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.
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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.