{"basis":"claimed","id":"CVE-2021-43890","name":"Microsoft Windows AppX Installer Spoofing Vulnerability","vendorProject":"Microsoft","product":"Windows","publishedAt":"2021-12-15","cvss":{"score":7.1,"severity":"HIGH","version":"3.1"},"exploited":{"kevAdded":"2021-12-15","dueDate":"2021-12-29","ransomware":true},"epss":{"score":0.11294,"percentile":0.95975,"date":"2026-11-02"},"coverage":{"cloudflare":{"status":"no_history","firstOn":null,"daysToRule":null},"aws":{"status":"none","firstOn":null,"daysToRule":null},"azure":{"status":"none","firstOn":null,"daysToRule":null},"google":{"status":"no_history","firstOn":null,"daysToRule":null},"fastly":{"status":"no_history","firstOn":null,"daysToRule":null},"f5":{"status":"no_history","firstOn":null,"daysToRule":null},"akamai":{"status":"no_source","firstOn":null,"daysToRule":null},"crs":{"status":"no_history","firstOn":null,"daysToRule":null}},"title":"Windows AppX Installer Spoofing Vulnerability","description":"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.","cwes":[],"vendors":[{"vendor":"cloudflare","status":"no_history","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"aws","status":"none","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"azure","status":"none","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"google","status":"no_history","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"fastly","status":"no_history","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"f5","status":"no_history","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"akamai","status":"no_source","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]},{"vendor":"crs","status":"no_history","basis":"claimed","firstOn":null,"firstIsNew":false,"daysToRule":null,"daysAtMost":null,"freePlan":false,"claims":[]}],"updatedAt":"2026-11-04T02:17:45.826Z","cvssVector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C","ssvc":{"exploitation":"active","automatable":"no","technicalImpact":"total"},"kevAction":"Apply updates per vendor instructions.","kevNotes":"https://nvd.nist.gov/vuln/detail/CVE-2021-43890","references":[{"url":"https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890","name":null,"tags":["x_refsource_MISC"]},{"url":"https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/","name":null,"tags":[]},{"url":"https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html","name":null,"tags":[]},{"url":"https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/","name":null,"tags":[]},{"url":"https://github.com/ChrisTitusTech/winutil/pull/26","name":null,"tags":[]}],"affected":[{"vendor":"Microsoft","product":"App Installer","affected":["1.0.0.0 to before publication"],"fixed":[]}],"briefing":{"status":{"id":"known_exploited","label":"Known exploited"},"why":"CISA lists CVE-2021-43890 as exploited in the wild since 2021-12-15, including known use in ransomware campaigns. No edge WAF vendor names it in a changelog, so default managed rules may or may not catch it.","who":"Teams running Microsoft App Installer (1.0.0.0 to before publication). Prioritise internet-facing instances.","what_to_do":[{"text":"Find the version you run. In PowerShell (build and latest updates):","command":{"where":"In PowerShell (build and latest updates)","command":"Get-ComputerInfo -Property OsName, OsBuildNumber; Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5"}},{"text":"Apply the vendor's fix; the advisory lists the affected and fixed versions."},{"text":"CISA's required action: Apply updates per vendor instructions. US federal agencies must act by 2021-12-29."},{"text":"Review exposed instances for signs of compromise: it was exploited before many systems were patched, and a later patch does not undo access already gained."},{"text":"Do not rely on WAF defaults: no edge vendor names this CVE. Restrict access to the vulnerable component or add a custom rule until it is patched.","href":"#edge","linkText":"Edge coverage"},{"text":"Check which WAF sits in front of your site and whether it covers this CVE.","href":"#check","linkText":"Check your domain"}],"evidence":[{"heading":"CVE record","text":"CVSS 3.1 7.1 (high).","links":[{"label":"CVE-2021-43890","href":"https://www.cve.org/CVERecord?id=CVE-2021-43890"},{"label":"NVD","href":"https://nvd.nist.gov/vuln/detail/CVE-2021-43890"}]},{"heading":"Exploit evidence","text":"No public exploit code found in Exploit-DB, Metasploit, nuclei-templates or GitHub. That does not mean none exists.","links":[]},{"heading":"CISA KEV","text":"Added 2021-12-15, federal due date 2021-12-29, known ransomware use (catalog 2026.11.03, checked 2026-11-04).","links":[{"label":"CISA KEV catalog","href":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-43890"}]},{"heading":"EPSS","text":"11.3% probability of exploitation activity in the next 30 days, higher than 95% of CVEs (2026-11-02).","links":[{"label":"FIRST EPSS","href":"https://api.first.org/data/v1/epss?cve=CVE-2021-43890"}]},{"heading":"WAF vendor statements","text":"No edge vendor changelog names it.","links":[]}],"agent_prompt":"Assess whether the infrastructure available to you is affected by CVE-2021-43890 (Microsoft Windows AppX Installer Spoofing Vulnerability). Perform a read-only assessment within your authorized access. Do not send exploit traffic, scan third-party systems, change configuration or restart services.\n\n1. Verify the CVE record at https://www.cve.org/CVERecord?id=CVE-2021-43890 and the vendor advisory. Establish affected and fixed versions, including vendor backports. The record lists: Microsoft App Installer: affected 1.0.0.0 to before publication. Cite sources and retrieval dates; flag anything you cannot verify.\n2. Inventory deployments of Microsoft Windows: asset, environment, owner where known, installed version or build, and how you confirmed it (package metadata, a version endpoint, image tags), e.g. `Get-ComputerInfo -Property OsName, OsBuildNumber; Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5` (in PowerShell (build and latest updates)). An image tag alone does not prove the running version.\n3. Identify which deployments are reachable from the internet or untrusted networks, and through which paths (direct origin, load balancer, CDN or WAF).\n4. Assess compensating controls separately from patch status. Current edge WAF coverage for this CVE: no edge WAF vendor names it. Live data: https://cve.vigilbase.com/api/v1/cves/CVE-2021-43890. Check whether the rule is enabled and in block mode on each path, and whether any path bypasses the WAF. A WAF rule is not a patch.\n5. It is known exploited (CISA KEV): review logs on exposed instances for signs of exploitation before and after patching.\n\nReturn a short summary and a table: asset | version/build | assessment (affected, fixed, not applicable, unknown) | exposure | compensating control | evidence | next action. List what you could not assess and the access needed to resolve it. Do not infer absence of compromise from patch status or a lack of alerts.\n\nRemediation guidance: Apply the vendor's fix; the advisory lists the affected and fixed versions. CISA's required action: Apply updates per vendor instructions. US federal agencies must act by 2021-12-29.\nMore: https://cve.vigilbase.com/cve/CVE-2021-43890"},"measured":[],"sightings":null,"social":{"total":0,"first_seen":null,"daily":[{"day":"2026-10-22","posts":0},{"day":"2026-10-23","posts":0},{"day":"2026-10-24","posts":0},{"day":"2026-10-25","posts":0},{"day":"2026-10-26","posts":0},{"day":"2026-10-27","posts":0},{"day":"2026-10-28","posts":0},{"day":"2026-10-29","posts":0},{"day":"2026-10-30","posts":0},{"day":"2026-10-31","posts":0},{"day":"2026-11-01","posts":0},{"day":"2026-11-02","posts":0},{"day":"2026-11-03","posts":0},{"day":"2026-11-04","posts":0}],"top_posts":[],"exploit_repositories":[]}}