An attacker that has valid credentials can use a Sieve script with the editheader extension to trigger a use-after-free in the mail editing code, and to write memory contents beyond the intended buffer into the delivered mail. This causes memory leak and opportunity to do memory corruption during mail delivery, which can crash the delivery process and may allow execution of arbitrary code in the context of that process. Disable the Sieve editheader extension. Update to non-vulnerable version. No publicly available exploits are known.

CVE-2026-42007 affects Dovecot mail server, where an authenticated attacker can exploit the Sieve script editheader extension to trigger a use-after-free vulnerability in the mail editing code. The flaw allows writing memory contents beyond intended buffer boundaries into delivered mail, resulting in memory leaks and potential memory corruption during mail delivery. This can crash the delivery process and may allow execution of arbitrary code within the context of that process. The vulnerability requires valid credentials to exploit, limiting the attack surface somewhat. Mitigations include disabling the Sieve editheader extension or updating to a non-vulnerable version of Dovecot. No publicly available exploits are currently known. The advisory is published by Open-Xchange, the maintainer of Dovecot.