Threats / Microsoft / CVE-2021-31166
CVE-2021-31166
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
Microsoft HTTP Protocol Stack vulnerability
Microsoft HTTP Protocol Stack (http.sys) use-after-free vulnerability enabling remote code execution. Actively exploited in the wild.
Verdict
Today item — known-exploited.
A use-after-free flaw in the HTTP Protocol Stack allows unauthenticated remote attackers to execute arbitrary code on affected systems. High exploitation prevalence and severity warrant immediate patching.
CISA KEV Yes · 2022-04-065EPSS 0.99657 (verify live)6Exploit Public PoC7CVSS 9.83CISA SSVC Exploitation: active · Automatable: yes · Impact: total4
01
Is it exploitable?
— the evidence, ranked above the scoreExploit available
Public proof-of-concept exploit code is cataloged for this vulnerability.We link the existence of the exploit; we do not host or redistribute payloads.
Reported exploitation
3 independent public reports of in-the-wild exploitation are cataloged.Distinct reporting sources (vendor, incident response, government); open them for the underlying claims.
Exploited in the wild
Listed in the CISA Known Exploited Vulnerabilities catalog (added 2022-04-06).
Probability (EPSS)
EPSS 0.99657 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C) · Affected: Microsoft, HTTP Protocol Stack. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
Weakness (CWE)
Mapped to CWE-416 Use After Free — weakness family: Memory safety.CWE assignment from the public NVD record (may be stale); the weakness class drives how the flaw is exploited.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: yes · Technical Impact: totalCISA's own stakeholder-specific triage inputs, published in the CVE record (CISA Coordinator role). NOTE: this is CISA's three decision points; the final Act/Attend/Track decision also requires a Mission & Well-being input not published here, so no overall decision is asserted.
Currency of the record
CISA added this to its KEV catalog 330 days after the CVE record was published.KEV lag = CISA-KEV dateAdded − CVE.org datePublished; a floor on how long the known-exploited flaw sat in the public record before the federal catalog listed it. Severity/weakness here is CNA- or CISA-assigned (current at disclosure), independent of NVD's analysis backlog.
Public exploit available
Public exploit tooling is catalogued for this CVE (a public proof-of-concept / detection template). Catalog entries: Metasploit module. We link the catalog page(s) only — no payload or code is hosted or restated here.Link, never host — the catalog page is the source; basicsecurity.net does not redistribute payloads.
02
Who’s exploiting it?
— attribution turns risk into urgencyAttribution not established
No confirmed (advisory-backed) threat-actor attribution is established for this record. Absence of a named actor is not absence of compromise — see Coverage & confidence.
03
Why it matters
— the attack path, told twice: adversary, then board1
Front door — unauthenticated access narrative 1
Attacker
I craft a malicious HTTP request that triggers improper memory handling in http.sys.
Business
Exposed internet-facing servers become entry points for system compromise and lateral movement.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I achieve code execution with system privileges by exploiting the freed memory region.
Business
Attackers gain full control of web servers, enabling data theft, malware deployment, and infrastructure takeover.
3
Lateral reach — past segmentation narrative 3
Attacker
I establish persistence and pivot to internal networks from the compromised host.
Business
Breach scope expands beyond the initial server, threatening enterprise networks and sensitive assets.
04
What to do
— defensible action- Remediate per the vendor advisory — confirm the fixed build for your version and verify exposure.1
Say it to the boardA vulnerability with this evidence profile is a defensible budget line, not a backlog ticket — fund the change against the proof above.
05