CVE-2021-43890Microsoft Windows AppX Installer Spoofing Vulnerability

What it is

The record does not classify the weakness yet; the description below says what the reporter found.

How it is reached

An attacker can exploit it over the network, with an ordinary user account, but someone has to act first, such as opening a file or following a link. Success also depends on conditions the attacker does not fully control.

What an attacker gains

A successful attack can read the data it protects, change data or behaviour and take it offline.

Is it being exploited?

CISA added it to the Known Exploited Vulnerabilities catalog on 2021-12-15: it is being exploited in the wild. CISA links it to ransomware campaigns. EPSS estimates a 11% chance of exploitation activity in the next 30 days, higher than 96% of scored CVEs.

What to do

1. Treat it as urgent: it is on CISA's KEV list. CISA's deadline for US federal agencies: 2021-12-29.
2. No fixed version is listed for Microsoft Windows itself; fixed versions for the other affected products are listed on this page.
3. Until it is patched, review who holds accounts that can reach it.
4. Warn the people who use it, and filter files and links on the way in.
5. Check exposed instances for signs of compromise, not only for the version.

Description from NVD

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.