An Internet hacker succeeded in implanting some copies of the source code for the Sendmail package with a Trojan horse, allowing intruders to access computers on which the popular mail-server was compiled and open computer networks to attack, security experts warned Tuesday.

The Trojan horse versions of the Sendmail package contain malicious code that creates a back door when the program is compiled from its source code, said the Computer Emergency Response Team Coordination Center (CERT/CC) in a statement.

CERT/CC urges Web sites that employ, redistribute or mirror the Sendmail package to immediately verify the integrity of their distribution.

Modified versions of the files sendmail.8.12.6.tar.z and sendmail.8.12.6.tar.gz began to appear in downloads from the FTP (file transfer protocol) server on or around Sept. 28, 2002, according to CERT/CC.

The Sendmail development team disabled FTP access to the server on Oct. 6.

Although CERT/CC said it doesn’t appear that copies downloaded via HTTP contained the Trojan horse, it encourages users who may have downloaded the source code via this protocol to verify software authenticity.

In particular, CERT/CC urges users to verify the cryptographic signatures of the packages. The Sendmail development team confirmed that Trojan horse copies failed signature checks.