Vulnerability Monitor

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CVE-2021-43890

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.

Security Impact Summary

This vulnerability carries a HIGH severity rating with a CVSS v3.1 score of 7.1, indicating it can be exploited remotely over the network but requires specific conditions to be met though user interaction is required requiring only low-level privileges . The vulnerability impacts confidentiality (data exposure), integrity (unauthorized modifications), and availability (service disruption) for affected systems. Impacting 12 products from microsoft, from microsoft, from microsoft and 9 others, organizations running these solutions should prioritize assessment and patching.

Historical Context

Reported in 2021, this vulnerability emerged during an era marked by increased sophistication in supply chain attacks, cloud infrastructure vulnerabilities, and software-as-a-service (SaaS) security challenges. Security practices during this period emphasized zero-trust architectures, container security, and API protection.

Published

2021-12-15T15:15:11.207

Last Modified

2026-02-25T13:46:56.090

Status

Analyzed

Source

[email protected]

Severity

CVSSv3.1: 7.1 (HIGH)

CVSSv2 Vector

AV:N/AC:M/Au:S/C:P/I:P/A:P

Access Vector: NETWORK
Access Complexity: MEDIUM
Authentication: SINGLE
Confidentiality Impact: PARTIAL
Integrity Impact: PARTIAL
Availability Impact: PARTIAL

Exploitability Score

6.8

Impact Score

6.4

Weaknesses

Type: Primary
NVD-CWE-noinfo

Affected Vendors & Products

Type	Vendor	Product	Version/Range	Vulnerable?
Application	microsoft	app_installer	< 1.16	Yes
Operating System	microsoft	windows_10_1809	-	No
Operating System	microsoft	windows_10_1903	-	No
Operating System	microsoft	windows_10_1909	-	No
Operating System	microsoft	windows_10_2004	-	No
Operating System	microsoft	windows_10_20h2	-	No
Operating System	microsoft	windows_10_21h1	-	No
Operating System	microsoft	windows_10_21h2	-	No
Operating System	microsoft	windows_11_21h2	-	No
Application	microsoft	app_installer	< 1.11	Yes
Operating System	microsoft	windows_10_1507	-	No
Operating System	microsoft	windows_10_1709	-	No
Operating System	microsoft	windows_10_1803	-	No

References

https://github.com/ChrisTitusTech/winutil/pull/26 Issue Tracking ([email protected])
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 Patch, Vendor Advisory ([email protected])
https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html Press/Media Coverage, Third Party Advisory ([email protected])
https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/ Press/Media Coverage, Third Party Advisory ([email protected])
https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ Exploit, Vendor Advisory ([email protected])
https://github.com/ChrisTitusTech/winutil/pull/26 Issue Tracking (af854a3a-2127-422b-91ae-364da2661108)
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 Patch, Vendor Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html Press/Media Coverage, Third Party Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/ Press/Media Coverage, Third Party Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ Exploit, Vendor Advisory (af854a3a-2127-422b-91ae-364da2661108)
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890 US Government Resource (134c704f-9b21-4f2e-91b3-4a467353bcc0)

How SecUtils Interprets This CVE

SecUtils normalizes and enriches National Vulnerability Database (NVD) records by standardizing vendor and product identifiers, aggregating vulnerability metadata from both NVD and MITRE sources, and providing structured context for security teams. For microsoft's affected products, we extract Common Platform Enumeration (CPE) data, Common Weakness Enumeration (CWE) classifications, CVSS severity metrics, and reference data to enable rapid vulnerability prioritization and asset correlation. This record contains no exploit code, proof-of-concept instructions, or attack methodologies—only defensive intelligence necessary for patch management, risk assessment, and security operations.