Estate agents and small businesses are being warned to take greater care over passwords to stop hackers breaking into their systems.
David Nicholls, managing director of Better-IT, says over 15 billion passwords are available on the dark web. Most of them are available for free, but bank details can be sold for up to £55.
The dark web is part of the internet that can't be reached by normal browsers. It's known for being somewhere that people buy and sell illegal items, as well as personal data.
Hackers are increasingly able to break into businesses' systems and download huge databases of data, with Twitter just one firm to suffer a high-profile hack recently.
"If your password is stolen and you use the same password across multiple systems, that’s when you can quickly get into really big problems," says Nicholls.
"If hackers can use stolen data to get into one system used by your business, they can sometimes leverage this to get into other systems."
"The worst thing they can do is access your email, as this lets them reset passwords and monitor all your communications," he adds.
Nicholls says agents can protect themselves by taking two steps. The first is to use a different randomly generated password for every login. The second is to use a password manager to keep track of all passwords.