Microsoft · Windows
CVE-2021-43890
Windows AppX Installer Spoofing Vulnerability
What it is
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.
CISA: Microsoft Windows AppX Installer contains a spoofing vulnerability which has a high impacts to confidentiality, integrity, and availability.
What to do
Apply updates per vendor instructions.
Deadline for US federal agencies: December 29, 2021
- Added to the CISA list
- December 15, 2021
- Published
- December 15, 2021
- Severity
- High 7.1 CVSS 3.1
- Chance of attacks in the next 30 days (EPSS)
- 11% in the top 4.0% of all known vulnerabilities
- More details
- NVD · CVE.org · CISA
All references (5)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890
- https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-at…
- https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html
- https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-ins…
- https://github.com/ChrisTitusTech/winutil/pull/26
More exploited flaws from Microsoft
| Added | Vulnerability | Product | Severity | Attack chance |
|---|---|---|---|---|
| Oct 13, 2026 | Command Injection | Windows | High 8.8 | 9.8% |
| Sep 8, 2026 | Link Following | Windows | High 7.8 | 0.4% |
| Sep 8, 2026 | Heap-Based Buffer Overflow | Windows | High 7.8 | 0.4% |
| May 20, 2026 | Buffer Overflow | Windows | Critical 9.8 | 99% |
| Apr 28, 2026 | Protection Mechanism Failure | Windows | Medium 4.3 | 71% |
| Apr 13, 2026 | Out-of-Bounds Read | Windows | High 7.8 | 12% |
| Apr 13, 2026 | Link Following | Windows | High 7.8 | 4.6% |
| Feb 17, 2026 | Microsoft Windows Video ActiveX Control Remote Code Execution | Windows | High 8.8 | 77% |
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.