Micropatches for two Windows Print Spooler Elevation of Privilege issues (CVE-2022-30206, CVE-2022-21997)

 

by Mitja Kolsek, the 0patch Team

On September 24, 2022 we were made aware of a POC for a Print Spooler elevation of privilege vulnerability discovered by security researcher luckyu with NSFOCUS TIANYUAN LAB. It turned out to be another symbolic link issue that Print Spooler has quite a history of.

The POC sets up a new printer with a custom spool directory as a non-admin user. This directory is a symbolic link to another directory, which contains a .SHD file that is itself again a symbolic link to some existing file which a non-admin user lacks permissions to delete. Then, by using file locking and performing some operations on the printer, the Print Spooler process (running as Local System) is made to delete said .SHD file, which in fact deletes the file it points to. A non-admin can therefore delete any local file that Local System is able to delete. This can, surprisingly, lead to arbitrary code execution using a trick earlier discovered by another researcher Abdelhamid Naceri and described in this ZDI article.

Microsoft assigned CVE-2022-30206 to this issue and fixed it with July 2022 updates. Not for everyone, of course: we determined that older Windows versions that had stopped receiving updates by then were also affected, and therefore needed our patch.

Having decided to patch this issue, we analyzed it, found the root cause and determined that the best approach would be similar to what we've done with other symbolic link issues in Print Spooler before: add a check in localspl.dll before the DeleteFile call to see if a path to the .SHD file is a symbolic link - and skip the deletion in case it was. This would not break the typical use case without symbolic links, and we doubt that any legitimate use case would involve a .SHD file being a symbolic link.

Having written such micropatch, we started testing it and discovered something peculiar: the linked-to file still got deleted even if our patch properly bypassed the DeleteFile call. What was going on?

It turned out that while we have patched the issue at hand, there was another very similar issue elsewhere in the code that also led to deleting the .SHD file. Interestingly though, this issue was not present on a Windows 10 v1909 updated to its last updates in May 2022. This version of Windows 10 was the last security-adopted Windows version at the time and received Windows updates a bit longer than others - which meant that it must have received a patch for this "newly discovered" issue while others haven't.

To identify this second issue, we tested the POC with our patch on Windows 10 v1909 with different Windows updates starting with May 2022 (the last updates for v1909) and going back in time. This revealed that the issue was fixed with February 2022 updates.

Looking at all vulnerabilities fixed with February 2022 updates, only one fit the description: CVE-2022-21997. It was a "Windows Print Spooler Elevation of Privilege Vulnerability", with the Microsoft advisory stating that "an attacker would only be able to delete targeted files on a system." The issue was reported by Bo Wu, whom we tried to contact for confirmation but unfortunately so far failed to reach.

In any case, it would make no sense to only patch the originally intended CVE-2022-30206, so we also went on to patch this "bonus" issue CVE-2022-21997. We did that with an identical patch, just in another location in localspl.dll. With both patches in place, the POC and the technique it was using no longer worked.

These micropatches were written for the following Versions of Windows with all available Windows Updates installed:

1. Windows 10 v2004
2. Windows 10 v1909 (CVE-2022-30206 only)
   
3. Windows 10 v1903
   
4. Windows 10 v1809
   
5. Windows 10 v1803
6. Windows 7 without ESU, with year 1 of ESU and with year 2 of ESU
7. Windows Server 2008 R2 without ESU, with year 1 of ESU and with year 2 of ESU
   

 

 

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We'd like to thank luckyu for publishing their analysis with a proof-of-concept that allowed us to reproduce both vulnerabilities and create a micropatch. We also encourage security researchers to privately share their analyses with us for micropatching.