Threat Signal Report

Newly Patched Windows Vulnerability (CVE-2021-43890) Being Exploited to Deliver Malware

description-logo Description

FortiGuard Labs is aware of a report that a newly patched Windows vulnerability (CVE-2021-43890) is being exploited in the wild to deliver malware such as Emotet, Trickybot and Bazaloader. Exploiting CVE-2021-43890 allows an attacker to create a malicious package file that looks like a legitimate application. The vulnerability is patched as part of MS Tuesday in December 2021.

Why is this Significant?

This is significant because CVE-2021-43890 was abused as a zero-day to deliver Emotet, Trickybot and Bazaloader. Those malware typically deploy additional malware including ransomware to a compromised machine.

What is CVE-2021-43890?

CVE-2021-43890 is Windows AppX Installer Spoofing Vulnerability that allows an attack to spoof a malicious package as legitimate software. For example, an attacker can abuse CVE-2021-43890 to create a fake malicious package that has an icon of legitimate software, a valid certificate that marks the package as a Trusted App along with fraudulent publisher information. These pieces increase the chance of convincing the victim to run the file.

Image of "Windows AppX Installer abuse to install Emotet" courtesy of BleepingComputer

Microsoft rates this vulnerability as important.

Has the Vendor Released a Fix for the Vulnerability?

Yes, Microsoft released a fix on December 14th, 2021, as part of December Patch Tuesday.

What is the Status of Coverage?

There is not sufficient information available yet that enables FortiGuard Labs to develop IPS protection for CVE-2021-43890.

FortiGuard Labs provides the following AV coverage against malware that abuses CVE-2021-43890:




Traffic Light Protocol

Color When Should it Be used? How may it be shared?


Not for disclosure, restricted to participants only.
Sources may use TLP:RED when information cannot be effectively acted upon by additional parties, and could lead to impacts on a party's privacy, reputation, or operations if misused. Recipients may not share TLP:RED information with any parties outside of the specific exchange, meeting, or conversation in which it was originally disclosed. In the context of a meeting, for example, TLP:RED information is limited to those present at the meeting. In most circumstances, TLP:RED should be exchanged verbally or in person.


Limited disclosure, restricted to participants’ organizations.
Sources may use TLP:AMBER when information requires support to be effectively acted upon, yet carries risks to privacy, reputation, or operations if shared outside of the organizations involved. Recipients may only share TLP:AMBER information with members of their own organization, and with clients or customers who need to know the information to protect themselves or prevent further harm. Sources are at liberty to specify additional intended limits of the sharing: these must be adhered to.


Limited disclosure, restricted to the community.
Sources may use TLP:GREEN when information is useful for the awareness of all participating organizations as well as with peers within the broader community or sector. Recipients may share TLP:GREEN information with peers and partner organizations within their sector or community, but not via publicly accessible channels. Information in this category can be circulated widely within a particular community. TLP:GREEN information may not be released outside of the community.


Disclosure is not limited.
Sources may use TLP:WHITE when information carries minimal or no foreseeable risk of misuse, in accordance with applicable rules and procedures for public release. Subject to standard copyright rules, TLP:WHITE information may be distributed without restriction.