CVE-2021-31166 — Windows 10 20h2
CRITICALCVE-2021-31166 is a critical-severity (CVSS 9.8) Use After Free vulnerability in microsoft windows 10 20h2. It is in CISA's Known Exploited Vulnerabilities catalog (added 2022-04-06) — treat it as actively exploited and patch now. A fix is available — see the affected versions and patch details below.
HTTP Protocol Stack Remote Code Execution Vulnerability
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2022-04-06. Federal agencies were required to remediate by 2022-04-27.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-31166.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-31166 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 395,249 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
HTTP Protocol Stack Remote Code Execution Vulnerability
Affected Products
windows 10 20h2microsoftwindows 10 2004microsoftwindows server 20h2microsoftwindows server 2004microsoftResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
HTTP Protocol Stack Remote Code Execution Vulnerability
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft windows 10 20h2 deployment and check each version against the affected-products list above.
Fix
Apply the microsoft windows 10 20h2 security patch or hotfix for CVE-2021-31166 on the affected version, following the vendor advisory for your exact build.
Workarounds
Constrain what reaches the vulnerable code: limit the size and shape of untrusted input, isolate the affected component in a sandboxed or least-privileged process, and enable the platform's memory-safety mitigations (ASLR, stack protector, hardened allocator) so an out-of-bounds access is more likely to fail closed than to be exploitable. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
Frequently Asked Questions
Is CVE-2021-31166 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.