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

CISA Known Exploited Vulnerabilities

Name

Microsoft Windows AppX Installer Spoofing Vulnerability

Vendor

Microsoft

Product

Windows

Added to the catalog

2021-12-15

Due date for US federal agencies

2021-12-29

Catalog version

2026.10.30

Microsoft Windows AppX Installer contains a spoofing vulnerability which has a high impacts to confidentiality, integrity, and availability.

Required action (CISA)

Apply updates per vendor instructions.

Ransomware use: CISA marks this vulnerability as known to be used in ransomware campaigns. CISA does not say which group used it.

Links in CISA's notes

• https://nvd.nist.gov/vuln/detail/CVE-2021-43890

Read on 2026-11-02T23:39:54.475Z under licence cisa-kev-cc0@b0067899 from https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json

NVD analysis

Status

Analyzed

Published

2021-12-15T15:15:11.207

Last modified

2026-08-06T05:16:36.653

Weaknesses

NVD-CWE-noinfo

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.

References

• https://github.com/ChrisTitusTech/winutil/pull/26 (Issue Tracking)
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 (Patch, Vendor Advisory)
• https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html (Press/Media Coverage, Third Party Advisory)
• https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/ (Press/Media Coverage, Third Party Advisory)
• https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ (Exploit, Vendor Advisory)
• https://github.com/ChrisTitusTech/winutil/pull/26 (Issue Tracking)
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 (Patch, Vendor Advisory)
• https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html (Press/Media Coverage, Third Party Advisory)
• https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/ (Press/Media Coverage, Third Party Advisory)
• https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ (Exploit, Vendor Advisory)
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890 (US Government Resource)

This product uses the NVD API but is not endorsed or certified by the NVD.

Read on 2026-11-02T23:41:59.479Z under licence nvd-api-terms-of-use@ff2a5288 from https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2021-43890

CVE record

Title (CNA)

Windows AppX Installer Spoofing Vulnerability

Assigner

microsoft

State

PUBLISHED

Published

2021-12-15T14:15:35.000Z

Updated

2026-08-06T03:55:23.978Z

Problem types

Spoofing

References

• 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-attacks/
• 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-installer/
• https://github.com/ChrisTitusTech/winutil/pull/26

Read on 2026-11-03T01:09:14.403Z under licence cve-terms-of-use@81bfa2f3 from https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2021/43xxx/CVE-2021-43890.json