{"id":"CVE-2021-36955","kind":"vulnerability","aliases":["CVE-2021-36955"],"related":[],"title":"Microsoft Windows: Microsoft Windows Common Log File System (CLFS) Driver Privilege Escalation Vulnerability","description":"Microsoft Windows Common Log File System (CLFS) driver contains an unspecified vulnerability that allows for privilege escalation.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Microsoft Windows Common Log File System (CLFS) driver has a flaw that lets a local user escalate privileges. CISA lists it as actively exploited and the tag set includes ransomware use.","severity":"high","cvss":7.8,"epss":0.04071,"exploited":true,"exploited_at":"2021-11-03T00:00:00+00:00","products":[{"name":"Microsoft Windows","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","infrastructure","ransomware","self-hosted","windows"],"not_affected_if":["patched-windows"],"how_to_check":null,"fix":"Apply the Microsoft Windows security update for CVE-2021-36955 per vendor instructions (CISA required action).","status":"open","published_at":"2021-09-15T12:15:13+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:37:30+00:00","updated_at":"2026-10-23T21:57:34+00:00","category":"infrastructure","confidence":0.7,"triage":{"at":"2026-10-23T21:37:30Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft Windows security update for CVE-2021-36955 per vendor instructions (CISA required action).","reach":"local","impact":"high","summary":"Microsoft Windows Common Log File System (CLFS) driver has a flaw that lets a local user escalate privileges. CISA lists it as actively exploited and the tag set includes ransomware use.","category":"infrastructure","who_cares":["windows","self-hosted"],"confidence":0.7,"needs_auth":"yes","worst_case":"Local attacker with a low-privilege account gains SYSTEM-level control of the machine.","analyst_note":"Find the patched Windows build numbers and KB articles for CVE-2021-36955 in the Microsoft Security Update Guide, and a detection method, since the sources do not say.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["patched-windows"],"user_interaction":"no","needs_special_setup":"no","severity_if_unknown":"high"},"cost_usd":0.000457,"input_tokens":2843,"output_tokens":345,"severity_before":"unknown"},"analyst_status":"queued","analyst_note":"Find the patched Windows build numbers and KB articles for CVE-2021-36955 in the Microsoft Security Update Guide, and a detection method, since the sources do not say.","search":"'-2021':15A,18A '-36955':16A,19A 'action':79C 'activ':51B 'allow':73C 'appli':80C 'cisa':47B,77C 'clfs':9A,35B,66C 'common':5A,31B,62C 'contain':68C 'cve':14A,17A 'driver':10A,36B,67C 'escal':12A,45B,76C 'exploit':22B,52B 'file':7A,33B,64C 'flaw':39B 'host':27B 'includ':57B 'infrastructur':23B 'instruct':84C 'let':41B 'list':48B 'local':43B 'log':6A,32B,63C 'microsoft':1A,3A,20A,29B,60C 'per':82C 'privileg':11A,46B,75C 'ransomwar':24B,58B 'requir':78C 'self':26B 'self-host':25B 'set':56B 'system':8A,34B,65C 'tag':55B 'unspecifi':70C 'updat':81C 'use':59B 'user':44B 'vendor':83C 'vulner':13A,71C 'window':2A,4A,28B,30B,61C 'windows':21A","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-09-15T12:15:13+00:00","kev":{"dateAdded":"2021-11-03","dueDate":"2021-11-17","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Known"},"cvss_vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-36955","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-36955","severity":null,"published_at":"2021-11-03T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:53+00:00","fetched_at":"2026-10-25T13:39:33+00:00","note":null},{"source":"epss","external_id":"CVE-2021-36955","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-36955","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-23T19:13:43+00:00","fetched_at":"2026-10-23T19:13:43+00:00","note":null},{"source":"nvd","external_id":"CVE-2021-36955","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-36955","severity":null,"published_at":"2021-09-15T12:15:13+00:00","modified_at":"2026-08-10T20:19:11+00:00","first_seen_at":"2026-10-23T21:57:34+00:00","fetched_at":"2026-10-23T21:57:34+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}