Cisco is warning users that nearly 80 of its routers are vulnerable to a hack tactic first revealed last week.

Dubbed "drive-by pharming" by Symantec and university researchers who first publicised the danger, the attack involves luring users to malicious sites where a device's default password is used to redirect them to bogus sites. Once they are at those sites, their identities could be stolen or malware could be force-fed to their computers.

In an advisory, Cisco listed 77 vulnerable routers in the lines sold to small offices, home offices, branch offices and telecommuters. The advisory recommended that users change the default username and password required to access the router's configuration settings and disable the device's HTTP server feature.

The paper, co-written by a Symantec researcher and two other researchers from Indiana University, urged a similar move by router owners.

"Owners of home routers who set a moderately secure password – one that is non-default and non-trivial to guess – are immune to router manipulation via JavaScript," the report said.

The researchers also argued that router makers should stop using blank or easy-to-guess passwords, such as "admin," and switch to the device's serial number. "This value, which is unique to each individual router, would comprise a very secure and unpredictable password," the report stated.