CVE-2021-43890 — Microsoft Windows AppX Installer Spoofing Vulnerability

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/Bazaloa

01What is this vulnerability?

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…

02Affected products

03Active exploitation status

Yes — actively exploited. Added to the CISA KEV catalog on 2021-12-15. Ransomware use: Known.

04Recommended remediation

• Patch to a fixed version listed in the vendor advisory (see references below).
• Mitigate with WAF rules, network egress filters, or feature flags where the patch is not yet available.
• Hunt historical logs for exploitation indicators — see Detection signatures below.

05Technical details

For the full vendor write-up, exploit chains, and reference implementations, see the references list in section 09.

06Detection signatures

Open the Sigma generator with a pre-filled prompt for this CVE to draft a starting detection in your stack of choice:

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07Related CVEs

No directly-cited follow-up CVEs in the KB record for this advisory. The references list in section 09 carries the vendor cross-references.

08Timeline

• Published: 2021-12-15T14:15:35.000Z
• Last modified: 2025-10-21T23:25:22.309Z
• Added to CISA KEV: 2021-12-15
• BOD 22-01 due: 2021-12-29

09References

• portal.msrc.microsoft.com — https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-4389…
• www.bleepingcomputer.com — https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol…
• thehackernews.com — https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html
• www.microsoft.com — https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-t…
• github.com — https://github.com/ChrisTitusTech/winutil/pull/26