{"id":"CVE-2021-43890","published":"2021-12-15T15:15:11Z","modified":"2026-08-06T05:16:36Z","title":"Microsoft Windows AppX Installer Spoofing Vulnerability","vendor":"Microsoft","product":"Windows","tier":"core","eol":null,"description":{"text":"We have investigated reports of a spoofing vulnerability in AppX installer that affects Microsoft Windows. Microsoft is aware of attacks that attempt to exploit this vulnerability by using specially crafted packages that include the malware family known as Emotet/Trickbot/Bazaloader.\nAn attacker could craft a malicious attachment to be used in phishing campaigns. The attacker would then have to convince the user to open the specially crafted attachment. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.\nPlease see the Security Updates table for the link to the updated app. Alternatively you can download and install the Installer using the links provided in the FAQ section.\nPlease see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability.\nDecember 27 2023 Update:\nIn recent months, Microsoft Threat Intelligence has seen an increase in activity from threat actors leveraging social engineering and phishing techniques to target Windows OS users and utilizing the ms-appinstaller URI scheme.\nTo address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations.","provenance":"source","from":"NVD"},"summary":{"text":"The App Installer component (AppX installer) in Microsoft Windows contains a spoofing flaw that malicious actors have exploited using specially crafted packages, including ones tied to the Emotet, Trickbot and Bazaloader malware families. An attacker would need to deliver a malicious attachment through phishing and convince a user to open it, with impact being greater for users running with administrative rights than for those with fewer user privileges.","provenance":"ai","model":"claude-haiku-5-5","generatedAt":"2026-10-23T07:00:35Z"},"cvss":{"score":7.1,"severity":"HIGH","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H","provenance":"source","from":"NVD"},"epss":{"score":0.11294,"percentile":0.95963,"date":"2026-10-29","provenance":"source"},"kev":{"dateAdded":"2021-12-15","dueDate":"2021-12-29","ransomware":true,"requiredAction":"Apply updates per vendor instructions.","notes":"https://nvd.nist.gov/vuln/detail/CVE-2021-43890","provenance":"source"},"vulncheckKev":{"dateAdded":"2021-12-14","exploitedFirstSeen":"2021-12-14","ransomware":true,"evidence":[{"url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Dec","date":"2021-12-14"},{"url":"https://www.darkreading.com/vulnerabilities-threats/microsoft-patches-zero-day-spreading-emotet-malware","date":"2021-12-14"},{"url":"https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json","date":"2021-12-15"}],"evidenceTotal":3,"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://github.com/ChrisTitusTech/winutil/pull/26","tags":["Issue Tracking"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890","tags":["Patch","Vendor Advisory"]},{"url":"https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html","tags":["Press/Media Coverage","Third Party Advisory"]},{"url":"https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/","tags":["Press/Media Coverage","Third Party Advisory"]},{"url":"https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/","tags":["Exploit","Vendor Advisory"]},{"url":"https://github.com/ChrisTitusTech/winutil/pull/26","tags":["Issue Tracking"]},{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890","tags":["Patch","Vendor Advisory"]},{"url":"https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html","tags":["Press/Media Coverage","Third Party Advisory"]},{"url":"https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/","tags":["Press/Media Coverage","Third Party Advisory"]},{"url":"https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/","tags":["Exploit","Vendor Advisory"]},{"url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890","tags":["US Government Resource"]}],"techniques":[{"id":"T1204.002","name":"Malicious File","tactics":["execution"],"provenance":"ai","basis":"ai-mapping","bases":["ai-mapping"],"basisDetail":"claude-haiku-5-5: User must open malicious attachment","mappingType":"exploitation_technique","evidence":[{"from":"description","quote":"The attacker would then have to convince the user to open the specially crafted attachment","start":366,"end":456}],"model":"claude-haiku-5-5","confidence":"medium","confidenceNote":"AI-mapped from the description (quote-validated, not expert-reviewed)"}],"reports":[],"affected":[],"advisories":[],"euvd":{"id":"EUVD-2021-30752","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-30752","exploited":true,"exploitedSince":"2021-12-15","provenance":"source"},"exploits":[],"exploitSources":null,"exploitRefs":["https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/"],"flow":[{"stage":"exploitation","techniques":["T1204.002"],"provenance":"ai"}],"timeline":[{"date":"2021-12-14","kind":"exploited-reported","label":"Exploitation first reported (VulnCheck KEV evidence)","url":"https://api.msrc.microsoft.com/cvrf/v3.0/cvrf/2021-Dec","provenance":"source"},{"date":"2021-12-15","kind":"published","label":"Published (NVD)","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-43890","provenance":"source"},{"date":"2021-12-15","kind":"kev","label":"Added to CISA KEV (known ransomware use)","url":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","provenance":"source"},{"date":"2021-12-15","kind":"euvd-exploited","label":"ENISA EUVD: exploited","url":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2021-30752","provenance":"source"}],"speed":{"daysToPublicExploit":null,"firstExploitFrom":null,"daysToKev":0},"priority":{"score":77.2,"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 1780 days ago"},{"name":"Known ransomware use","points":10.0,"detail":"CISA KEV and VulnCheck KEV: Known"},{"name":"EPSS percentile","points":19.19,"detail":"percentile 0.9596"},{"name":"Named in threat reports","points":0.0,"detail":"0 reports"},{"name":"Public exploit code","points":5.0,"detail":"1 NVD reference tagged Exploit"},{"name":"Mentioned in OTX pulses","points":3.0,"detail":"50 pulses"},{"name":"CVSS severity","points":4.97,"detail":"base score 7.1"},{"name":"End-of-life product","points":0.0,"detail":"supported or unknown"}]}}