Victims of malware infection often have little chance of researching what has hit them using search engine results, security company Prevx has discovered.

The company analysed 250,000 malicious filenames from a total database of 30 million listed by search engines during 2006, and found that a growing number were using clever file-naming techniques to avoid easy search detection.

Tactics included mimicking common filenames to avoid searchers noticing them, using customised filenames that changed after specified numbers of PC infections, and exploiting the time it can take search engines to update, commonly reckoned to be over a week in some cases.

Even filenames that could successfully be found using search engines could take anything from two to fifteen days to be indexed, a lag that offered malware a vital infection window.

One example of deliberate obfuscation given by the company is an executable file associated with the Backdoor.Win32.IRCBot.BV data-stealing Trojan from last summer. Search engines were unable to find the file because it had been named ‘.EXE’, an extension so common it returned a reported 176 million name matches on Google. Another had used the same technique, this time for the equally common '.DLL’.

Self-replicating worms such as Trojan.URDVXC were able to generate as many as 1,000 random filenames on a single PC, within seconds of infection, the company said. Prevx had logged 350,000 filenames for this Trojan alone within its first 24 hours. Samples of these can be found on the company’s website.

“If a user searches for a suspect file name on Google or Yahoo! and nothing is found, then the assumption is that the file is probably nothing to worry about,” said Mel Morris, CEO of Prevx.

“Such is the power of search today. Sadly, users must be more vigilant, a blank result on a top search engine is more likely to point to it being malicious.”

Even expert users would find tracking down the origins of suspicious files on their system to be almost impossible at times.

Prevx’s Prevx1 anti-malware tool was reviewed by Techworld last year.