CVE-2021-43890
Windows AppX Installer Spoofing Vulnerability
- Published
- Updated
- Reserved
- Assigned by
- microsoft
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.
Exploited in the wild
CISA lists this vulnerability in its Known Exploited Vulnerabilities catalog as Microsoft Windows AppX Installer Spoofing Vulnerability.
- Added
- 15 Dec 2021
- Fix due (US federal)
- 29 Dec 2021
- Used by ransomware
- Known
Apply updates per vendor instructions.
Public exploit
An exploit or proof of concept is public: anyone can try it against an unpatched system. CISA's SSVC rates its exploitation: it is being exploited.
Timeline
ID reserved
By microsoft
- 29 days later
· 29 days later
Published
Added to CISA KEV
Known exploited, also by ransomware
- 5 years later
· 5 years later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| microsoftCNA (assigner) | 3.1 | High7.1 | 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 |
| NVDNVD · Analyzed | 3.1 | High7.1 | CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H |
- EPSS (exploitation in 30 days)
- 11.3% · top 4.0% of all CVEs (7 Nov 2026)
- CISA SSVC
- Exploitation: active · Automatable: no · Technical impact: total
The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.1.
Weakness
- Spoofing
- CWE-noinfo Not enough information
Affected products
- affected 1.0.0.0 – < publication
References (6)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-43890 (opens in a new tab)x_refsource_MISCx_transferred
- https://www.bleepingcomputer.com/news/microsoft/microsoft-disables-msix-protocol-handler-abused-in-malware-attacks/ (opens in a new tab)x_transferred
- https://thehackernews.com/2023/12/microsoft-disables-msix-app-installer.html (opens in a new tab)x_transferred
- https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ (opens in a new tab)x_transferred
- https://github.com/ChrisTitusTech/winutil/pull/26 (opens in a new tab)x_transferred
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-43890 (opens in a new tab)government-resource
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- CISA KEV catalog (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2021-43890)Record last updated .