CVE-2021-43890: Technical Deep-Dive (Auto Refreshed)

Generated on 2026-03-22T22:15:13.187Z. This file is automatically regenerated every 30 minutes by the CVE AI enrichment job using web sources (NVD, MITRE, CISA KEV, GitHub).

Executive Technical Summary

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. An 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. Please 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. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. December 27 2023 Update: In 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. To address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations.

• Context preserved from previous revision: 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. An 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. Please 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. Please see the Mitigations and Workaround sections for important information about steps you can take to protect your system from this vulnerability. December 27 2023 Update: In 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. To address this increase in activity, we have updated the App Installer to disable the ms-appinstaller protocol by default and recommend other potential mitigations. 1. Use only isolated environments and assets you own or are explicitly authorized to test.

Technical Details

• CVE: CVE-2021-43890
• KEV date added: 2021-12-15
• KEV due date: 2021-12-29
• NVD published: 2021-12-15
• NVD modified: 2026-02-25
• MITRE modified: 2025-10-21
• CVSS base score: 7.1
• CVSS vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
• CVSS exploitability score: 1.2
• CVSS impact score: 5.9
• Attack vector: Network
• Attack complexity: High
• Privileges required: Low
• User interaction: Required
• Scope: Unchanged
• Confidentiality impact: High
• Integrity impact: High
• Availability impact: High

Versions and Products Impacted

• microsoft / app installer (versions: < 1.16)
• microsoft / app installer (versions: < 1.11)
• Microsoft / App Installer (versions: 1.0.0.0)

Weakness Classification

• NVD-CWE-noinfo
• CWE-noinfo Not enough information

Repositories for Lab Validation (Public Examples)

• Ostorlab/KEV | stars: 607 | updated: 2026-03-16 | https://github.com/Ostorlab/KEV
  Notes: Ostorlab KEV: One-command to detect most remotely known exploitable vulnerabilities. Sourced from CISA KEV, Google's Tsunami, Ostorlab's Asteroid and Bug Bounty programs.

People and Organizations Mentioned

• microsoft
• Windows
• Ostorlab

Practical Defensive Validation (Authorized Only)

1. Use only isolated environments and systems you own or are explicitly authorized to test.
2. Snapshot infrastructure before validation and preserve baseline logs (EDR, SIEM, OS, app).
3. Inventory microsoft / app installer (versions: < 1.16) assets and confirm exact vulnerable versions with automated checks.
4. Patch in staged environments and validate closure with scanners + service health checks.
5. Map detections to MITRE ATT&CK tactics relevant to your environment and tune alert quality.

References

• NVD record: https://nvd.nist.gov/vuln/detail/CVE-2021-43890
• MITRE CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43890
• CISA KEV Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• CISA KEV JSON feed: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json
• KEV notes: https://nvd.nist.gov/vuln/detail/CVE-2021-43890
• https://github.com/ChrisTitusTech/winutil/pull/26
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890
• https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html
• https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/
• https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890
• Repository example: https://github.com/Ostorlab/KEV

This content is for defensive security training and authorized validation only.