Threats / Microsoft / CVE-2021-31166
CVE-2021-31166
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-06-22
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.
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
Affected: Microsoft, HTTP Protocol Stack. Confirm exact fixed builds in the vendor advisory.
Weakness (CWE)
Mapped to CWE-416 Use After Free — weakness family: Memory safety.CWE assignment from the public NVD record; the weakness class drives how the flaw is exploited.
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