Systems running Sendmail are at risk of hacker attacks because of a flaw in the way the commonly used email server software handles long e-mail addresses, experts have warned.
This is the second serious bug announced this month and has some Sendmail users looking for alternatives.
Sendmail does not adequately check the length of email addresses. An email 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 Centre warned in an advisory.
Sendmail servers that weren't directly connected to the Internet were also at risk, since the vulnerability was triggered by the contents of a malicious email message that could be handed on from server to server, CERT said.
Many vendors distributed 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 Inc and the Sendmail Consortium urged 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 Inc sells a commercial product based on the open source sendmail software.
The problem affected 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 email address parser flaw was 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.
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," writes one user. "I'm changing to another MTA."
"Sendmail: The IIS of Open Source," writes another, referring to Microsoft's Internet Information Server (IIS) Web server software, which has had its share of security issues.
However, other forum postings criticise 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 was possible to execute code on some systems by exploiting this flaw.