{"id":"CVE-2021-43226","kind":"vulnerability","aliases":["CVE-2021-43226"],"related":[],"title":"Windows Common Log File System Driver Elevation of Privilege Vulnerability","description":"Microsoft Windows Common Log File System Driver contains a privilege escalation vulnerability that could allow a local, privileged attacker to bypass certain security mechanisms.\n\nCISA required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.","summary":"Windows Common Log File System (CLFS) driver has a privilege escalation flaw that lets a local user with some access gain higher privileges. CISA lists it as actively exploited, with ransomware use noted in the catalog.","severity":"high","cvss":7.8,"epss":null,"exploited":true,"exploited_at":"2025-10-06T00:00:00+00:00","products":[{"name":"Microsoft Windows","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows 10 1507","fixed":["10.0.10240.19145"],"affected":"< 10.0.10240.19145","ecosystem":"vendor"},{"name":"microsoft windows 10 1607","fixed":["10.0.14393.4825"],"affected":"< 10.0.14393.4825","ecosystem":"vendor"},{"name":"microsoft windows 10 1809","fixed":["10.0.17763.2366"],"affected":"< 10.0.17763.2366","ecosystem":"vendor"},{"name":"microsoft windows 10 1909","fixed":["10.0.18363.1977"],"affected":"< 10.0.18363.1977","ecosystem":"vendor"},{"name":"microsoft windows 10 2004","fixed":["10.0.19041.1415"],"affected":"< 10.0.19041.1415","ecosystem":"vendor"},{"name":"microsoft windows 10 20h2","fixed":["10.0.19042.1415"],"affected":"< 10.0.19042.1415","ecosystem":"vendor"},{"name":"microsoft windows 10 21h1","fixed":["10.0.19043.1415"],"affected":"< 10.0.19043.1415","ecosystem":"vendor"},{"name":"microsoft windows 10 21h2","fixed":["10.0.19044.1415"],"affected":"< 10.0.19044.1415","ecosystem":"vendor"},{"name":"microsoft windows 11 21h2","fixed":["10.0.22000.376"],"affected":"< 10.0.22000.376","ecosystem":"vendor"},{"name":"microsoft windows 7","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows 8.1","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows rt 8.1","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows server 2004","fixed":["10.0.19041.1415"],"affected":"< 10.0.19041.1415","ecosystem":"vendor"},{"name":"microsoft windows server 2008","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows server 2008","fixed":[],"affected":"= r2","ecosystem":"vendor"},{"name":"microsoft windows server 2012","fixed":[],"affected":"","ecosystem":"vendor"},{"name":"microsoft windows server 2012","fixed":[],"affected":"= r2","ecosystem":"vendor"},{"name":"microsoft windows server 2016","fixed":["10.0.14393.4825"],"affected":"< 10.0.14393.4825","ecosystem":"vendor"},{"name":"microsoft windows server 2019","fixed":["10.0.17763.2366"],"affected":"< 10.0.17763.2366","ecosystem":"vendor"},{"name":"microsoft windows server 2022","fixed":["10.0.20348.405"],"affected":"< 10.0.20348.405","ecosystem":"vendor"},{"name":"microsoft windows server 20h2","fixed":["10.0.19042.1415"],"affected":"< 10.0.19042.1415","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","infrastructure","ransomware","self-hosted","windows"],"not_affected_if":["patched-systems"],"how_to_check":null,"fix":"Apply the Microsoft patch for CVE-2021-43226 per vendor instructions; the sources do not name the KB or build numbers.","status":"open","published_at":"2021-12-15T15:15:09+00:00","first_seen_at":"2026-10-23T19:11:51+00:00","triaged_at":"2026-10-23T20:44:38+00:00","updated_at":"2026-10-23T23:44:51+00:00","category":"infrastructure","confidence":0.6,"triage":{"at":"2026-10-23T20:44:38Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft patch for CVE-2021-43226 per vendor instructions; the sources do not name the KB or build numbers.","reach":"local","summary":"Windows Common Log File System (CLFS) driver has a privilege escalation flaw that lets a local user with some access gain higher privileges. CISA lists it as actively exploited, with ransomware use noted in the catalog.","category":"infrastructure","who_cares":["windows","self-hosted"],"confidence":0.6,"needs_auth":"yes","worst_case":"A local attacker with limited access gains SYSTEM-level control of the machine.","analyst_note":"Find the Microsoft security advisory for CVE-2021-43226 to get affected Windows builds, the patch KB numbers and a check method.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["patched-systems"],"user_interaction":"no","needs_special_setup":"no","severity_if_unknown":"high"},"cost_usd":0.000422,"input_tokens":2539,"output_tokens":337,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-01':180C '-2021':12A,15A '-43226':13A,16A '10':21A,25A,29A,33A,37A,41A,45A,49A '11':53A '1507':22A '1607':26A '1809':30A '1909':34A '2004':38A,68A '2008':72A,76A '2012':80A,84A '2016':88A '2019':92A '2022':96A '20h2':42A,100A '21h1':46A '21h2':50A,54A '22':179C '7':57A '8.1':60A,64A 'access':127B 'action':170C 'activ':135B 'allow':158C 'appli':171C 'applic':177C 'attack':162C 'bod':178C 'bypass':164C 'catalog':143B 'certain':165C 'cisa':131B,168C 'clfs':113B 'cloud':183C 'common':2A,109B,146C 'contain':151C 'could':157C 'cve':11A,14A 'discontinu':186C 'driver':6A,114B,150C 'elev':7A 'escal':118B,154C 'exploit':101B,136B 'file':4A,111B,148C 'flaw':119B 'follow':176C 'gain':128B 'guidanc':181C 'higher':129B 'host':106B 'infrastructur':102B 'instruct':175C 'let':121B 'list':132B 'local':123B,160C 'log':3A,110B,147C 'mechan':167C 'microsoft':17A,19A,23A,27A,31A,35A,39A,43A,47A,51A,55A,58A,61A,65A,69A,73A,77A,81A,85A,89A,93A,97A,144C 'mitig':172C,192C 'note':140B 'per':173C 'privileg':9A,117B,130B,153C,161C 'product':190C 'ransomwar':103B,138B 'requir':169C 'rt':63A 'secur':166C 'self':105B 'self-host':104B 'server':67A,71A,75A,79A,83A,87A,91A,95A,99A 'servic':184C 'system':5A,112B,149C 'unavail':194C 'use':139B,187C 'user':124B 'vendor':174C 'vulner':10A,155C 'window':1A,107B,108B,145C 'windows':18A,20A,24A,28A,32A,36A,40A,44A,48A,52A,56A,59A,62A,66A,70A,74A,78A,82A,86A,90A,94A,98A","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-12-15T15:15:09+00:00","kev":{"dateAdded":"2025-10-06","dueDate":"2025-10-27","requiredAction":"Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.","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-2021-43226","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-43226","severity":null,"published_at":"2025-10-06T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:51+00:00","fetched_at":"2026-10-25T15:39:40+00:00","note":null},{"source":"nvd","external_id":"CVE-2021-43226","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-43226","severity":null,"published_at":"2021-12-15T15:15:09+00:00","modified_at":"2026-08-22T04:16:54+00:00","first_seen_at":"2026-10-23T23:44:51+00:00","fetched_at":"2026-10-23T23:44:51+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"}