{"id":"CVE-2021-42321","published":"2021-11-10T01:19:50Z","modified":"2026-08-19T19:23:07Z","title":"Microsoft Exchange Server Remote Code Execution Vulnerability","vendor":"Microsoft","product":"Exchange","tier":"core","eol":null,"description":{"text":"Microsoft Exchange Server Remote Code Execution Vulnerability","provenance":"source","from":"NVD"},"summary":{"text":"The flaw is a remote code execution vulnerability in Microsoft Exchange Server, affecting the Exchange component and involving incomplete input validation and deserialization of untrusted data. An attacker with network access and low-privilege authentication, requiring no user interaction, could execute code on the affected server.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-23T06:55:40Z"},"cvss":{"score":8.8,"severity":"HIGH","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","provenance":"source","from":"NVD"},"epss":{"score":0.91737,"percentile":0.99815,"date":"2026-10-29","provenance":"source"},"kev":{"dateAdded":"2021-11-17","dueDate":"2021-12-01","ransomware":true,"requiredAction":"Apply updates per vendor instructions.","notes":"https://nvd.nist.gov/vuln/detail/CVE-2021-42321","provenance":"source"},"vulncheckKev":{"dateAdded":"2021-11-09","exploitedFirstSeen":"2021-11-09","ransomware":true,"evidence":[{"url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","date":"2021-11-09"},{"url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Nov","date":"2021-11-17"},{"url":"https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json","date":"2021-11-17"},{"url":"https://www.ic3.gov/Media/News/2022/221117.pdf","date":"2022-11-17"},{"url":"https://cisa.gov/news-events/cybersecurity-advisories/aa22-321a","date":"2022-11-25"},{"url":"https://www.prodaft.com/m/reports/FIN7_TLPCLEAR.pdf","date":"2022-12-21"},{"url":"https://4502402.fs1.hubspotusercontent-na1.net/hubfs/4502402/Ransomware%20-%20Index%20Update%20Q1%202023.pdf","date":"2023-05-15"},{"url":"https://css.ethz.ch/content/dam/ethz/special-interest/gess/cis/center-for-securities-studies/pdfs/CyberDefenseReport_%20From%20Vegas%20to%20Chengdu.pdf","date":"2024-06-06"},{"url":"https://intel471.com/blog/a-look-at-trending-chinese-apt-techniques","date":"2024-11-18"},{"url":"https://research.cert.orangecyberdefense.com/hidden-network/report.html","date":"2024-11-22"}],"evidenceTotal":11,"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-184","name":"Incomplete List of Disallowed Inputs"},{"id":"CWE-502","name":"Deserialization of Untrusted Data"}],"references":[{"url":"https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-42321","tags":["Patch","Vendor Advisory"]},{"url":"http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html","tags":["Exploit","Third Party Advisory","VDB Entry"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321","tags":["Patch","Vendor Advisory"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321","tags":["US Government Resource"]}],"techniques":[{"id":"T1078","name":"Valid Accounts","tactics":["initial-access","persistence","privilege-escalation","stealth"],"provenance":"source","basis":"ctid-kev","bases":["ctid-kev"],"basisDetail":"CTID Mappings Explorer KEV 07/28/2025 (ATT&CK 16.1); capability: Code Execution","mappingType":"exploitation_technique","evidence":[],"comment":"This vulnerability is exploited by an adversary who has gained authentication to the Exchange Server and exploited validation issues in command-let arguments. This gives the adversary access to perform remote code execution on the server. ","confidence":"medium","confidenceNote":"CTID KEV capability group \"code execution\" is generic"},{"id":"T1059","name":"Command and Scripting Interpreter","tactics":["execution"],"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":"This vulnerability is exploited by an adversary who has gained authentication to the Exchange Server and exploited validation issues in command-let arguments. This gives the adversary access to perform remote code execution on the server. ","confidence":"medium","confidenceNote":"CTID KEV capability group \"code execution\" is generic"}],"reports":[{"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":"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-2021-29296","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-29296","exploited":true,"exploitedSince":"2021-11-17","provenance":"source"},"exploits":[],"exploitSources":{"pocGithub":{"count":3,"skipped":0,"earliest":"2021-11-23","earliestUrl":"https://github.com/DarkSprings/CVE-2021-42321","repos":[{"name":"DarkSprings/CVE-2021-42321","url":"https://github.com/DarkSprings/CVE-2021-42321","stars":83,"created":"2021-11-23"},{"name":"7BitsTeam/exch_CVE-2021-42321","url":"https://github.com/7BitsTeam/exch_CVE-2021-42321","stars":10,"created":"2022-10-08"},{"name":"timb-machine-mirrors/testanull-CVE-2021-42321_poc.py","url":"https://github.com/timb-machine-mirrors/testanull-CVE-2021-42321_poc.py","stars":0,"created":"2022-01-05"}]},"metasploit":[{"name":"Microsoft Exchange Server ChainedSerializationBinder RCE","module":"exploit/windows/http/exchange_chainedserializationbinder_rce","type":"exploit","disclosureDate":"2021-12-09","url":"https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_chainedserializationbinder_rce.rb"}],"first":{"date":"2021-11-23","source":"PoC-in-GitHub","url":"https://github.com/DarkSprings/CVE-2021-42321"},"provenance":"source"},"exploitRefs":["http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html","http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html"],"flow":[{"stage":"exploitation","techniques":["T1078"],"provenance":"source"},{"stage":"primary","techniques":["T1059"],"provenance":"source"}],"timeline":[{"date":"2021-11-09","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://docs.google.com/spreadsheets/d/1lkNJ0uQwbeC1ZTRrxdtuPLCIl7mlUreoKfSIgajnSyY/edit","provenance":"source"},{"date":"2021-11-10","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-42321","provenance":"source"},{"date":"2021-11-17","kind":"kev","label":"Added to CISA KEV (known ransomware use)","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","provenance":"source"},{"date":"2021-11-17","kind":"euvd-exploited","label":"ENISA EUVD: exploited","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-29296","provenance":"source"},{"date":"2021-11-23","kind":"exploit-public","label":"First public exploit: PoC-in-GitHub; later: Metasploit 2021-12-09","url":"https://github.com/DarkSprings/CVE-2021-42321","provenance":"source"}],"speed":{"daysToPublicExploit":13,"firstExploitFrom":"PoC-in-GitHub","daysToKev":7},"priority":{"score":83.0,"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 1815 days ago"},{"name":"Known ransomware use","points":10.0,"detail":"CISA KEV and VulnCheck KEV: Known"},{"name":"EPSS percentile","points":19.96,"detail":"percentile 0.9981"},{"name":"Named in threat reports","points":3.87,"detail":"1 report"},{"name":"Public exploit code","points":5.0,"detail":"2 NVD references tagged Exploit; 3 PoC-in-GitHub repos; 1 Metasploit module"},{"name":"Mentioned in OTX pulses","points":3.0,"detail":"50 pulses"},{"name":"CVSS severity","points":6.16,"detail":"base score 8.8"},{"name":"End-of-life product","points":0.0,"detail":"supported or unknown"}]},"lineage":{"familyId":"fam-class-microsoft-exchange-server-cwe-502"}}