The trouble with Microsoft’s Troubleshooters
Recent versions of Windows are equipped with a “Troubleshooting” feature that can be found in the Control Panel or the new Settings application. You might find this tool useful if you have some trouble with your computer as it can quickly fix some common issues in a user-friendly way. This write up demonstrates an implementation flaw that can be used to compromise a computer where a crafted diagnostic package is opened. Microsoft pushed back providing a fix for the current versions of Windows; to avoid trouble, pay special attention to not open .diagcab files. Mail server operators are recommended to include this file type to their blacklists.
Windows is shipped with a bunch of Troubleshooters by default. The technology is flexible so additional diagnostic packages can be downloaded from the internet. Integrity of these packages are protected to avoid external tampering, if signature verification fails, the tool refuses to execute the package.
So where is the problem then? A flaw in the implementation allows attackers to save any files to any locations on the file system (in line with the permissions of the current user) and this takes place before the integrity of the package is checked. Even though overwriting of files is not possible via this vulnerability due to another security measure, an attacker could still gain code execution here by dropping a file to the Startup folder of Windows, which will be executed by the Operating System next time when the user logs in.
Threat actors usually distribute malicious files like this in email, via their shady websites or packaged in a torrent file. As in case of every vulnerability, there are several constraints of successful exploitation:
- The email attachments are monitored and scanned by decent email providers — so they could detect and reject messages with attachments like this
- Certain file extensions are blocked by the mail reader or web browser applications. According to Microsoft, Outlook and Internet Explorer are protected since .diagcab is blacklisted by these two softwares if the file is coming from the internet zone.
- User interaction to trigger processing of the diagnostic file by clicking on it
- The user must be using a Microsoft Windows operating system
During my testing, I concluded that neither Gmail nor Outlook Live blocked .diagcab files at all, so users of these services could be potential targets. I encountered the filtering mechanism of some MS Exchange based corporate servers blocking my attachments, however, by linking to a webdav share, I could circumvent this protection so the diagcab file could be executed in Outlook. But not even links like this can be used ultimately, they are deactivated by providers like Gmail or Outlook Live and blocked by other security measures of Internet Explorer.
Other popular products like Google Chrome, Mozilla Firefox or Thunderbird simply pop up the standard open and execute dialog box when they see .diagcab files. Actually, even Microsoft Edge. Using one of these browsers for reading emails on a web based platform is probably the biggest attack vector of this vulnerability — especially given that users of those platforms are used to the high quality pre-filtering of the emails received.
There is a Man-in-the-Middle attack vector as well; if a .diagcab file is downloaded over a clear-text network channel (like via the plain http links found here), the same attack could be performed by operators of a hostile network (like public Wi-Fi or similar).
The table below summarizes the potential attack vectors that have been tested and found the flaw to be exploitable:
The software behind the Troubleshooters technology is called Microsoft Support Diagnostics Tool and located at %WINDIR%\System32\msdt.exe. It is associated to the following file types:
Files with .diagcfg extension are simple XML files that hold reference to one or more diagnostic packages and provide meta information about them. They are packaged into Microsoft cabinet (.cab) file archives and saved with .diagcab extension. The XML content itself could look something like this:
<?xml version="1.0" encoding="utf-8"?><PackageConfiguration xmlns="http://www.microsoft.com/schemas/dcm/configuration/2008"><Execution><Package Path="%windir%\diagnostics\system\Audio"><Answers Version="1.0"><Interaction ID="IT_GetDeviceType"><Value>microphone/headset microphone</Value></Interaction></Answers></Package><Name>@%windir%\diagnostics\system\Audio\DiagPackage.dll,-33</Name><Description>@%windir%\diagnostics\system\Audio\DiagPackage.dll,-31</Description><Icon>@%windir%\diagnostics\system\Audio\DiagPackage.dll,-1003</Icon></Execution><Index><Id>AudioRecordingDiagnostic</Id><RequiresAdminPrivileges>true</RequiresAdminPrivileges><PrivacyUrl>http://go.microsoft.com/fwlink/?LinkId=190175</PrivacyUrl><Version>1.0</Version><PublisherName>Microsoft Corporation</PublisherName><Category>@%windir%\system32\DiagCpl.dll,-401</Category><Keyword>@%windir%\system32\DiagCpl.dll,-24</Keyword></Index></PackageConfiguration>
The Path attribute of the referenced packages point to a directory in the file system. Packages under %WINDIR%\Diagnostics folder are considered to be valid without any further checking, but anything else is subject to signature verification. This is implemented in the helper library sdiageng.dll. Before verifying the signature, the implementation makes a local copy of the referenced directory to a temporary random destination folder, something like “C:\Users\John Doe\AppData\Local\Temp\SDIAG_0636db01-fabd-49ed-bd1d-b3fbbe5fd0ca”. Note this path has a static number of components which will make the attack deterministic.
The function responsibly for the file transfer is SdpCopyDirectory of sdiageng.dll and its skeleton code looks something like this:
TCHAR attackerControlledSourcePath[MAXPATH]; // this is the "user input"
hFind = FindFirstFile(allFilesFromAttackerControlledSourcePath, &FindFileData);
StringCchPrintfW(srcFile, MAXPATH, L"%s\\%s",
StringCchPrintfW(dstFile, MAXPATH, L"%s\\%s", tempDirectory,
CopyFileW(srcFile, dstFile, TRUE);
while (FindNextFile(hFind, &FindFileData) != 0);FindClose(hFind);
After the copy is done, the dll verifies the integrity based on the file DiagPackage.cat. If everything is correct and the user proceeds with the wizard on the GUI, the embedded powershell scripts are executed under the hood.
Here comes the trick: since the data source is controlled by the attacker and network file systems are supported by the Windows OS natively, this pattern can be exploited before the integrity check had chance to detect the malformed package. A rogue, network attached file system can gain controlled write over the destination file system by returning ..\.. components in the file name.
For proof of concept, I chose to spin up a webdav server of my own. Webdav is a firewall friendly, lightweight protocol that is easy to script, but I believe the same attack could be mounted with CIFS as well. The UNC (Universal Naming Convention) path of a webdav share looks something like this: \\localhost@80\DavWWWRoot. An example session of interaction with such a malicious webdav server from the command prompt:
C:\Projects\diagcab>dir \\127.0.0.1@80\DavWWWRoot\packageVolume in drive \\127.0.0.1@80\DavWWWRoot has no label.Volume Serial Number is 0000-0000Directory of \\127.0.0.1@80\DavWWWRoot\package2017. 07. 12. 11:10 <DIR> .2017. 07. 12. 11:10 <DIR> ..2017. 07. 12. 10:48 27 648 ..\..\..\..\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\calc.exe1 File(s) 27 648 bytes2 Dir(s) 251 292 504 064 bytes free
Putting this altogether, the crafted .diagcab file has the package path set to the rogue webdav server. Once it is opened by the victim, a new file is saved under the Startup directory, and is executed by the operating system on the next startup.
Exploitation of the vulnerability described above was tested on Windows 10 version 1903 (OS Build 18362.535), probably other versions are affected as well.
I believe the correct remediation at MS side would be discarding file system entries that have a path separator character in their name. This could be done in the implementation of the FindFirstFile/FindNextFile API calls, which reside in kernel32.dll, so in the kernel space. This would protect all applications having a similar vulnerable pattern in their code, no matter which file system backend the nasty file system entries were returned by, providing sufficient protection for both local and remote file systems.
Since this won’t happen for a while, I don’t recommend opening .diagcab files anymore. Mail system administrators are advised to tweak the blacklist in their configurations.
Diagcab files and a rogue webdab PoC server is hosted online for demonstrational purposes. (This latter will go down once my free dyno hours are exhausted at Heroku.)
If you wish, you can test the same via this webpage hosted on Github.io.
You can inspect the webdav backend server by running the following command:
If you execute the diagcab file hosted here, it will configure your Windows to launch the calculator at login. To revert the original status of your computer press CTRL+R, type shell:startup and remove the calc.exe file from the folder.
Sources of the webdav server exploit can be found on Github:
12/22/2019: issue reported
12/23/2019: case is opened
6/1/2020: Microsoft responded to not fix the issue
9/1/2020: Draft of this write up shared with Microsoft as they requested
15/1/2020: Public advisory
MS: Microsoft has decided that it will not be fixing this vulnerability in the current version and we are closing this case. …
IR: Clicking on a non-executable file driven by an official Microsoft technology could save attacker controlled content to any attacker controlled location on the file system including shell:startup and thus accomplishing code execution. Please confirm Microsoft does not consider this being a vulnerability. …
MS: There are a number of file types that can execute code in such a way but aren’t technically “executables”. And a number of these are considered unsafe for users to download/receive in email, even .diagcab is blocked by default in Outlook on the web and other places. This is noted a number of places online by Microsoft.
The issue is that to make use of this attack an attacker needs to create what amounts to a virus, convince a user to download the virus, and then run it. Yes, it doesn’t end in .exe, but these days most viruses don’t. Some protections are already put into place, such as standard files extensions to be blocked, of which this is one. We are also always seeking to improve these protections. But as written this wouldn’t be considered a vulnerability. No security boundaries are being bypassed, the PoC doesn’t escalate permissions in any way, or do anything the user couldn’t do already.