# CVE-2021-43890: Microsoft Windows AppX Installer Spoofing

> Attackers can spoof AppX packages to execute malware via phishing, impacting Windows users, especially those with administrative rights.

- **Severity:** High 7.1 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Yes, since 15 Dec 2021 (CISA KEV), used in ransomware
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 15 Dec 2021
- **Updated:** 6 Aug 2026
- **Also known as:** GHSA-hvcq-2mcq-rg5f

## What is CVE-2021-43890?

CVE-2021-43890 is a high-severity vulnerability in Microsoft App Installer, published on 15 Dec 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official 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.

## Which versions are affected by CVE-2021-43890?

It affects 1 product.


Products: Microsoft App Installer.

## How do I fix CVE-2021-43890?

No fix has been published for CVE-2021-43890 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 29 Dec 2021.

## Is CVE-2021-43890 being exploited?

Yes. Attackers have exploited CVE-2021-43890 in the wild since at least 15 Dec 2021, including in ransomware campaigns.

## Other vulnerabilities in Microsoft App Installer

- [CVE-2024-38177](https://insights.sbomify.com/CVE-2024-38177/): Windows App Installer Spoofing Vulnerability (high 7.8)
- [CVE-2026-68821](https://insights.sbomify.com/CVE-2026-68821/): Windows Package Manager Elevation of Privilege Vulnerability (high 7.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-43890 and fix it if it is affected.

What it is: CVE-2021-43890 is a high-severity vulnerability in Microsoft App Installer, published on 15 Dec 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: Yes. Attackers have exploited CVE-2021-43890 in the wild since at least 15 Dec 2021, including in ransomware campaigns.

Affected product: Microsoft App Installer. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-43890/ (Markdown: https://insights.sbomify.com/CVE-2021-43890/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-43890
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-43890
- OSV: https://osv.dev/vulnerability/CVE-2021-43890

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-43890/
Summaries are generated and may contain mistakes; the official description above is the record.