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Vulnerabilities

Vulnerability · CVE-2021-43890

Microsoft Windows AppX Installer Spoofing Vulnerability

Published Dec 14, 2021 · catalog updated Aug 5, 2026 · Microsoft Windows

Patch now

CVE-2021-43890 is in the CISA known-exploited catalog, due Dec 29, 2021. Confirm Microsoft Windows against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Gaps on this record

  • No version range is stored. A customer cannot check an installed version against this record.
Severity
High7.1
Known exploitation
CISA KEV and VulnCheck KEV
EPSS
11.29%
Remediation
Remediation guidance
Affected products
cpe microsoft/app installer · vendor Microsoft Windows

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe microsoft/app installerversion unspecified
  • vendor Microsoft Windowsversion unspecified

How to fix CVE-2021-43890

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

CVE-2021-43890 is in the CISA known-exploited catalog, due Dec 29, 2021. Confirm Microsoft Windows against the vendor advisory and apply that fix. The CVSS score does not change the priority.

Which versions are affected?

Affected version ranges are not on this record. Apply updates per vendor instructions.

What should an analyst verify?

No weakness class is stored. EPSS 11.29%. The CISA KEV listing is the exploitation evidence. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Technology hubs

Windows security

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Dec 15, 2021 · Catalog

    Detected

    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 co

  2. Dec 15, 2021 · CISA KEV

    Known exploited

    Apply updates per vendor instructions.

  3. Dec 29, 2021 · CISA KEV

    Remediation due

    Apply updates per vendor instructions.

  4. Aug 5, 2026 · Catalog

    Updated

    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 co

Sources

  • CISA KEVConfirmed exploitationCVE-2021-43890

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

    cisa.gov
  • CVE.orgVulnerability recordCVE-2021-43890

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2021-43890

    30-day exploitation probability 11.3%

    first.org
  • NIST NVDVulnerability recordCVE-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/Bazaloa

    nvd.nist.gov
  • VulnCheck KEVConfirmed exploitationCVE-2021-43890

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

    vulncheck.com

References

Freshness

Last verified in this catalog Oct 17, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.