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Systems running Sendmail are at risk of hacker attacks because of a flaw in the way the commonly used e-mail server software handles long e-mail addresses, experts warned Saturday. This second serious bug announced this month has some Sendmail users looking for alternatives.

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 Coordination Center warned in an advisory Saturday.

Sendmail servers that aren't 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, CERT said.

Patching the Problem

Many vendors 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 urge users of their versions to upgrade to Sendmail 8.12.9 or apply a patch, they said in a joint advisory Saturday. 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 prior to 8.12.9, and several versions of Sendmail Switch and Sendmail for NT, according to CERT.

The e-mail address parser flaw is the second "critical" bug in 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, experts said.

E-Mail Alternatives

Some users have had it with Sendmail, according to postings on the topic on "News for nerds" Web site Slashdot.org.

"This is the straw that breaks the camel's back. I'm changing to another MTA," writes one user. "Sendmail: The IIS of Open Source," writes another, referring to Microsoft's Internet Information Server Web server software, which has had its share of security issues.

However, other forum postings criticize qmail and Postfix, two Sendmail alternatives, and point out that this latest Sendmail flaw is likely difficult to exploit.

CERT said it successfully exploited the flaw to knock the Sendmail server offline and that it is possible to execute code on some systems by exploiting this flaw.

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