The fallout from a hacking attack on Gawker Media a week ago underscores a basic security risk online – using the same username and password for multiple sites is convenient, but costly.

After the attack on the publisher of such blogs as Gawker, Gizmodo and Jezebel exposed account information on as many as 1.4 million people, several unrelated companies had to freeze their accounts and force users to reset passwords.

Gawker Media itself didn’t have all that much sensitive information about its users. But the usernames and passwords obtained there could open doors to more valuable accounts elsewhere, including e-mail and banking.

Twitter, Google and Yahoo, among others, saw the potential damage and began resetting their passwords en masse, disrupting users as they tried to check their e-mail or post a tweet.

“It shows one of the fundamental problems with passwords - they get reused and shared across multiple sites,” said Jeff Burstein, a senior product manager with the Symantec security firm.

Despite repeated warnings from security companies not to do so, users tend to reuse passwords anyway because they can be hard to remember and manage. Users may have dozens, perhaps hundreds, of accounts – for e-mail, Facebook, Twitter, e-retailers, banks and the growing number of news websites and blogs requiring registration.

Although account information gets compromised all the time, the infiltration of Gawker’s servers is noteworthy because the hacked data were posted online, for free. In most other breaches, the stolen data are never made public, but sold underground to criminals.

Because the databases were freely available, other sites were able to score the data and look for matches with their users.

Twitter acknowledged resetting some passwords for its 175 million users after hackers used the Gawker data to break into Twitter accounts and pump out links to a site selling acai berry drinks.

At least two of the biggest web e-mail providers, Yahoo and Google, also reset some passwords. Neither would say how many of its users were affected. Google described it as a “small subset” of its users. Job-networking service LinkedIn also changed a small number of its 85 million users’ passwords.