New Print Spooler Vulnerability - CVE-2021-36958

THE THREAT

On August 11th, Microsoft announced a new Windows Print Spooler vulnerability, tracked as CVE-2021-36958. This vulnerability is related to the recent class of vulnerabilities referred to as PrintNightmare. Microsoft has classified CVE-2021-36958 as a Remote Code Execution vulnerability; exploitation would allow an adversary to install programs, alter data, or create new user accounts.

Proof-of-Concept exploit code is publicly available. eSentire's security staff have tested the public exploit and confirmed that exploitation allows for Local Privilege Escalation.

Attacks have not been observed at the time of writing but are expected in the near future. Organizations are recommended to apply the mitigations, provided by Microsoft, until security patches are made available.

What we’re doing about it

• eSentire Managed Vulnerability Service (MVS) will automatically add the relevant plugins for this vulnerability once details are made available
• eSentire's security teams continue to track this topic for additional details and detection opportunities

What you should do about it

• Until security patches are released, Microsoft recommends organizations disable the Windows Print Spooler service.
  • This solution will impact the ability to print both locally and remotely. Consider disabling the service on critical systems where it is not needed, such as domain controllers.
• Alternative mitigation options have been proposed, and if considered, should be tested prior to implementation. These options reduce the attack surface but they do not fully mitigate the issue.
  • CERT/CC advises blocking SMB traffic at the network boundary.
  • Restrict package point and print connections to a list of approved servers. Security researcher Benjamin Delphi demonstrated this was effective in blocking his PoC exploit.

Additional information

CVE-2021-36958 is the latest vulnerability in a long list of recent vulnerabilities impacting Windows Print Spooler. Previous PrintNightmare security patches are not effective in mitigating this latest vulnerability. In an attack scenario, a threat actor may exploit CVE-2021-36958 by connecting to a remote printer under the attacker’s control. By exploiting the CopyFile directive, the attacker can copy arbitrary files to the client’s system which are subsequently executed with SYSTEM privileges.

Despite being classified as a Remote Code Execution vulnerability, successful exploitation does require local access. Public reporting claims that this vulnerability is misclassified and should be considered a Local Privilege Escalation (LPE) vulnerability. eSentire's security staff have tested the exploit and confirmed that LPE is possible.

Previous PrintNightmare vulnerabilities have already been exploited by a variety of threat actor groups. At least two separate ransomware groups (Conti & Magniber) have been confirmed to target PrintNightmare vulnerabilities. The known exploitation of similar vulnerabilities increases the importance of ensuring that mitigations are in place to prevent exploitation of CVE-2021-36958.

Timeline of PrintNightmare Vulnerabilities

CVE-2021-1675 - Patched June 8, 2021
CVE-2021-34527 - Patched July 7, 2021
CVE-2021-34481 - Patched August 10, 2021
CVE-2021-34483 - Patched August 10, 2021
CVE-2021-36936 - Patched August 10, 2021
CVE-2021-36947 - Patched August 10, 2021
CVE-2021-36958 - Microsoft Advisory August 11, 2021. Mitigations only.

References:

[1] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36958
[2] https://www.bleepingcomputer.com/news/microsoft/microsoft-confirms-another-windows-print-spooler-zero-day-bug/
[3] https://www.kb.cert.org/vuls/id/131152
[4] https://github.com/gentilkiwi/mimikatz/blob/master/mimispool/README.md