Many websites once used by US banks have been taken over by spammers and virus writers, reveals research.
The websites have become available after the banks owning them were bought, went bust or did not renew their rental of the domain name.
The cyber-thieves are trading on the good reputation of the sites to spread spam or influence search results.
A small number have been seeded with malware to infect the computers of any people who visit.
Almost 3,200 US banks have shut up shop over the last decade, according to the research by Prof Tyler Moore from the Southern Methodist University in Dallas and Dr Richard Clayton from the University of Cambridge.
About one-third of the domains the banks used, 1,030 sites, have now passed into the hands of people who are abusing their formerly good reputation, found the research.
Many of the sites are hosting adverts in a bid to make use of the small number of people who visit the sites even though many of the banks that once used them have been closed for years.
The two researchers found that a small number of the sites were being actively abused by cyber-thieves.
Some had been turned into fake online pharmacies, some to help influence the results that people see when they search for certain topics, some to promote porn sites, and others to help control networks of hijacked home computers.
A very small number, 11 sites in total, were found to be seeded with viruses.
The most "harmful" re-use of these banking sites was when they were overhauled to resemble a legitimate bank, said the researchers. Such sites could be used to lend legitimacy to phishing campaigns, they warned.
Banking regulators in the US should work harder to police these domains to ensure they were not put to "sinister" use, concluded the professors.