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Fencer Vulns
high 7.1

CVE-2021-43890 kev+ ransomware in the critical feed json

Windows AppX Installer Spoofing Vulnerability

published 2021-12-15 4 y ago modified 2026-08-06 3 mo ago also EUVD-2021-30752

cvss
7.1
version 3.1, CNA
cisa kev
Exploited
added 2021-12-15, used in ransomware
epss
11.3%
EPSS 11.3% chance of exploitation in 30 days, percentile 96%
public exploits
0
none known

Summary

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.

Patch / fix

cisa required action Apply updates per vendor instructions.

  • microsoft app_installer 1.11, 1.16

Timeline

  1. CISA SSVC: exploitation active
  2. Added to CISA KEV
  3. Published
  4. CISA patch deadline

Known exploited

Added
4 y ago
Patch by
2021-12-29 (4 y ago)
Ransomware
Known to be used in ransomware campaigns
Required action
Apply updates per vendor instructions.
Vendor, product
Microsoft, Windows
CISA name
Microsoft Windows AppX Installer Spoofing Vulnerability

CISA SSVC

Exploitation
Active, exploited in the wild
Automatable
no
Technical impact
total
Assessed
4 y ago

Exploit prediction

11.29% chance of exploitation in the next 30 days, higher than 96% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • microsoft.com https://www.microsoft.com/en-us/security/blog/2023/12/28/financially-motivated-threat-actors-misusing-app-installer/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 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
CNA 3.1 7.1 CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
NVD 3.1 7.1 CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H
NVD 2.0 6.0 AV:N/AC:M/Au:S/C:P/I:P/A:P

Affected products

  • Microsoft App Installer

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.