Reference page — cumulative record through Wednesday, August 19, 2026 UTC. Reference pages update as the archive grows; only dated daily editions are immutable pages of record.
Microsoft App Installer — Windows AppX Installer Spoofing Vulnerability
AV AC PR UI S C I A CVSS EPSS %ile KEV
N H L R U H H H 7.1 .1029 95.3 YES
AFFECTED
Product Versions Fixed
App Installer 1.0.0.0 – —
TIMELINE
Nov 16 Reserved by microsoft
Dec 15 Added to CISA KEV, remediation due 2021-12-29
Dec 15 Published (CNA: microsoft)
Aug 6 EXPLOIT PUBLISHED — CVE-2021-43890 (Microsoft App Installer). Public exploit reference added.
Description
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.
Lifecycle
Complete event history — 4 events, chronological
| Date | Event | Detail |
|---|
| November 16, 2021 | Reserved | Reserved by microsoft |
| December 15, 2021 | KEV ADDED | Added to CISA KEV, remediation due 2021-12-29 |
| December 15, 2021 | Published | Published (CNA: microsoft) |
| August 6, 2026 | EXPLOIT PUBLISHED | EXPLOIT PUBLISHED — CVE-2021-43890 (Microsoft App Installer). Public exploit reference added. |
Affected
Affected products and packages — 1 row
| Vendor | Product / Package | Ecosystem | Version introduced | Fixed |
|---|
| Microsoft | App Installer | — | 1.0.0.0 | — |
About this page
This is a reference page, not a dated page of record. It assembles the complete lifecycle of CVE-2021-43890 from the CVE Program record, NVD enrichment, the CISA KEV catalog, EPSS, and OSV advisories. The box score's numbers (CVSS, EPSS, KEV status) are current as of Wednesday, August 19, 2026 UTC and are re-derived as the archive grows; only dated daily editions are immutable pages of record. The authoritative source for this identifier is cve.org.