CVE-2021-43890: Used in ransomware campaigns — CISA KEV

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 CISA says about 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…

02Required action — verbatim from CISA

Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.

03Notes & references

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

04SARA's analyst layer — why this matters

This vulnerability is currently on the CISA KEV list, which CISA only adds CVEs to when there is reliable evidence of active exploitation in the wild. For federal civilian agencies, BOD 22-01 requires remediation by the due date above. For everyone else, KEV is the strongest "patch immediately" signal you can get from public threat intel.

05Affected products (summary)

No structured affected-product list available — see references below for vendor advisories.

06Detection

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