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CVE-2021-43890

7.1, high Known exploited Used in ransomware

Microsoft Windows AppX Installer Spoofing Vulnerability

NVD (opens in a new tab) CVE.org (opens in a new tab) CISA KEV catalogue (opens in a new tab)
CVSS
7.1, high high CVSS v3.1
EPSS, next 30 days
11% higher than 95.9% of scored CVEs
Exploitation
Confirmed in the wild
Published
15 Dec 20214 years ago

Known exploited

CISA added this to the Known Exploited Vulnerabilities Catalog on 15 Dec 2021 and links it to ransomware activity. US federal agencies had to remediate by 29 Dec 2021, a deadline that has passed.

Required action. Apply updates per vendor instructions.

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.

Details

Vendor
Microsoft
Product
App Installer
Weakness
n/a
Status
Analyzed
Updated
6 Aug 2026
Assigner
secure@microsoft.com

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H

References 6

Exploit code 1

Patches and fixes 2

Advisories 2

Other references 1

The kind is read from NVD's tags where present and from the shape of the URL otherwise, so treat it as a hint.

Other Microsoft CVEs

Every Microsoft CVE we hold

Same vendor in what we hold, not a known link between the flaws.

  • CVE-2026-96951 Microsoft Windows Command Injection Vulnerability KEV EPSS 2% 8.8, high
  • CVE-2026-81963 Microsoft Windows Link Following Vulnerability KEV EPSS 0.4% 7.8, high
  • CVE-2026-85880 Microsoft Windows Heap-Based Buffer Overflow Vulnerability KEV EPSS 4% 7.8, high
  • CVE-2026-65660 Microsoft SharePoint Code Injection Vulnerability KEV EPSS 2% 8.8, high
  • CVE-2026-68820 Microsoft Windows Ancillary Function Driver for WinSock Use-After-Free Vulnerability KEV EPSS 0.3% 7, high
  • CVE-2026-50522 Microsoft SharePoint Deserialization of Untrusted Data Vulnerability KEV EPSS 3% 9.8, critical
  • CVE-2026-56155 Microsoft Active Directory Federation Services Insufficient Granularity of Access Control Vulnerability KEV EPSS 0.3% 7.8, high
  • CVE-2026-56164 Microsoft SharePoint Server Missing Authentication for Critical Function Vulnerability KEV EPSS 1% 9.8, critical

Beyond this page

Is CVE-2021-43890 in your network?

This page tells you how dangerous CVE-2021-43890 is out there. Whether it sits on one of your devices, only a scan of your own network can tell. That is what our systems do, every day, for the companies we protect.

CVE-2021-43890 is in the CISA catalogue of exploited vulnerabilities: attackers are already exploiting it.

  • EXYS7600 OCELOT

    Find it before attackers do

    Ocelot scans your network on a schedule and reports every vulnerability it finds by its CVE identifier, with its CVSS score and how many of your hosts are affected. A finding like CVE-2021-43890 comes with the devices to patch.

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  • EXYS7950 KONDRA

    Detect it if it's exploited

    Until the patch is applied, Kondra monitors your network, events and endpoints in real time. It keeps an inventory of the software installed on every endpoint, and its anomaly detection rules flag what an attacker does once inside.

    Learn more about EXYS7950 KONDRA
Check your exposure

Assembled from the NVD 2.0 API, the CISA Known Exploited Vulnerabilities Catalog and FIRST EPSS. Scores are as of our last collection on 17 Oct 2026, not the live upstream state.