Researchers from Akamai have discovered that a slight modification to a Microsoft Exchange zero-day attack can bypass a patch introduced by Microsoft in March. The modified attack, identified as CVE-2023-29324, was patched by Microsoft but labeled as “important” rather than “critical.” Akamai researchers argue that the vulnerability should be considered critical as the addition of a single character renders the patch ineffective.
The original bug, CVE-2023-23397, allowed remote attackers to leak hashed Windows account passwords through a specially crafted email, and it has been exploited by the Russian APT28 group since spring 2022.
The original vulnerability exploited the custom notification sound feature in Microsoft Outlook. By specifying the custom sound as a universal naming convention (UNC) path, the attacker could trick Outlook into retrieving the sound file from a remote server.
The patch introduced by Microsoft verifies that the specified path does not refer to an internet URL. However, Akamai researcher Ben Barnea found a way to bypass the fix by tricking Outlook into treating a UNC path pointing to the internet as a local path with the addition of an extra backward slash.
Akamai’s analysis highlights that Microsoft claims Exchange servers updated in March no longer invoke a custom sound file. The root cause of the vulnerability lies in the complex handling of paths in Windows, and Barnea suggests that removing the custom reminder feature would be the ultimate solution, as it poses more security risks than value to users.
