Microsoft Windows AppX Installer Spoofing

What to do

1. Update App Installer to publication or a later version.
2. CISA's required action: Apply updates per vendor instructions.
3. Read the advisory on portal.msrc.microsoft.com for workarounds and exact fixed builds.
4. Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.

Affected products and versions

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector

NetworkCan be exploited remotely over a network, including the internet.

Attack complexity

HighSuccess depends on conditions outside the attacker's control.

Privileges required

LowA basic user account is enough.

User interaction

RequiredA user must take an action such as opening a file or link.

Scope

UnchangedImpact stays within the vulnerable component.

Confidentiality

HighAll data handled by the component can be read.

Integrity

HighData or code can be modified at will.

Availability

HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:H. Scored by NVD.

Official 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.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

Timeline

1. December 14, 2021CISA published its SSVC assessment (exploitation: active, automatable: no, technical impact: total).
2. December 15, 2021CVE record published.
3. December 15, 2021Added to CISA's Known Exploited Vulnerabilities catalogue.
4. December 29, 2021Deadline for US federal agencies to remediate, set by CISA.
5. August 6, 2026Record last changed materially (score, affected products, status or references).

References

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