During several intrusions Nov. 19, the flaw enabled an attacker who already had access to a server to remove the limitations that protected the system from everyday users. The technique is known as a privilege escalation.
Members of the development team found the flaw in September and fixed the latest version of the core Linux software, or kernel. The fix came a bit late, however. The latest version of the kernel, 2.4.23, was released Friday, eight days after the Debian breach.
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"Fortunately, we require developers to sign the upload (software) digitally," said Martin Schulze, a developer and member of the project. "These files are stored off-site as well, which were used as a basis for a recheck."
The development team promised to lock all developer accounts until the flaw had been found and fixed. The team published patches for the flaw on Monday as well but didn't specify when the accounts would be unlocked.
The unknown attacker compromised at least four servers. The systems--known as Master, Murphy, Gluck and Klecker--had maintained the open-source project's bug tracking system, source code database, mailing lists, Web site and security patches.
The attacker gained access to one of the systems by compromising a developer's computer and installing a program to sniff out the characters typed on the developer's keyboard, according to a postmortem analysis the team published Friday. When the programmer logged into the klecker system, the attacker recorded his password.
Using the September flaw, the attacker gained owner privileges on Klecker. This is frequently referred to as "owning" the system. The flaw--in a part of the kernel that manages memory--allows only users that already have access to the system to raise their privileges. Such flaws are less critical than vulnerabilities that give an outside attacker access to a server and so are fixed less quickly.
The attacks have been the latest leveled at open-source software. In early November, an attacker attempted tothat would have created a flaw similar to the one that affected the Debian Project. A year ago, malicious attackers placed spyware into a . Several other known attacks have also been executed against other open-source projects.
The latest bug has been fixed in the most recent version of the Linux kernel, 2.4.23, and has also been patched in the next generation of Linux since 2.6.0-test6, which was released in late September.
Despite a two-month delay in releasing a patch, Ian Murdock, the founder of Debian and the chairman of Linux distribution maintenance provider Progeny, praised the project team.
"All in all, the way the Debian guys handled the situation has been admirable: They have been open with what they found out, and the speed at which they have found things out has been quite quick," he said. Murdock is a developer on the team but no longer has day-to-day administration duties.