It was discovered that the SIEVE component of cyrus-imapd, a highly scalable enterprise mail system, is vulnerable to a buffer overflow when processing SIEVE scripts. Due to incorrect use of the sizeof() operator an attacker is able to pass a negative length to snprintf() calls resulting in large positive values due to integer conversion. This causes a buffer overflow which can be used to elevate privileges to the cyrus system user. An attacker who is able to install SIEVE scripts executed by the server is therefore able to read and modify arbitrary email messages on the system.