Criminal hackers have found a way round the latest generation of online banking security devices given out by banks, the BBC has learned.
Experts say customers should follow banks' official advice, use up-to-date anti-virus software and be vigilant.
Spencer Kelly explains, with the help of some rather outlandish wigs, how malware changes and disguises itself to avoid detection by security software.
You can watch the full investigation on a special edition of BBC Click, BBC News Channel, Saturday 4 February 1130 GMT or
afterwards on iPlayer.
Hackers outwit online banking identity security systems
Follow the Click team on Twitter - @bbcclick