---
id: "CVE-2021-43890"
kind: "vulnerability"
aliases: ["CVE-2021-43890"]
title: "Microsoft Windows: Microsoft Windows AppX Installer Spoofing Vulnerability"
severity: "high"
cvss: 7.1
epss: 0.11294
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "Microsoft Windows", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "infrastructure", "ransomware", "windows"]
not_affected_if: ["unknown"]
published_at: "2021-12-15T00:00:00+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-43890", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-43890", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-43890", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# Microsoft Windows: Microsoft Windows AppX Installer Spoofing Vulnerability

Microsoft Windows AppX Installer has a spoofing flaw that CISA lists as actively exploited, and ransomware use is tagged in the KEV entry. The sources say it has high impact on confidentiality, integrity and availability, but do not say exactly what a spoofed installer lets an attacker do.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Microsoft Windows affected ? fixed ?

## Fix

Apply the Microsoft security update for CVE-2021-43890 per vendor instructions, as CISA requires; the specific KB or build numbers are not in the sources.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-43890
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-43890
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-43890

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_