Affected product: Dovecot IMAP Server Internal reference: DOV-6464 Vulnerability type: CWE-770 (Allocation of Resources Without Limits or Throttling) Vulnerable version: 2.2, 2.3 Vulnerable component: lib-mail Report confidence: Confirmed Solution status: Fixed in 2.3.21.1 Researcher credits: Vendor internal discovery Vendor notification: 2024-01-30 CVE reference: CVE-2024-23184 CVSS: 5.0 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:N)

Vulnerability Details: Having a large number of address headers (From, To, Cc, Bcc, etc.) becomes excessively CPU intensive. With 100k header lines CPU usage is already 12 seconds, and in a production environment we observed 500k header lines taking 18 minutes to parse. Since this can be triggered by external actors sending emails to a victim, this is a security issue.

The main problem is that each header line's address is added to the end of a linked list. This is done by walking the whole linked list, which becomes more inefficient the more addresses there are.

Workaround: One can implement restrictions on address headers on MTA component preceding Dovecot.

Fix: Install non-vulnerable version of Dovecot. Patch can be found at https://github.com/dovecot/core/compare/8e4c42d%5E...1481c04.patch