This second vulnerability announced this month occurs because Sendmail does not adequately check the length of e-mail addresses.
An e-mail message with a specially crafted address can trigger a stack overflow, potentially allowing an attacker to gain control of a vulnerable sendmail server, the CERT Co-ordination Center warned in an advisory on Saturday.
Sendmail servers that are not directly connected to the internet are also at risk, since the vulnerability is triggered by the contents of a malicious e-mail message that can be handed on from server to server.
Many suppliers distribute vulnerable versions of Sendmail and users should check with their vendors for a security patch, CERT said. Sendmail is the most commonly used MTA (mail transfer agent) and handles the majority of all internet e-mail traffic.
Sendmail and the Sendmail Consortium are urging users of their versions to upgrade to sendmail 8.12.9 or apply a patch.
The Sendmail Consortium develops an open-source version of Sendmail that is distributed with both free and commercial operating systems. Sendmail sells a commercial product based on the open-source Sendmail software.
The problem affects all versions of Sendmail Pro, all versions of open-source Sendmail before 8.12.9, and several versions of Sendmail Switch and Sendmail for NT.
The e-mail address parser flaw is the second "critical" bug Sendmail announced and patched this month.
The earlier vulnerability occurred because of an error in a function that checks whether addresses in the e-mail message header are valid. This could also allow an attacker to take over a Sendmail server.