CyberSecBulletin
CVE-2021-40449Exploited in the wildNVD status: Analyzed

Microsoft Windows Win32k Privilege Escalation

Published Updated By the CyberSecBulletin desk

CVE-2021-40449 is a high-severity use after free vulnerability in Windows 10 Version 1809 from Microsoft, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists version 10.0.17763.2237 as the first without the flaw, so updating Windows 10 Version 1809 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on November 17, 2021, which means attacks have been observed in the wild, including in ransomware campaigns.

Patch now

Attackers are already exploiting this vulnerability, according to CISA.

What to do

  1. Update Windows 10 Version 1809 to 10.0.17763.2237 or a later version.
  2. Update Windows Server 2019 to 10.0.17763.2237 or a later version.
  3. Update Windows Server 2019 (Server Core installation) to 10.0.17763.2237 or a later version.
  4. CISA's required action: Apply updates per vendor instructions.
  5. Read the advisory on portal.msrc.microsoft.com for workarounds and exact fixed builds.

Affected products and versions

VendorProductAffected versionsFixed in
MicrosoftWindows 10 Version 180910.0.0 up to, but not including, 10.0.17763.223710.0.17763.2237
MicrosoftWindows Server 201910.0.0 up to, but not including, 10.0.17763.223710.0.17763.2237
MicrosoftWindows Server 2019 (Server Core installation)10.0.0 up to, but not including, 10.0.17763.223710.0.17763.2237
MicrosoftWindows 10 Version 190910.0.0 up to, but not including, 10.0.18363.185410.0.18363.1854
MicrosoftWindows 10 Version 21H110.0.0 up to, but not including, 10.0.19041.128810.0.19041.1288
MicrosoftWindows Server 202210.0.0 up to, but not including, 10.0.20348.28810.0.20348.288
MicrosoftWindows 10 Version 200410.0.0 up to, but not including, 10.0.19041.128810.0.19041.1288
MicrosoftWindows Server version 200410.0.0 up to, but not including, 10.0.19041.128810.0.19041.1288
MicrosoftWindows 10 Version 20H210.0.0 up to, but not including, 10.0.19041.128810.0.19041.1288
MicrosoftWindows Server version 20H210.0.0 up to, but not including, 10.0.19042.128810.0.19042.1288
MicrosoftWindows 11 version 21H210.0.0 up to, but not including, 10.0.22000.25810.0.22000.258
MicrosoftWindows 10 Version 150710.0.0 up to, but not including, 10.0.10240.1908610.0.10240.19086
MicrosoftWindows 10 Version 160710.0.0 up to, but not including, 10.0.14393.470410.0.14393.4704
MicrosoftWindows Server 201610.0.0 up to, but not including, 10.0.14393.470410.0.14393.4704
MicrosoftWindows Server 2016 (Server Core installation)10.0.0 up to, but not including, 10.0.14393.470410.0.14393.4704
MicrosoftWindows 76.1.0 up to, but not including, 6.1.7601.257406.1.7601.25740
MicrosoftWindows 7 Service Pack 16.1.0 up to, but not including, 6.1.7601.257406.1.7601.25740
MicrosoftWindows 8.16.3.0 up to, but not including, 6.3.9600.201446.3.9600.20144
MicrosoftWindows Server 2008 Service Pack 26.0.0 up to, but not including, 6.0.6003.212516.0.6003.21251
MicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)6.0.0 up to, but not including, 6.0.6003.212516.0.6003.21251
MicrosoftWindows Server 2008 R2 Service Pack 16.1.0 up to, but not including, 6.1.7601.257406.1.7601.25740
MicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)6.0.0 up to, but not including, 6.1.7601.257406.1.7601.25740
MicrosoftWindows Server 20126.2.0 up to, but not including, 6.2.9200.234906.2.9200.23490
MicrosoftWindows Server 2012 (Server Core installation)6.2.0 up to, but not including, 6.2.9200.234906.2.9200.23490
MicrosoftWindows Server 2012 R26.3.0 up to, but not including, 6.3.9600.201446.3.9600.20144

Version data comes from the organisation that assigned the CVE; 14 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
LocalThe attacker needs local access or must trick a user into running something.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
LowA basic user account is enough.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.17.8HighCNA (microsoft.com)
CVSS 3.17.8HighNVD
CVSS 2.04.6MediumNVD

Official description

Win32k Elevation of Privilege Vulnerability

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Use After Free

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.

How developers prevent this class of flaw

  • Choose a language that provides automatic memory management.
  • When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.

More Use After Free vulnerabilities (CWE-416)

Timeline

  1. CVE record published.
  2. Added to CISA's Known Exploited Vulnerabilities catalogue.
  3. Deadline for US federal agencies to remediate, set by CISA.
  4. CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
  5. Record last changed materially (score, affected products, status or references).

References

Third-party and government advisories

Technical write-ups (may include proof of concept)

Recent Use After Free vulnerabilities

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