Exim-tremely Critical? Do you need to worry about CVE-2024-39929

A critical security vulnerability in Exim MTA, identified as CVE-2024-39929, has been discovered due to a bug in RFC 2231 header parsing. This flaw could potentially allow remote attackers to deliver malicious attachments directly to user inboxes, bypassing filename extension blocking protections.

Overview of the Exim

Exim is a widely-used mail transfer agent (MTA) on Unix-like operating systems. The vulnerability, with a CVSS score of 9.1, affects Exim versions up to and including 4.97.1. Given that Exim is used on approximately 74% of public-facing SMTP mail servers, an estimated 4,830,719 instances out of 6,540,044 online, the impact of this vulnerability is substantial.

Exploiting CVE-2024-39929

The vulnerability could enable remote attackers to bypass filename extension blocking measures, allowing them to deliver executable attachments to end-users’ mailboxes. If a user were to download or run one of these malicious files, their system could be compromised, leading to unauthorized access, data theft, or further exploitation.

The demographic for this attack will mostly be smaller organizations, as larger corporations typically use Microsoft 365 or Google Workspace, which will significantly mitigate the threat

- Olivier Beg, Chief Hacking Officer @ Hadrian

While a proof-of-concept (PoC) is available, there are no known instances of active exploitation at this time. Organizations should follow best practices and promptly to mitigate potential threats.

Mitigating CVE-2024-39929

Patch Systems: Exim 4.98, released on July 10, 2024, addresses this vulnerability. Organizations should ensure their Exim servers are updated to this version to protect against potential exploitation.
Network Controls: Utilize email filtering and attachment scanning services like Microsoft 365 or Google Workspace to mitigate the risk. Endpoint and network monitoring tools can also help detect malicious activity caused by a compromise.
User Awareness: Educating users about the dangers of downloading and running attachments from unknown or untrusted sources can reduce the risk of successful exploitation.

CVE-2024-39929 represents a critical security threat due to its potential to bypass attachment filtering protections and deliver malicious executables to end-users. Despite the lack of active exploitation reports, the availability of a PoC necessitates prompt action. Updating to Exim 4.98 and reinforcing network defenses are crucial steps in mitigating this risk.