LastPass is a service that lets users store their usernames, passwords and form-fill data online. The service then automatically fills in the information when the user visits a site that requires the information. The company offers a free service as well as a fee-based service.

Such services are designed to let people create strong and unique passwords for each site they use without having to worry about remembering each one of them. Users tend to use the same passwords for multiple sites because of this worry.

With services such as LastPass, users need to only remember one master password for logging into the service.

In its blog post, LastPass noted that the possible compromise is unlikely to affect anyone with a "strong, non-dictionary"-based master password or pass phrase.

The reason that LastPass is requiring everyone to change their master password is because of the potential for the intruders to use brute-force methods to guess at weaker master passwords, the company noted. "Unfortunately, not everyone picks a master password that's immune to brute forcing."