Executive brief
Dovecot's Sieve mail filtering service tracks CPU usage to prevent runaway scripts from consuming server resources. An attacker with valid mail credentials can repeatedly switch between Sieve scripts to reset the accounting counters, bypassing the configured CPU limit and causing sustained CPU consumption. Additionally, compiled script files are not cleaned up when deleted, wasting disk space. Together, these issues can degrade mail delivery performance and availability for legitimate users.
Technical details
The vulnerability is a resource accounting bypass in the Sieve script engine. CPU usage is tracked per compiled script object, and an authenticated attacker can reset the accounting by repeatedly changing the active Sieve script; each script change creates a new compiled script object with zeroed counters. Additionally, compiled script files are not removed when scripts are deleted or renamed, leading to disk space exhaustion over time. The attacker requires valid mail account credentials to change their Sieve scripts, making this an authenticated attack. The combined effects allow sustained CPU consumption beyond configured limits and increased disk I/O contention, degrading mail delivery service. Patches are available in OX Dovecot Pro 2.3.22.2, 3.0.7, and 3.1.6 or later.
Affected products
- Open-Xchange Dovecot Pro 2.3.0 to 2.3.22.1, 3.0.0 to 3.0.6, 3.1.0 to 3.1.5
Timeline
- 2026-08-28: disclosed