{"id":"CVE-2021-31207","published":"2021-05-11T19:15:10Z","modified":"2026-06-17T03:51:27Z","title":"Microsoft Exchange Server Security Feature Bypass Vulnerability","vendor":"Microsoft","product":"Exchange Server","tier":"core","eol":null,"description":{"text":"Microsoft Exchange Server Security Feature Bypass Vulnerability","provenance":"source","from":"NVD"},"summary":{"text":"Microsoft Exchange Server contains a security feature bypass vulnerability, classified under unrestricted upload of files with dangerous types and improper input validation. An attacker with network access and high-privilege authentication, and with no user interaction required, can exploit this flaw to bypass security controls and achieve high impact to confidentiality, integrity and availability, with high attack complexity.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-23T06:54:26Z"},"cvss":{"score":6.6,"severity":"MEDIUM","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H","provenance":"source","from":"NVD"},"epss":{"score":0.99782,"percentile":0.99955,"date":"2026-10-29","provenance":"source"},"kev":{"dateAdded":"2021-11-03","dueDate":"2021-11-17","ransomware":true,"requiredAction":"Apply updates per vendor instructions.","notes":"https://nvd.nist.gov/vuln/detail/CVE-2021-31207","provenance":"source"},"vulncheckKev":{"dateAdded":"2021-08-30","exploitedFirstSeen":"2021-08-30","ransomware":true,"evidence":[{"url":"https://www.cybereason.com/blog/threat-alert-microsoft-exchange-proxyshell-exploits-and-lockfile-ransomware","date":"2021-08-30"},{"url":"https://www.fireeye.com/blog/threat-research/2021/09/proxyshell-exploiting-microsoft-exchange-servers.html","date":"2021-09-03"},{"url":"https://www.securin.io/microsoft-exchange-proxyshell-and-windows-petitpotam-vulnerabilities-chained-in-new-attack/","date":"2021-09-15"},{"url":"https://threatpost.com/apt-chamelgang-targets-russian-energy-aviation/175272/","date":"2021-10-01"},{"url":"https://news.sophos.com/en-us/2021/10/04/atom-silo-ransomware-actors-use-confluence-exploit-dll-side-load-for-stealthy-attack/","date":"2021-10-04"},{"url":"https://threatpost.com/tortilla-exchange-servers-proxyshell/175967/","date":"2021-11-03"},{"url":"https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json","date":"2021-11-03"},{"url":"https://cybersecurityworks.com/howdymanage/uploads/file/csw_final_ransomware_index-update-q321-csw_.pdf","date":"2021-11-10"},{"url":"https://blog.cyble.com/2022/01/17/avoslocker-ransomware-linux-version-targets-vmware-esxi-servers/","date":"2022-01-17"},{"url":"https://cybersecurityworks.com/howdymanage/uploads/file/ransomware-_-2022-spotlight-report_compressed.pdf","date":"2022-01-26"}],"evidenceTotal":100,"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":[{"id":"CWE-434","name":"Unrestricted Upload of File with Dangerous Type"},{"id":"CWE-20","name":"Improper Input Validation"}],"references":[{"url":"http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207","tags":["Patch","Vendor Advisory"]},{"url":"https://www.zerodayinitiative.com/advisories/ZDI-21-819/","tags":["Third Party Advisory","VDB Entry"]},{"url":"http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207","tags":["Patch","Vendor Advisory"]},{"url":"https://www.zerodayinitiative.com/advisories/ZDI-21-819/","tags":["Third Party Advisory","VDB Entry"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31207","tags":["US Government Resource"]}],"techniques":[{"id":"T1548.002","name":"Bypass User Account Control","tactics":["privilege-escalation"],"provenance":"source","basis":"ctid-kev","bases":["ctid-kev"],"basisDetail":"CTID Mappings Explorer KEV 07/28/2025 (ATT&CK 16.1); capability: Security Feature Bypass","mappingType":"exploitation_technique","evidence":[],"comment":"This vulnerability is exploited via authentication bypass, allowing the adversary to write to files.","confidence":"high"},{"id":"T1565","name":"Data Manipulation","tactics":["impact"],"provenance":"source","basis":"ctid-kev","bases":["ctid-kev"],"basisDetail":"CTID Mappings Explorer KEV 07/28/2025 (ATT&CK 16.1); capability: Security Feature Bypass","mappingType":"primary_impact","evidence":[],"comment":"This vulnerability is exploited via authentication bypass, allowing the adversary to write to files.","confidence":"high"},{"id":"T1190","name":"Exploit Public-Facing Application","tactics":["initial-access"],"provenance":"machine","basis":"report-attack-id","bases":["report-attack-id","report-passage"],"basisDetail":"ATT&CK ID T1190 introduces a list containing CVE-2021-31207; ATT&CK ID T1190 written in the same sentence as CVE-2021-31207; keyword query KW:T1190:2","mappingType":null,"evidence":[{"reportId":"SRC-58d99d8240c0597214b0","quote":"Hive actors have also gained initial access to victim networks by distributing phishing emails with malicious attachments [T1566.001] and by exploiting the following vulnerabilities against Microsoft Exchange servers [T1190]:\nCVE-2021-31207 - Microsoft Exchange Server Security Feature Bypass Vulnerability","start":3168,"end":3474},{"reportId":"SRC-f094fce42fddf5f29cd2","quote":"BlackByte ransomware threat actors exploit ProxyShell vulnerabilities (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207) found in Microsoft Exchange Server to gain initial access to the target network.","start":3973,"end":4172},{"reportId":"SRC-f5b6ae14f78161a4d74c","quote":"Tactic;Technique;Description TA0001 Initial Access;T1190 Exploit Public-Facing Application;In a number of attacks, the threat actors used ProxyShell vulnerablilities (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207).","start":57,"end":272}],"confidence":"low","confidenceNote":"machine-matched from report text"}],"reports":[{"id":"SRC-55c3db7cc26d9cd5018b","title":"The five-day job: A BlackByte ransomware intrusion case study","publisher":"microsoft.com","url":"https://www.microsoft.com/en-us/security/blog/2023/07/06/the-five-day-job-a-blackbyte-ransomware-intrusion-case-study/","date":"2023-07-06","mentions":[{"quote":"To obtain initial access into the victim’s environment, the threat actor was observed exploiting the ProxyShell vulnerabilities CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207 on unpatched Microsoft Exchange Servers.","start":2873,"end":3092},{"quote":"‘CVE-2021-31207’ exploit malware was detected","start":18416,"end":18461},{"quote":"CVE-2021-31207","start":19211,"end":19225}]},{"id":"SRC-f144807c3c05fbc98a1a","title":"Ransomware as a service: Understanding the cybercrime gig economy and how to protect yourself","publisher":"microsoft.com","url":"https://www.microsoft.com/en-us/security/blog/2022/05/09/ransomware-as-a-service-understanding-the-cybercrime-gig-economy-and-how-to-protect-yourself/","date":"2022-05-09","mentions":[{"quote":"ELBRUS has also been abusing CVE-2021-31207 in Exchange to compromise organizations in April of 2022, an interesting pivot to using a less popular authenticated vulnerability in the ProxyShell cluster of vulnerabilities.","start":31768,"end":31988}]},{"id":"SRC-f5b6ae14f78161a4d74c","title":"Blackcat","publisher":"blog.group-ib.com","url":"https://blog.group-ib.com/blackcat","date":null,"mentions":[{"quote":"Tactic;Technique;Description TA0001 Initial Access;T1190 Exploit Public-Facing Application;In a number of attacks, the threat actors used ProxyShell vulnerablilities (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207).","start":57,"end":272}]},{"id":"SRC-f094fce42fddf5f29cd2","title":"TTPs used by BlackByte Ransomware Targeting Critical Infrastructure","publisher":"picussecurity.com","url":"https://www.picussecurity.com/resource/ttps-used-by-blackbyte-ransomware-targeting-critical-infrastructure","date":null,"mentions":[{"quote":"The ransomware threat actors scan the network of their targets and check whether the network has CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207 vulnerabilities.","start":3744,"end":3908},{"quote":"BlackByte ransomware threat actors exploit ProxyShell vulnerabilities (CVE-2021-34473, CVE-2021-34523, CVE-2021-31207) found in Microsoft Exchange Server to gain initial access to the target network.","start":3973,"end":4172},{"quote":"CVE-2021-31207","start":4526,"end":4540}]},{"id":"SRC-e030a140d352155c7a50","title":"RaaS AvosLocker Incident Response Analysis","publisher":"linkedin.com","url":"https://www.linkedin.com/pulse/raas-avoslocker-incident-response-analysis-fl%C3%A1vio-costa?trk=articles_directory","date":null,"mentions":[{"quote":"CVEs involved in these exploits are CVE-2021-34473, CVE-2021-31206, CVE-2021-34523, and CVE-2021-31207.","start":1949,"end":2052}]},{"id":"SRC-db754fc6f8e3b44fee8e","title":"People’s Republic of China (PRC) Ministry of State Security APT40 Tradecraft in Action","publisher":"cisa.gov","url":"https://www.cisa.gov/news-events/cybersecurity-advisories/aa24-190a","date":null,"mentions":[{"quote":"APT40 rapidly exploits newly public vulnerabilities in widely used software such as Log4J (CVE-2021-44228), Atlassian Confluence (CVE-2021-31207, CVE-2021-26084) and Microsoft Exchange (CVE-2021-31207, CVE-2021-34523, CVE-2021-34473).","start":3713,"end":3947}]},{"id":"SRC-63da12bf8b0e9f1434a5","title":"2022OverWatchThreatHuntingReport","publisher":"go.crowdstrike.com","url":"https://go.crowdstrike.com/rs/281-OBQ-266/images/2022OverWatchThreatHuntingReport.pdf","date":null,"mentions":[{"quote":"https:/ /msrc.microsoft.com/update-guide/vulnerability/CVE-2021-31207.","start":39722,"end":39792}]},{"id":"SRC-5a1fc38d97e601a01ee4","title":"Exbyte: BlackByte Ransomware Attackers Deploy New Exfiltration Tool","publisher":"security.com","url":"https://www.security.com:443/threat-intelligence/blackbyte-exbyte-ransomware","date":null,"mentions":[{"quote":"In recent BlackByte attacks investigated by Symantec, the attackers exploited the ProxyShell (CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207) and ProxyLogon (CVE-2021-26855 and CVE-2021-27065) vulnerabilities in Microsoft Exchange Servers to gain initial access.","start":2846,"end":3112}]},{"id":"SRC-58d99d8240c0597214b0","title":"#StopRansomware: Hive Ransomware","publisher":"cisa.gov","url":"https://www.cisa.gov/news-events/cybersecurity-advisories/aa22-321a","date":null,"mentions":[{"quote":"CVE-2021-31207 - Microsoft Exchange Server Security Feature Bypass Vulnerability","start":3394,"end":3474},{"quote":"Hive actors gain access to victim network by exploiting the following Microsoft Exchange vulnerabilities: CVE-2021-34473, CVE-2021-34523, CVE-2021-31207, CVE-2021-42321.","start":9678,"end":9847}]}],"affected":[],"advisories":[],"euvd":{"id":"EUVD-2021-18120","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-18120","exploited":true,"exploitedSince":"2021-11-03","provenance":"source"},"exploits":[],"exploitSources":{"metasploit":[{"name":"Microsoft Exchange ProxyShell RCE","module":"exploit/windows/http/exchange_proxyshell_rce","type":"exploit","disclosureDate":"2021-04-06","url":"https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb"}],"first":{"date":"2021-04-06","source":"Metasploit","url":"https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb"},"provenance":"source"},"exploitRefs":["http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html"],"flow":[{"stage":"exploitation","techniques":["T1548.002"],"provenance":"source"},{"stage":"primary","techniques":["T1565"],"provenance":"source"},{"stage":"observed","techniques":["T1190"],"provenance":"machine","lowConfidence":true}],"timeline":[{"date":"2021-04-06","kind":"exploit-public","label":"First public exploit: Metasploit","url":"https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb","provenance":"source"},{"date":"2021-05-11","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-31207","provenance":"source"},{"date":"2021-08-30","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://www.cybereason.com/blog/threat-alert-microsoft-exchange-proxyshell-exploits-and-lockfile-ransomware","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-2021-18120","provenance":"source"},{"date":"2022-05-09","kind":"report","label":"Named in microsoft.com: Ransomware as a service: Understanding the cybercrime gig economy and how to protect yourself","url":"https://www.microsoft.com/en-us/security/blog/2022/05/09/ransomware-as-a-service-understanding-the-cybercrime-gig-economy-and-how-to-protect-yourself/","provenance":"machine"},{"date":"2023-07-06","kind":"report","label":"Named in microsoft.com: The five-day job: A BlackByte ransomware intrusion case study","url":"https://www.microsoft.com/en-us/security/blog/2023/07/06/the-five-day-job-a-blackbyte-ransomware-intrusion-case-study/","provenance":"machine"}],"speed":{"daysToPublicExploit":null,"firstExploitFrom":"Metasploit","daysToKev":176},"priority":{"score":87.6,"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 1886 days ago"},{"name":"Known ransomware use","points":10.0,"detail":"CISA KEV and VulnCheck KEV: Known"},{"name":"EPSS percentile","points":19.99,"detail":"percentile 0.9996"},{"name":"Named 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