CVE-2021-31166 — Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

• Documented patch — portal.msrc.microsoft.com (NIST National Vulnerability Database)

CISA required action CISA

Apply updates per vendor instructions.

US federal deadline: Apr 27, 2022 — passed 1633 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

1. Identify exposed assets running the affected vendor/product/version combinations (Microsoft Windows 10 2004, Microsoft Windows 10 20h2, Microsoft Windows 10 Version 2004…).
2. Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
3. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
4. Apply the fixed version or the patch documented under Remediation.
5. EPSS ahead of 100% of scored CVEs: prioritize it in the patch queue.
6. Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-416 · Use After Free

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

Public exploits, PoCs and detection templates

Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.

Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.

EPSS over time FIRST

Oct 13, 2026 → Oct 15, 2026 · 3 reading(s) · max 99.87%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

• Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
• Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.
• Hunt retroactively in logs from before Apr 6, 2022 (KEV listing): exploitation may have started earlier.

Timeline

What happened to this CVE, newest first.

• 2026
• Affected products added · microsoft/windows 10 version 2004, microsoft/windows server version 2004, microsoft/windows 10 version 20h2, microsoft/w
  Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
• SSVC assessment updated
  Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
• Record published
  Oct 13, 2026
• Fix documented
  Oct 13, 2026 · NIST National Vulnerability Database
• Affected products added · microsoft/windows 10 2004, microsoft/windows 10 20h2, microsoft/windows server 2004, microsoft/windows server 20h2
  Oct 13, 2026 · NIST National Vulnerability Database
• CVSS changed · 9.8
  Oct 13, 2026 · NIST National Vulnerability Database
• Source status changed · → Analyzed
  Oct 13, 2026 · NIST National Vulnerability Database
• Added to CISA KEV (detected)
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• Record created in this database
  Oct 13, 2026 · CISA Known Exploited Vulnerabilities
• 2022
• Added to CISA KEV catalog
  Apr 5, 2022 · CISA
• 2021
• CVE published
  May 11, 2021 · microsoft

Source activity

Repeated readings are counted rather than listed.

• 2× References added · Oct 13, 2026 – Oct 13, 2026
• 1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

5 on the record

• portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31166
  Patch, Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
• www.cisa.gov/known-exploited-vulnerabilities-catalog
  US Government Resource, NIST National Vulnerability Database
• packetstormsecurity.com/files/162722/Microsoft-HTTP-Protocol-Stack-Remote-Code-Execution.html
  Third Party Advisory, VDB Entry, x_refsource_MISC, NIST National Vulnerability Database
• github.com/advisories/GHSA-3mj9-c62w-jw5w
  GitHub Security Advisories
• nvd.nist.gov/vuln/detail/CVE-2021-31166
  GitHub Security Advisories

Elsewhere on this site

Related CVEs (same products)

• High CVE-2026-50508 Windows NTLM Spoofing Vulnerability Jun 9, 2026
• High CVE-2023-28252 Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability KEV Apr 11, 2023
• High CVE-2023-28229 Microsoft Windows CNG Key Isolation Service Privilege Escalation Vulnerability KEV Apr 11, 2023
• Medium CVE-2023-24880 Microsoft Windows SmartScreen Security Feature Bypass Vulnerability KEV Mar 14, 2023
• High CVE-2023-21823 Microsoft Windows Graphic Component Privilege Escalation Vulnerability KEV Feb 14, 2023
• High CVE-2023-23376 Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability KEV Feb 14, 2023

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

• No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
• No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
• No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2021-31166 is "Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability", rated Critical. It affects Windows 10 2004 (Microsoft); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 9.8 describes technical severity. EPSS 99.87% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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