{"id":"CVE-2021-26857","kind":"vulnerability","aliases":["CVE-2021-26857"],"related":[],"title":"Microsoft Exchange Server: Microsoft Exchange Server Remote Code Execution Vulnerability","description":"Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Microsoft Exchange Server has an unspecified flaw in its code that allows remote code execution, and it is part of the ProxyLogon chain. Attackers can run code on the Exchange server, which is a known ransomware target, and it is actively exploited in the wild.","severity":"critical","cvss":7.8,"epss":0.95762,"exploited":true,"exploited_at":"2021-11-03T00:00:00+00:00","products":[{"name":"Microsoft Exchange Server","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["exchange-server","exploited","infrastructure","ransomware","self-hosted","windows"],"not_affected_if":["exchange-online"],"how_to_check":null,"fix":"Apply Microsoft's updates for this CVE per vendor instructions (CISA required action). Exact patched build numbers: unknown from sources.","status":"open","published_at":"2021-03-03T00:15:12+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:41:20+00:00","updated_at":"2026-10-23T21:57:34+00:00","category":"infrastructure","confidence":0.7,"triage":{"at":"2026-10-23T21:41:20Z","model":"claude-haiku-5-5","output":{"fix":"Apply Microsoft's updates for this CVE per vendor instructions (CISA required action). Exact patched build numbers: unknown from sources.","reach":"internet","impact":"high","summary":"Microsoft Exchange Server has an unspecified flaw in its code that allows remote code execution, and it is part of the ProxyLogon chain. Attackers can run code on the Exchange server, which is a known ransomware target, and it is actively exploited in the wild.","category":"infrastructure","who_cares":["exchange-server","self-hosted","windows"],"confidence":0.7,"needs_auth":"unknown","worst_case":"Attacker runs code on the Exchange server and may deploy ransomware.","analyst_note":"Pull Microsoft's security update guide entry for CVE-2021-26857 to get affected build ranges, patched builds and the Exchange Health Checker or IOC check steps.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["exchange-online"],"user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"critical"},"cost_usd":0.000475,"input_tokens":2848,"output_tokens":381,"severity_before":"unknown"},"analyst_status":"queued","analyst_note":"Pull Microsoft's security update guide entry for CVE-2021-26857 to get affected build ranges, patched builds and the Exchange Health Checker or IOC check steps.","search":"'-2021':12A,15A '-26857':13A,16A 'action':99C 'activ':70B 'allow':41B,83C 'appli':100C 'attack':53B 'chain':52B,96C 'cisa':97C 'code':8A,39B,43B,56B,86C 'contain':78C 'cve':11A,14A 'exchang':2A,5A,21B,31B,59B,76C 'exchange':18A 'exchange-serv':20B 'execut':9A,44B,87C 'exploit':23B,71B,95C 'flaw':36B 'host':28B 'infrastructur':24B 'instruct':104C 'known':64B 'microsoft':1A,4A,17A,30B,75C 'part':48B,91C 'per':102C 'proxylogon':51B,94C 'ransomwar':25B,65B 'remot':7A,42B,85C 'requir':98C 'run':55B 'self':27B 'self-host':26B 'server':3A,6A,19A,22B,32B,60B,77C 'target':66B 'unspecifi':35B,80C 'updat':101C 'vendor':103C 'vulner':10A,81C,89C 'wild':74B 'window':29B","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-03-03T00:15:12+00:00","kev":{"dateAdded":"2021-11-03","dueDate":"2022-05-03","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Known"},"cvss_vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-26857","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-26857","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-2021-26857","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-26857","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-2021-26857","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-26857","severity":null,"published_at":"2021-03-03T00:15:12+00:00","modified_at":"2026-08-19T19:22:27+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"}