SAN FRANCISCO (AFP) -- A nasty worm has wriggled into millions of computers and continues to spread, leaving security experts Tuesday wondering whether the attack is a harbinger of evil deeds to come. U.S. software protection firm F-Secure said a computer worm known as "Conficker" or "Downadup" had infected more than nine million computers by Tuesday and was spreading at a rate of one million machines daily. The malicious software had yet to do any noticeable damage, prompting debate as to whether it is impotent, waiting to detonate, or a test run by cybercriminals intent on profiting from the weakness in the future. "This is enormous; possibly the biggest virus we have ever seen," said software security specialist David Perry of Trend Micro. "I think the bad guys are field testing a new technology. If Conficker proves to work well, they could go out and sell malware (malicious software) to people. There is a huge market for selling criminal malware," he said. The worm, a self-replicating program, takes advantage of networks or computers that haven't kept up to date with security patches for Windows RPC Server Service. It can infect machines from the Internet or by hiding on USB memory sticks carrying data from one computer to another. Once in a computer, it digs deep, setting up defenses that make it hard to extract. Malware could be triggered to steal data or turn control of infected computers over to hackers amassing "zombie" machines into "botnet" armies. "Here we are with a big, big outbreak and they keep revamping their methodology to increase the size of it," Perry said. "They could be growing this huge botnet to slice it up and sell it on the criminal market." Microsoft said it is aware of the Conficker "worm family" and has modified its free Malicious Software Removal Tool to detect and get rid of infections. The U.S. software giant also advised people to stay up-to-date on anti-virus tools and Windows updates, and to protect computers and files with strong passwords. A troubling aspect of Conficker is it harnesses computing power of a botnet to crack passwords. Repeated "guesses" at passwords by a botnet have caused some computer users to be locked out of files or machines that automatically disable access after certain numbers of failed tries. "Downadup uses brute force from the infected network of botnets to break the password of the machine being attacked," Perry said. "That is something never seen before and I find it disturbing." Perry urges people to harden passwords by mixing in numbers, punctuation marks and upper-case letters. Doing so makes it millions of times harder for passwords to be deduced, according to Perry. "This is necessary in a world where malware hacks passwords," Perry said.