An odd attack on a download server for the open-source software replaced every tenth copy of Sendmail with a Trojan horse, according to the Sendmail development group.
The apparent attack on Sendmail didn't leave a back door in the popular open-source e-mail software package, as previously believed, but compromised the download software on the Sendmail consortium's primary server so that every tenth request for source code would receive a modified copy in reply.
"The exploited code that we see is not in our (development) tree at all," said Eric Allman, chief technology officer of Sendmail Inc., which sells a version of the open-source e-mail server program, and a member of the Sendmail Consortium, the development group for the software. "It seemed to be going to the (Sendmail) host, but it was delivering a corrupted file that wasn't on our server anywhere."
The problem apparently only affects source code for version 8.12.6 of Sendmail downloaded between Sept. 28 and Oct. 6, according to an advisory posted by the Computer Emergency Response Team (CERT) Coordination Center on Tuesday.
While the Sendmail development group is only just starting its forensic analysis of the computer that hosted the files, Allman said that its current theory is that the FTP (file transfer protocol) server had been hacked. If a user tried to download the latest Sendmail source code from the ftp.sendmail.org server, a compromised copy of the code would be sent instead about 10 percent of the time.
"It was a little bizarre that way," said Allman.
If the evidence confirms the theory, the hack would definitely be a strange way to compromise a downloadable file, said Marc Maiffret, chief hacking officer for security software firm eEye Digital Security.
"I'm not sure why they would want to do that," he said.
A Trojan horse--like the instrument that led to the downfall of the city of Troy--is a program that appears to be a legitimate piece of software but in fact has unwanted functions that allow a company or hacker to access the victim's computer.
The FTP server compromised by this attack apparently provided people who requested downloads not with the Sendmail source file, but with a Trojan-horse copy. This copy included a non-Sendmail test component that, when compiled, started a program that opens a covert channel to another server on the Internet. That server has since been configured to block the covert connection, according to messages posted to the Bugtraq security list.
Taking into account the 1-in-10 ratio, about 200 people may have downloaded the corrupted software over that eight-day period, said Sendmail's Allman. The development group is trying to contact everyone who downloaded the source code.
Both Sendmail and the CERT Coordination Center stressed that any software that is downloaded from the Internet should be verified using common cryptographic tools and the file's signature.
"Anyone that downloaded the code and followed good software practices would have found that this software was bogus," said Marty Linder, team leader for incident handling for CERT Coordination Center.
Linder stressed that, while the open development projects that give open-source its name may seem to invite problems like those of Sendmail, companies working on proprietary software have also run into problems.
In October 2000, Microsoft's source code may have been compromised by a hacker that penetrated the company's network allegedly with the help of a malicious program known as the Qaz Trojan.
"The same thing can happen if an intruder compromises the source tree of a private company," Linder said. "It's just another method for injecting badness into software."