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Stuxnet worm can reinfect PCs even after disinfection

Beware of poisoned Step7 files

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A security researcher has found yet another way the Stuxnet worm infiltrates computers used in nuclear plants and other industrial facilities, a technique that has the ability to reinfect machines even after they've been cleaned of the malware.

Stuxnet has already proven itself as one of the most sophisticated pieces of known malware ever. Its ability to target four vulnerabilities that until recently were unknown and unpatched allowed it to spread through USB sticks, Windows file shares, and other vectors. The worm is especially adept at targeting industrial-control applications developed by German software maker Siemens, allowing it to act as a guided missile of sorts that sabotages plants that meet very specific criteria.

Now, a researcher from anti-virus provider Symantec has discovered that Stuxnet also spreads by targeting files that administrators use to configure Siemens software. If present on a targeted Windows PC, the so-called Step7 files are automatically poisoned as soon as it is infected, Symantec's Nicolas Falliere wrote on Monday. When the files are opened later, they touch off a new round of infections.

“Stuxnet's ability to infect project files and run when they are opened is yet another propagation vector to add to the list,” Falliere said. “While we advise operators and programmers to be wary of project files from untrusted sources — internet forums, for instance — the most likely source of infection is likely to be a trusted party whose systems have been compromised by the threat.”

The technique can be especially effective in environments where Step7 files are located on a central computer and then copied to and executed on other machines. If Stuxnet is able to take hold of that central server, it can then infect local machines that are downstream from it.

Falliere also warned that the technique potentially allows Stuxnet to come back from the dead.

“Infected projects restored from backups may reintroduce the infection to previously cleaned machines so administrators should exercise caution when restoring files in this manner.”

His report is here. ®

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