CVE-2021-43890
Microsoft Windows AppX Installer Spoofing Vulnerability
Published Dec 15, 2021 · updated Aug 5, 2026 · microsoft app installer
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.
Timeline
Detection, updates, and resolution for this record.
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
Dec 15, 2021 · CISA KEV
Known exploited
Apply updates per vendor instructions.
Dec 29, 2021 · CISA KEV
Remediation due
Apply updates per vendor instructions.
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
Required action
Apply updates per vendor instructions.
CISA due date Dec 29, 2021
Affected
- cpe microsoft/app installerversion unspecified
- vendor Microsoft Windowsversion unspecified
Sources
- CISA KEVCVE-2021-43890
Microsoft Windows AppX Installer contains a spoofing vulnerability which has a high impacts to confidentiality, integrity, and availability.
cisa.gov - CVE.orgCVE-2021-43890
Official CVE identifier
cve.org - FIRST EPSSCVE-2021-43890
30-day exploitation probability 11.3%
first.org - NIST NVDCVE-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