Microsoft Windows AppX Installer Spoofing Vulnerability

What is CVE-2021-43890?

Microsoft Windows AppX Installer contains a spoofing vulnerability which has a high impacts to confidentiality, integrity, and availability.

Known to be used in ransomware campaigns. Active threat actors have chained this vulnerability into ransomware operations — treat patching as a same-week priority, not a "next maintenance window" task. The fix is the same one below; the urgency is higher.

VulnCheck tracked exploitation 1 day before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.

Who is affected by CVE-2021-43890?

Affects anyone running Windows workstations or servers. In a small CPA, legal, or dental practice, Windows is typically the platform your accounting, document management, or practice management software runs on — exploitation gives an attacker access to whatever client files and credentials live on those machines.

What should I do about CVE-2021-43890?

Apply updates per vendor instructions.

If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Request a free review.

Microsoft fix

• Release Notes (Security Update)

Sourced from Microsoft Security Update Guide (CVRF document 2021-Dec).

CISA due date

CISA's listed due date: December 29, 2021.

Federal civilian agencies work under CISA's Binding Operational Directive 26-04, which replaced BOD 22-01 on June 10, 2026. It sets the deadline for each affected system by:

• How exposed that system is to the internet
• Whether the attack can be automated
• How much control it gives an attacker

Its table gives catalogued vulnerabilities 3 or 14 days.

None of this binds a private business, but treat the date as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).

EPSS

EPSS score: 11.3% — higher than 96% of all scored CVEs.

EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.

Score dated 2026-11-10, from the FIRST.org Exploit Prediction Scoring System.

Other exploited Microsoft Windows entries

More CISA KEV entries for Microsoft Windows. Same product line, same actively-exploited status.

• CVE-2026-96951: Microsoft Windows Command Injection Vulnerability
• CVE-2026-81963: Microsoft Windows Link Following Vulnerability
• CVE-2026-85880: Microsoft Windows Heap-Based Buffer Overflow Vulnerability
• CVE-2008-4250: Microsoft Windows Buffer Overflow Vulnerability
• CVE-2026-32202: Microsoft Windows Protection Mechanism Failure Vulnerability
• CVE-2023-36424: Microsoft Windows Out-of-Bounds Read Vulnerability
• CVE-2025-60710: Microsoft Windows Link Following Vulnerability
• CVE-2008-0015: Microsoft Windows Video ActiveX Control Remote Code Execution Vulnerability

Browse more

• All Microsoft KEV entries
• Windows
• All SMB-relevant KEV entries

Source

This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.