{"id":"CVE-2021-34473","kind":"vulnerability","aliases":["CVE-2021-34473"],"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.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Microsoft Exchange Server has a remote code execution flaw (ProxyShell family, CVE-2021-34473) that is actively exploited, with ransomware use noted in the sources. An unauthenticated attacker on the internet can run code on the mail server.","severity":"critical","cvss":10.0,"epss":0.99999,"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","package","ransomware","self-hosted","windows"],"not_affected_if":["exchange-online","microsoft-365-cloud"],"how_to_check":null,"fix":"Apply the Microsoft updates for Exchange Server per vendor instructions (CISA required action); exact patch build is unknown from the sources.","status":"open","published_at":"2021-07-14T18:15:11+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:39:18+00:00","updated_at":"2026-10-23T21:57:34+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-23T21:39:18Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft updates for Exchange Server per vendor instructions (CISA required action); exact patch build is unknown from the sources.","reach":"internet","impact":"high","summary":"Microsoft Exchange Server has a remote code execution flaw (ProxyShell family, CVE-2021-34473) that is actively exploited, with ransomware use noted in the sources. An unauthenticated attacker on the internet can run code on the mail server.","category":"package","who_cares":["exchange-server","windows","self-hosted"],"confidence":0.7,"needs_auth":"unknown","worst_case":"Attacker runs code on the Exchange server and can take over the mail system.","analyst_note":"Find the exact fixed Exchange build and a detection method (e.g. Microsoft advisory or CISA KEV entry) since the sources do not give versions or checks.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["exchange-online","microsoft-365-cloud"],"user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"critical"},"cost_usd":0.000473,"input_tokens":2825,"output_tokens":380,"severity_before":"unknown"},"analyst_status":"queued","analyst_note":"Find the exact fixed Exchange build and a detection method (e.g. Microsoft advisory or CISA KEV entry) since the sources do not give versions or checks.","search":"'-2021':12A,15A,42B '-34473':13A,16A,43B 'action':83C 'activ':46B 'allow':76C 'appli':84C 'attack':57B 'cisa':81C 'code':8A,36B,63B,79C 'contain':71C 'cve':11A,14A,41B 'exchang':2A,5A,21B,31B,69C 'exchange':18A 'exchange-serv':20B 'execut':9A,37B,80C 'exploit':23B,47B 'famili':40B 'flaw':38B 'host':28B 'instruct':88C 'internet':60B 'mail':66B 'microsoft':1A,4A,17A,30B,68C 'note':51B 'packag':24B 'per':86C 'proxyshel':39B 'ransomwar':25B,49B 'remot':7A,35B,78C 'requir':82C 'run':62B 'self':27B 'self-host':26B 'server':3A,6A,19A,22B,32B,67B,70C 'sourc':54B 'unauthent':56B 'unspecifi':73C 'updat':85C 'use':50B 'vendor':87C 'vulner':10A,74C 'window':29B","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-07-14T18:15:11+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:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-34473","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-34473","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":"2026-10-25T14:39:41+00:00","note":null},{"source":"epss","external_id":"CVE-2021-34473","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-34473","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-2021-34473","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-34473","severity":null,"published_at":"2021-07-14T18:15:11+00:00","modified_at":"2026-08-10T19:58:52+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"}