Windows AppX Installer Spoofing Vulnerability

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Summary

CVECVE-2021-43890
StatePUBLISHED
Assignermicrosoft
Source PriorityCVE Program / NVD first with legacy fallback
Published2021-12-15 15:15:11 UTC
Updated2026-08-06 05:16:36 UTC
DescriptionWe 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.

Risk And Classification

Primary CVSS: v3.1 7.1 HIGH from [email protected]

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

EPSS: 0.102950000 probability, percentile 0.952920000 (date 2026-08-17)

CISA KEV: Listed on 2021-12-15; due 2021-12-29; ransomware use Known

Problem Types: NVD-CWE-noinfo | Spoofing | CWE-noinfo Not enough information

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary7.1HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
3.1[email protected]Secondary7.1HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
3.1CNACVSS7.1HIGHCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
2.0[email protected]Primary6AV:N/AC:M/Au:S/C:P/I:P/A:P

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
Low
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

CVSS v2.0 Breakdown

Access Vector
Network
Access Complexity
Medium
Authentication
Single
Confidentiality
Partial
Integrity
Partial
Availability
Partial

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

CISA Known Exploited Vulnerability

VendorMicrosoft
ProductWindows
NameMicrosoft Windows AppX Installer Spoofing Vulnerability
Required ActionApply updates per vendor instructions.
Noteshttps://nvd.nist.gov/vuln/detail/CVE-2021-43890

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Microsoft App Installer All All All All
Operating System Microsoft Windows 10 1809 - All All All
Operating System Microsoft Windows 10 1903 - All All All
Operating System Microsoft Windows 10 1909 - All All All
Operating System Microsoft Windows 10 2004 - All All All
Operating System Microsoft Windows 10 20h2 - All All All
Operating System Microsoft Windows 10 21h1 - All All All
Operating System Microsoft Windows 10 21h2 - All All All
Operating System Microsoft Windows 11 21h2 - All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Microsoft App Installer affected 1.0.0.0 publication custom Unknown

References

ReferenceSourceLinkTags
Microsoft Disables MSIX App Installer Protocol Widely Used in Malware Attacks af854a3a-2127-422b-91ae-364da2661108 thehackernews.com Press/Media Coverage, Third Party Advisory
Added Winget Check and Install by mrhaydendp · Pull Request #26 · ChrisTitusTech/winutil · GitHub af854a3a-2127-422b-91ae-364da2661108 github.com Issue Tracking
www.cisa.gov/known-exploited-vulnerabilities-catalog 134c704f-9b21-4f2e-91b3-4a467353bcc0 www.cisa.gov US Government Resource
Microsoft disables MSIX protocol handler abused in malware attacks af854a3a-2127-422b-91ae-364da2661108 www.bleepingcomputer.com Press/Media Coverage, Third Party Advisory
Financially motivated threat actors misusing App Installer | Microsoft Security Blog af854a3a-2127-422b-91ae-364da2661108 www.microsoft.com Exploit, Vendor Advisory
Security Update Guide - Microsoft Security Response Center af854a3a-2127-422b-91ae-364da2661108 portal.msrc.microsoft.com Patch, Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis
CISA Known Exploited Vulnerabilities catalog CISA www.cisa.gov kev

Legacy QID Mappings

  • 91848 Windows AppX Installer Spoofing Vulnerability

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