Monday, March 13, 2006

McAfee AntiVirus false positives - older, "reliable" signatures pose risk too

False positives are the bane of AntiVirus and IDS/IPS systems. On the one hand, hundreds and even thousands of new threats are released each week, where they must be discovered, submitted to vendors, analyzed by vendors, definitions, signature files or heuristic algorithms must be tweaked, tested, released to customers, and finally deployed to customer systems. All of this must be done in as short a time as possible, since the threats often spread in minutes and hours. AntiVirus signatures are often available within two days from the first appearance of a threat on the network. Polymorphic techniques, even simple ones like automatically generating dozens or more variants at the threat's compile time, are becoming more common making it more difficult for AntiVirus vendors to keep up with the expanding threat pool every year. Today we learned that an error in a signature file caused the McAfee AntiVirus system to delete good files from production systems. This unfortunate accident affected at least a hundred of their customers and probably thousands of PC systems. The final tally of affected systems probably won't be announced. (A similar problem recently caused Microsoft AntiSpyware to zap Symantec AntiVirus from systems.) This incident is receiving more press attention than they usually do. The real wonder is that things like this don't happen more often. McAfee update exterminates Excel
Such problems with security software are called false positives and they happen occasionally. McAfee typically has to do an emergency release of a virus definition file once every three months because of a false positive issue, Telafici said. "This is our once for the quarter I think," he said.
Similar rates of false positives are probably seen from other vendors, but this might be the first time that an AntiVirus vendor publicly disclosed information about their false positive rate. Not every customer is affected by every false positive. Many affect 3rd party applications which were previously unknown to the AntiVirus vendor. In cases like these, a DLL from a valid production software system accidentally matches a signature file developed by the AntiVirus vendor, who doesn't have the system to test against. Tracking down these problems sometimes includes a finger-pointing exercise between the AntiVirus vendor and the 3rd party application vendor -- the AntiVirus companies sometimes uncover viruses in shipping code, too, and it may be difficult to tell where the problem lies at first. McAfee update exterminates Excel
However, this time around it was a particularly big goof, because the company faulted Excel, Telafici admitted. "Usually, it is either custom applications or applications that did not exist at the time we wrote the signature file," he said.
That bit is particularly interesting. The implication is that after the initial creation and testing, a given signature may not be tested as thoroughly or as often down the line. Several months later, an update to your application software might cause a signature file to break, causing catastrophic damage. In retrospect it makes some sense, as full-on testing of this stuff takes time and resources, and the pressure to test and ship the newest definition or signature files is quite high. However, this revelation probably indicates that the ongoing risks from signature or heuristic approaches may be somewhat higher than previously thought. With the number of threats multiplying every year, and with the number of signature files which require testing increasing concomitantly, older signatures which have been "thoroughly tested and validated in the customer environment" may no longer be assumed to be benign beyond doubt. The current McAfee false positive incident is discussed here: McAfee Anti-Virus Causes Widespread File Damage [Slashdot] Excel = Virus ... At Least to McAfee [RealTechNews]

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