{"id":"CVE-2021-33766","kind":"vulnerability","aliases":["CVE-2021-33766"],"related":[],"title":"Microsoft Exchange Server: Microsoft Exchange Server Information Disclosure","description":"Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.\n\nCISA required action: Apply updates per vendor instructions.","summary":"Microsoft Exchange Server has an information disclosure flaw that lets an unauthenticated attacker read email traffic from the server. It is listed in CISA's known-exploited catalog and has a very high EPSS score, so attacks are happening.","severity":"high","cvss":7.5,"epss":0.98176,"exploited":true,"exploited_at":"2022-01-18T00:00:00+00:00","products":[{"name":"Microsoft Exchange Server","fixed":[],"affected":"","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["email","exchange-server","exploited","on-premises-exchange","vendor"],"not_affected_if":["exchange-online","microsoft-365-cloud-only"],"how_to_check":null,"fix":"Apply the Microsoft updates for Exchange Server per vendor instructions, as CISA requires.","status":"open","published_at":"2021-07-14T18:15:10+00:00","first_seen_at":"2026-10-23T19:11:53+00:00","triaged_at":"2026-10-23T21:31:33+00:00","updated_at":"2026-10-23T21:57:35+00:00","category":"vendor","confidence":0.6,"triage":{"at":"2026-10-23T21:31:33Z","model":"claude-haiku-5-5","output":{"fix":"Apply the Microsoft updates for Exchange Server per vendor instructions, as CISA requires.\nunknown","reach":"internet","impact":"medium","summary":"Microsoft Exchange Server has an information disclosure flaw that lets an unauthenticated attacker read email traffic from the server. It is listed in CISA's known-exploited catalog and has a very high EPSS score, so attacks are happening.","category":"vendor","who_cares":["exchange-server","on-premises-exchange","email"],"confidence":0.6,"needs_auth":"no","worst_case":"Attacker reads email traffic passing through the Exchange server without logging in","analyst_note":"Sources give no affected or fixed versions and no detection method; check Microsoft's security update guide entry for CVE-2021-33766 and the CISA KEV page.","how_to_check":"unknown","needs_analyst":true,"not_affected_if":["exchange-online","microsoft-365-cloud-only"],"user_interaction":"no","needs_special_setup":"unknown","severity_if_unknown":"high"},"cost_usd":0.000468,"input_tokens":2828,"output_tokens":370,"severity_before":"unknown"},"analyst_status":"queued","analyst_note":"Sources give no affected or fixed versions and no detection method; check Microsoft's security update guide entry for CVE-2021-33766 and the CISA KEV page.","search":"'-2021':10A,13A '-33766':11A,14A 'action':90C 'allow':78C 'appli':91C 'attack':40B,65B,81C 'catalog':56B 'cisa':51B,88C 'contain':71C 'cve':9A,12A 'disclosur':8A,34B,74C 'email':18B,42B,84C 'epss':62B 'exchang':2A,5A,20B,26B,29B,69C 'exchange':16A 'exchange-serv':19B 'exploit':22B,55B 'flaw':35B 'happen':67B 'high':61B 'inform':7A,33B,73C 'instruct':95C 'known':54B 'known-exploit':53B 'let':37B 'list':49B 'microsoft':1A,4A,15A,28B,68C 'on-premises-exchang':23B 'per':93C 'premis':25B 'read':41B 'requir':89C 'score':63B 'server':3A,6A,17A,21B,30B,46B,70C 'steal':83C 'target':87C 'traffic':43B,85C 'unauthent':39B,80C 'updat':92C 'vendor':27B,94C 'vulner':75C","nicknames":[],"mitigations":[],"who_affected":null,"tier":null,"verified":{},"analyst_revisit_at":null,"first_public_at":"2021-07-14T18:15:10+00:00","kev":{"dateAdded":"2022-01-18","dueDate":"2022-02-01","requiredAction":"Apply updates per vendor instructions.","knownRansomwareCampaignUse":"Unknown"},"cvss_vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L","cvss_by":"secure@microsoft.com","poc_count":0,"sources":[{"source":"kev","external_id":"CVE-2021-33766","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-33766","severity":null,"published_at":"2022-01-18T00:00:00+00:00","modified_at":null,"first_seen_at":"2026-10-23T19:11:53+00:00","fetched_at":"2026-10-25T15:39:43+00:00","note":null},{"source":"epss","external_id":"CVE-2021-33766","url":"https://api.first.org/data/v1/epss?cve=CVE-2021-33766","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-2021-33766","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-33766","severity":null,"published_at":"2021-07-14T18:15:10+00:00","modified_at":"2026-08-10T19:58:44+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"}