Threats / Microsoft / CVE-2021-36955
CVE-2021-36955
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-06-22
Microsoft Windows vulnerability
Microsoft Windows Common Log File System (CLFS) driver privilege escalation vulnerability exploited in ransomware campaigns.
Verdict
Today item, not a backlog item.
An unspecified privilege escalation flaw in the Windows CLFS driver enables attackers to escalate from limited to system-level access. Active exploitation in ransomware operations demonstrates material risk to Windows systems.
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
4 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 2021-11-03), flagged for known ransomware use.
Probability (EPSS)
EPSS 0.03054 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
Affected: Microsoft, Windows. Confirm exact fixed builds in the vendor advisory.
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 gain initial access to a Windows system through phishing, web exploitation, or other means, obtaining user-level code execution.
Business
Attacker establishes foothold on endpoint, bypassing perimeter controls.
2
Keys to the kingdom — privilege/identity takeover narrative 2
Attacker
I trigger the CLFS driver vulnerability to escalate my privileges from user to SYSTEM level.
Business
Attacker obtains unrestricted control over the compromised machine.
3
Lateral reach — past segmentation narrative 3
Attacker
I deploy ransomware payload with full system privileges, encrypting files and disabling recovery mechanisms.
Business
Organization experiences data encryption, operational shutdown, and extortion demands.
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