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Security advisories — Dec 15, 2021

Windows Zero-Day Vulnerability – CVE-2021-43890

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In Microsoft’s December 14, 2021, Patch Tuesday vulnerability release, security patches were released for a high severity zero-day vulnerability impacting the Windows AppX installer. The vulnerability is tracked as CVE-2021-43890 (CVSS: 7.1). Exploitation allows a threat actor to create a malicious file that appears to be a legitimate application.

Exploitation in the wild has been observed in the delivery of multiple malware types including: Emotet, Trickbot, and BazarLoader. Organizations are strongly recommended to apply the relevant security patches as exploitation is ongoing.

What we’re doing about it

What you should do about it

Additional information

In an attack scenario, threat actors could craft a malicious attachment and deliver it to victims via either email or a link inside of an email; the attachment would appear as a legitimate application. Opening the malicious attachment leads to the deployment of malware. eSentire has detected attacks exploiting this technique. In observed attacks, the malicious attachment mimicked Adobe PDF Reader. An example of Emotet malware being delivered in this fashion is outlined in the December 2021 Threat Intelligence Observations On-Demand webinar.

As exploitation requires user interaction, it is critical that staff are trained in best security practices, especially surrounding email security.


[1] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-43890

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