{"id":"CVE-2021-26858","published":"2021-03-03T00:15:12Z","modified":"2026-10-01T21:17:16Z","title":"Microsoft Exchange Server Remote Code Execution Vulnerability","vendor":"Microsoft","product":"Exchange Server","tier":"core","eol":null,"description":{"text":"Microsoft Exchange Server Remote Code Execution Vulnerability","provenance":"source","from":"NVD"},"summary":{"text":"An attacker who can get a user to interact with a crafted element, such as opening a malicious file or link, can execute code, requiring local access with no privileges needed and user interaction on the victim's side.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-29T08:31:07Z"},"cvss":{"score":7.8,"severity":"HIGH","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H","provenance":"source","from":"NVD"},"epss":{"score":0.93651,"percentile":0.99843,"date":"2026-10-30","provenance":"source"},"kev":{"dateAdded":"2021-11-03","dueDate":"2022-05-03","ransomware":true,"requiredAction":"Apply updates per vendor instructions.","notes":"Reference CISA's ED 21-02 (https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities) for further guidance and requirements. Note: The due date for addressing this vulnerability aligns with the requirements outlined in ED 21-02. https://nvd.nist.gov/vuln/detail/CVE-2021-26858","provenance":"source"},"vulncheckKev":{"dateAdded":"2021-03-02","exploitedFirstSeen":"2021-03-02","ransomware":true,"evidence":[{"url":"https://cisa.gov/news-events/alerts/2021/03/02/microsoft-releases-out-band-security-updates-exchange-server","date":"2021-03-02"},{"url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","date":"2021-03-02"},{"url":"https://www.microsoft.com/security/blog/2021/03/02/hafnium-targeting-exchange-servers","date":"2021-03-02"},{"url":"https://us-cert.cisa.gov/ncas/alerts/aa21-062a","date":"2021-03-03"},{"url":"https://www.crowdstrike.com/blog/falcon-complete-stops-microsoft-exchange-server-zero-day-exploits/","date":"2021-03-04"},{"url":"https://www.fireeye.com/blog/threat-research/2021/03/detection-response-to-exploitation-of-microsoft-exchange-zero-day-vulnerabilities.html","date":"2021-03-04"},{"url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Mar","date":"2021-03-10"},{"url":"https://www.welivesecurity.com/2021/03/10/exchange-servers-under-siege-10-apt-groups/","date":"2021-03-10"},{"url":"https://threatpost.com/microsoft-exchange-servers-apt-attack/164695/","date":"2021-03-11"},{"url":"https://www.cybereason.com/blog/research/prometei-botnet-exploiting-microsoft-exchange-vulnerabilities","date":"2021-04-22"}],"evidenceTotal":50,"provenance":"source"},"exploited":true,"otx":{"pulseCount":50,"pulses":[{"name":"Actively_Exploited_Kev | Oct 29, 2026","url":"https://otx.alienvault.com/pulse/6ae20c1db17e7057ae7d9ad1","created":"2026-10-28"},{"name":"Actively_Exploited_Kev | Oct 28, 2026","url":"https://otx.alienvault.com/pulse/6ae0ba7e0e2a97ff7b218819","created":"2026-10-27"},{"name":"Actively_Exploited_Kev | Oct 27, 2026","url":"https://otx.alienvault.com/pulse/6adf68b85c3e852f3c111d18","created":"2026-10-26"},{"name":"Actively_Exploited_Kev | Oct 24, 2026","url":"https://otx.alienvault.com/pulse/6adb740e397c3ad4799aa4cd","created":"2026-10-23"},{"name":"Actively_Exploited_Kev | Oct 23, 2026","url":"https://otx.alienvault.com/pulse/6ada236fcfba2dfa730ea482","created":"2026-10-22"},{"name":"Actively_Exploited_Kev | Oct 22, 2026","url":"https://otx.alienvault.com/pulse/6ad8d33a077cbb5cab524d80","created":"2026-10-21"},{"name":"Actively_Exploited_Kev | Oct 21, 2026","url":"https://otx.alienvault.com/pulse/6ad78130c4d1ac2be83a8b6d","created":"2026-10-20"},{"name":"Actively_Exploited_Kev | Oct 20, 2026","url":"https://otx.alienvault.com/pulse/6ad62f63c24a405a3423cc13","created":"2026-10-19"},{"name":"Actively_Exploited_Kev | Oct 19, 2026","url":"https://otx.alienvault.com/pulse/6ad4dda174e5797d0bd238b7","created":"2026-10-18"},{"name":"Actively_Exploited_Kev | Oct 18, 2026","url":"https://otx.alienvault.com/pulse/6ad38c22e1848f8189d1c121","created":"2026-10-17"}],"provenance":"machine"},"cwes":[],"references":[{"url":"https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26858","tags":["Patch","Vendor Advisory"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26858","tags":["Patch","Vendor Advisory"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858","tags":["US Government Resource"]}],"techniques":[{"id":"T1190","name":"Exploit Public-Facing Application","tactics":["initial-access"],"provenance":"source","basis":"ctid-kev","bases":["ctid-kev","report-passage"],"basisDetail":"CTID Mappings Explorer KEV 07/28/2025 (ATT&CK 16.1); capability: Code Execution; keyword query KW:T1190:2","mappingType":"exploitation_technique","evidence":[{"reportId":"SRC-ff02b0d97dbe7522c519","quote":"In early March 2021, APT actors exploited CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065 to install 17 China Chopper webshells on the Exchange Server.","start":4237,"end":4407}],"comment":"CVE-2021-26858, part of Proxy Logon, is a post-authentication arbitrary file write vulnerability in Exchange. CVE-2021-26858 and CVE-2021-27065 are similar post-authentication arbitrary write file vulnerabilities in Exchange. An attacker, authenticated either by using CVE-2021-26855 or via stolen admin credentials, could write a file to any path on the server.","confidence":"low","confidenceNote":"needs user interaction; not a public-facing exploit"},{"id":"T1505.003","name":"Web Shell","tactics":["persistence"],"provenance":"source","basis":"ctid-kev","bases":["ctid-kev"],"basisDetail":"CTID Mappings Explorer KEV 07/28/2025 (ATT&CK 16.1); capability: Code Execution","mappingType":"primary_impact","evidence":[],"comment":"CVE-2021-26858, part of Proxy Logon, is a post-authentication arbitrary file write vulnerability in Exchange. CVE-2021-26858 and CVE-2021-27065 are similar post-authentication arbitrary write file vulnerabilities in Exchange. An attacker, authenticated either by using CVE-2021-26855 or via stolen admin credentials, could write a file to any path on the server.","confidence":"medium","confidenceNote":"CTID KEV capability group \"code execution\" is generic"}],"reports":[{"id":"SRC-ff02b0d97dbe7522c519","title":"Impacket and Exfiltration Tool Used to Steal Sensitive Information from Defense Industrial Base Organization","publisher":"cisa.gov","url":"https://www.cisa.gov/news-events/cybersecurity-advisories/aa22-277a","date":null,"mentions":[{"quote":"In early March 2021, APT actors exploited CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065 to install 17 China Chopper webshells on the Exchange Server.","start":4237,"end":4407}]},{"id":"SRC-8b6e5f3c2592261e3f88","title":"Advisory further TTPs associated with SVR cyber actors","publisher":"ncsc.gov.uk","url":"https://www.ncsc.gov.uk/files/Advisory-further-TTPs-associated-with-SVR-cyber-actors.pdf","date":null,"mentions":[{"quote":"• CVE-2021-26858 (Arbitrary files)","start":3702,"end":3736}]},{"id":"SRC-14820bfbde5abb2b0561","title":"Us 21 Anatomy Of Native Iis Malware wp","publisher":"i.blackhat.com","url":"https://i.blackhat.com/USA21/Wednesday-Handouts/us-21-Anatomy-Of-Native-Iis-Malware-wp.pdf","date":null,"mentions":[{"quote":"pre-authentication RCE vulnerability chain (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-","start":9313,"end":9413},{"quote":"ProxyLogon vulnerability chain (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-","start":20128,"end":20221},{"quote":"authentication RCE vulnerability chain (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-","start":22404,"end":22500}]}],"affected":[],"advisories":[],"euvd":{"id":"EUVD-2021-13642","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-13642","exploited":true,"exploitedSince":"2021-11-03","provenance":"source"},"exploits":[],"exploitSources":null,"detections":{"cveRules":[{"title":"ProxyLogon Reset Virtual Directories Based On IIS Log","id":"effee1f6-a932-4297-a81f-acb44064fa3a","level":"critical","status":"test","path":"rules-emerging-threats/2021/Exploits/CVE-2021-26858/web_cve_2021_26858_iis_rce.yml","url":"https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-26858/web_cve_2021_26858_iis_rce.yml","techniques":["T1190"]},{"title":"CVE-2021-26858 Exchange Exploitation","id":"b06335b3-55ac-4b41-937e-16b7f5d57dfd","level":"high","status":"test","path":"rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml","url":"https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml","techniques":["T1203"]}]},"exploitRefs":[],"flow":[{"stage":"primary","techniques":["T1505.003"],"provenance":"source"},{"stage":"related","techniques":["T1190"],"provenance":"source","lowConfidence":true}],"timeline":[{"date":"2021-03-02","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://cisa.gov/news-events/alerts/2021/03/02/microsoft-releases-out-band-security-updates-exchange-server","provenance":"source"},{"date":"2021-03-03","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-26858","provenance":"source"},{"date":"2021-11-03","kind":"kev","label":"Added to CISA KEV (known ransomware use)","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","provenance":"source"},{"date":"2021-11-03","kind":"euvd-exploited","label":"ENISA EUVD: exploited","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-13642","provenance":"source"}],"speed":{"daysToPublicExploit":null,"firstExploitFrom":null,"daysToKev":245},"priority":{"score":81.2,"factors":[{"name":"In a KEV catalog","points":35.0,"detail":"listed in CISA KEV and VulnCheck KEV"},{"name":"Recent KEV listing","points":0.0,"detail":"added 2069 days ago"},{"name":"Known ransomware use","points":10.0,"detail":"CISA KEV and VulnCheck KEV: Known"},{"name":"EPSS percentile","points":19.97,"detail":"percentile 0.9984"},{"name":"Named in threat reports","points":7.74,"detail":"3 reports"},{"name":"Public exploit code","points":0.0,"detail":"none found"},{"name":"Mentioned in OTX pulses","points":3.0,"detail":"50 pulses"},{"name":"CVSS severity","points":5.46,"detail":"base score 7.8"},{"name":"End-of-life product","points":0.0,"detail":"supported or unknown"}]}}