Canonical, the London-based company that develops the free Ubuntu Linux open-source operating system and maintains the Ubuntu Forums, told tech blog Ars Technica that the passwords had been protected using a "salted" variant of the MD5 encryption algorithm.

Ars Technica writer Dan Goodin pointed out that while anything would be better than storing passwords in plaintext, MD5 is no longer considered secure, even when salted with random data to slow down brute-force decryption.

For its part, Canonical was quick to alert the Ubuntu Forums' users about the breach with a posting on the site's front page.