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Friday, February 16, 2024

New Outlook 0-day RCE Flaw Exploited within the Wild

Outlook has been found to have an attention-grabbing vulnerability whereas dealing with particular hyperlinks, which was discovered to be exploited by menace actors within the wild. This vulnerability has been assigned with CVE-2024-21413, and the severity was given as 9.8 (Crucial).

Nonetheless, Microsoft has addressed this vulnerability and glued it as a part of their Patch Tuesday launch of February 2024. Profitable exploitation of this vulnerability might enable a menace actor to bypass the Workplace-protected view and open a file in modifying mode as an alternative of the “protected mode.”


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Outlook 0-day RCE Flaw

Based on the Checkpoint report, if the hyperlink begins with http:// or https://, Outlook makes use of Home windows’s default browser to open the URL. Nonetheless, if there are every other protocols just like the “Skype” URL protocol, clicking on the hyperlink will show a safety warning.

Microsoft Warning Notice (Source: Checkpoint)
Microsoft Warning Discover (Supply: Checkpoint)

In different circumstances, just like the “file://” protocol, Outlook didn’t show a warning dialog field. As a substitute, it had an error message within the Home windows Notification Heart, and the useful resource that was tried to entry via the hyperlink was additionally not accessed.

If the file was accessed, there’s a excessive probability that the native NTLM credential info might have been leaked.

Windows Notification Center Warning Message (Source: Checkpoint)
Home windows Notification Heart Warning Message (Supply: Checkpoint)

A slight modification within the “file://” protocol hyperlink bypasses the beforehand proven safety restriction and proceeds to entry the useful resource. For testing functions, the under hyperlink was used, which efficiently accessed the “check.rtf” file on the distant useful resource.

<a href=”file:///!one thing”>CLICK ME</a>

As acknowledged by researchers, accessing this useful resource makes use of the SMB protocol that leaks the native NTLM credential info in the course of the course of. Furthermore, researchers additionally tried escalating this assault vector to arbitrary code execution. 

Moniker Hyperlink string makes use of the “search for” for COM (Element Object Mannequin) objects on Home windows. Outlook calls the ole32!MkParseDisplayName() API for doing this job. As per Microsoft’s API doc for Moniker, together with “!” makes it a composite moniker.


Researchers used this composite moniker with FileMoniker ( + ItemMoniker (one thing) for accessing Microsoft Phrase. Home windows runs Microsoft Phrase as a COM server within the background.

If the hyperlink is clicked, Phrase opens and parses the file “check.rtf” based mostly on the string “”. Nonetheless, this check.rtf is managed by the attacker, which was additional modified to carry out arbitrary code execution on the distant system utilizing “WINWORD.EXE”.

Researchers acknowledged this #MonikerLink bug/assault vector could also be current in different software program and in addition suggest builders examine and repair the difficulty.

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