Threats / Microsoft / CVE-2021-36955
CVE-2021-36955
· EUVD no mirror located
· GCVE no mirror located
Verified 2026-08-04
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.
CISA KEV Yes · 2021-11-035Ransomware use Flagged5EPSS 0.03229 (verify live)6Exploit Public PoC7CVSS 7.83CISA SSVC Exploitation: active · Automatable: no · 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
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 0.03229 — modeled likelihood of exploitation activity.EPSS is a daily-changing model output — open the source for today's value.
Severity / affected
CVSS 7.8 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C) · Affected: Microsoft, Windows. Confirm exact fixed builds in the vendor advisory.CVSS — CNA-assigned (CVE.org), version 3.1, set at disclosure.
CISA SSVC (triage disposition)
Exploitation: active · Automatable: no · 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 49 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.
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