{"id":"CVE-2021-31166","kind":"vulnerability","aliases":["CVE-2021-31166"],"related":[],"title":"Microsoft HTTP Protocol Stack: Microsoft HTTP Protocol Stack Remote Code Execution Vulnerability","description":"Microsoft HTTP Protocol Stack contains a vulnerability in http.sys that allows for remote code execution.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Microsoft's HTTP Protocol Stack (http.sys) contains a flaw that allows remote code execution. An attacker who can send crafted HTTP requests to a vulnerable Windows server can run code on it; CISA lists it as actively exploited.","severity":"critical","cvss":9.8,"epss":0.99867,"exploited":true,"exploited_at":"2022-04-06T00:00:00+00:00","products":[{"name":"Microsoft HTTP Protocol Stack","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exploited","iis","infrastructure","microsoft-http-stack","windows"],"not_affected_if":["unknown"],"how_to_check":null,"fix":"Apply the Microsoft updates for CVE-2021-31166 per vendor instructions (CISA required action). Affected build numbers: unknown from the sources.","status":"open","published_at":"2021-05-11T19:15:09+00:00","first_seen_at":"2026-10-23T19:11:52+00:00","triaged_at":"2026-10-23T21:18:06+00:00","updated_at":"2026-10-23T21:57:35+00:00","category":"infrastructure","confidence":0.7,"triage":{"at":"2026-10-23T21:18:06Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft updates for CVE-2021-31166 per vendor instructions (CISA required action). Affected build numbers: unknown from the sources.","reach":"network","impact":"high","summary":"Microsoft's HTTP Protocol Stack (http.sys) contains a flaw that allows remote code execution. An attacker who can send crafted HTTP requests to a vulnerable Windows server can run code on it; CISA lists it as actively exploited.","category":"infrastructure","who_cares":["windows","iis","microsoft-http-stack"],"confidence":0.7,"needs_auth":"unknown","worst_case":"Remote attacker runs code on Windows server via http.sys","analyst_note":"Find affected Windows builds and the Microsoft security update guide entry for CVE-2021-31166 to fill how_to_check and fix.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["unknown"],"user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"critical"},"cost_usd":0.000466,"input_tokens":2841,"output_tokens":364,"severity_before":"unknown"},"analyst_status":null,"analyst_note":null,"search":"'-2021':14A,17A '-31166':15A,18A 'action':86C 'activ':67B 'allow':41B,79C 'appli':87C 'attack':46B 'cisa':63B,84C 'code':10A,43B,60B,82C 'contain':37B,73C 'craft':50B 'cve':13A,16A 'execut':11A,44B,83C 'exploit':23B,68B 'flaw':39B 'http':2A,6A,20A,28B,33B,51B,70C 'http.sys':36B,77C 'ii':24B 'infrastructur':25B 'instruct':91C 'list':64B 'microsoft':1A,5A,19A,27B,31B,69C 'microsoft-http-stack':26B 'per':89C 'protocol':3A,7A,21A,34B,71C 'remot':9A,42B,81C 'request':52B 'requir':85C 'run':59B 'send':49B 'server':57B 'stack':4A,8A,22A,29B,35B,72C 'updat':88C 'vendor':90C 'vulner':12A,55B,75C 'window':30B,56B","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-05-11T19:15:09+00:00","kev":{"dateAdded":"2022-04-06","dueDate":"2022-04-27","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Unknown"},"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-31166","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-31166","severity":null,"published_at":"2022-04-06T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:52+00:00","fetched_at":"2026-10-25T20:35:36+00:00","note":null},{"source":"epss","external_id":"CVE-2021-31166","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-31166","severity":null,"published_at":null,"modified_at":null,"first_seen_at":"2026-10-23T19:13:44+00:00","fetched_at":"2026-10-23T19:13:44+00:00","note":null},{"source":"nvd","external_id":"CVE-2021-31166","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-31166","severity":null,"published_at":"2021-05-11T19:15:09+00:00","modified_at":"2026-06-17T03:51:21+00:00","first_seen_at":"2026-10-23T21:57:35+00:00","fetched_at":"2026-10-23T21:57:35+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"}