the TempDirectory property + ExecutableName property must exist.
  2. The file specified by TempDirectory property + ExecutableName property must be signed by HP.
  3. The path specified by TempDirectory property + ExecutableName property must pass the VerifyHPPath requirements: a. The path cannot contain relative path escapes. b. The path must be a rooted path. c. The path must not be a junction point. d. The path must start with HP path.
  4. The SilentInstallString property must contain the SPName property + .exe.
  5. The directory name is the TempDirectory property + swsetup\ + the ExecutableName property with .exe stripped.
  6. The Executable Name is specified in the SilentInstallString property split by double quote. If the split has a size greater than 1, it will take the second element as EXE Name, else the first element.
  7. The Executable Name must not contain .exe or .msi, otherwise, the Directory Name + \ + the Executable Name must exist.
  8. The function executes the binary specified at Directory Name + \ + Executable Name.

The core point of attack is outlined in step 6 and 7. We can control the Executable Name by formulating a malicious payload for the SilentInstallString item property that escapes a legitimate directory passed in the TempDirectory property into an attacker controlled directory.

For example, if we pass:

  1. C:\Program Files (x86)\Hewlett-Packard\HP Support Framework\ for the TempDirectory property.
  2. HPSF.exe for the the ExecutableName property.
  3. malware for the SPName property.
  4. "..\..\..\..\..\Malware\malware.exe (quote at beginning intentional) for the SilentInstallString property.

The service will execute the binary at C:\Program Files (x86)\Hewlett-Packard\HP Support Framework\swsetup\HPSF\..\..\..\..\..\Malware\malware.exe which ends up resolving to C:\Malware\malware.exe thus executing the attacker controlled binary as SYSTEM.

Local Privilege Escalation #3

The primary changes for the new InstallSoftpaqsDirectly method are:

  1. The untrusted ActionItemBase specifies the temporary directory.
  2. Invalid binaries are no longer deleted.

The only thing we need to change in our proof of concept is to provide our own TempDirectory path. That’s… it.

Local Privilege Escalation #4

For the new DownloadSoftpaq method, the primary changes are:

  1. The download URL cannot have query parameters.

The core problems that are persistent even after patching:

  1. Insecure validation of the download URL.
  2. Insecure validation of the downloaded binary.
  3. Open Redirect bug on ers.rssx.hp.com.

First of all, HP still only does a basic check on the hostname to ensure it ends with a whitelisted host, however, this does not address the core issue that this check in itself is insecure.

Simply checking hostname exposes attacks from Man-in-the-Middle attacks, other Open Redirect vulnerabilities, and virtual host based attacks (i.e a subdomain that redirects to 127.0.0.1).

For an unknown reason, HP still does not delete the binary downloaded if it does not have an HP signature because HP passes the constant false for the parameter that indicates whether or not to delete a bad binary.

At the end of the day, this is still a patched vulnerability since at this time you can’t quite exploit it, but I wouldn’t be surprised to see this exploited in the future.

Local Privilege Escalation #5

This vulnerability was completed untouched. Not much more to say about it.

Timeline

Here is a timeline of HP’s response:

10/05/2019 - Initial report of vulnerabilities sent to HP. 10/08/2019 - HP PSIRT acknowledges receipt and creates a case. 12/19/2019 - HP PSIRT pushes an update and claims to have “resolved the issues reported”. 01/01/2020 - Updated report of unpatched vulnerabilities sent to HP. 01/06/2020 - HP PSIRT acknowledges receipt. 02/05/2020 - HP PSIRT schedules a new patch for the first week of March. 03/09/2020 - HP PSIRT pushes the scheduled patch to 03/21/2020 due to Novel Coronavirus complications.

Protecting your machine

If you’re wondering what you need to do to ensure your HP machine is safe from these vulnerabilities, it is critical to ensure that it is up to date or removed. By default, HP Support Assistant does not have automatic updating by default unless you explicitly opt-in (HP claims otherwise).

It is important to note that because HP has not patched three local privilege escalation vulnerabilities, even if you have the latest version of the software, you are still vulnerable unless you completely remove the agent from your machine (Option 1).

Option 1: Uninstall

The best mitigation to protect against the attacks described in this article and future vulnerabilities is to remove the software entirely. This may not be an option for everyone, especially if you rely on the updating functionality the software provides, however, removing the software ensures that you’re safe from any other vulnerabilities that may exist in the application.

For most Windows installations, you can use the “Add or remove programs” component of the Windows control panel to uninstall the service. There are two pieces of software to uninstall, one is called “HP Support Assistant” and the other is called “HP Support Solutions Framework”.

Option 2: Update

The next best option is to update the agent to the latest version. The latest update fixes several vulnerabilities discussed except for three local privilege escalation vulnerabilities.

There are two ways to update the application, the recommended method is by opening “HP Support Assistant” from the Start menu, click “About” in the top right, and pressing “Check for latest version”. Another method of updating is to install the latest version from HP’s website here.

Written on April 3, 2020