Web giant Yahoo has confirmed it is investigating a breach of its system which may have exposed 400,000 user IDs.
US security firm Trustedsec said the attack appeared to have originated from servers connected to Yahoo Voices, a user-generated section of the site.
It said that hacking group D33DS had claimed to be behind the attack.
Hours after the attack came to light, Yahoo had not put a warning on its site.
In a statement Yahoo said: “We are currently investigating the claims of a compromise of Yahoo! user IDs,” adding that it encouraged users to “change their passwords on a regular basis”.
She said it was unclear which part of the network was affected. Initially a Yahoo spokesman told the BBC that the problem had originated at Yahoo Voice, its IP telephony service.
The document which gives details of the hack does not make clear which Yahoo service was targeted.
According to US security firm Trustedsec, the compromised Yahoo passwords were associated with a variety of email addresses including those from yahoo.com, gmail.com and aol.com.
It said that hackers used a well-established technique known as SQL injection to extract the sensitive information from the database.
“The most alarming part of the entire story was the fact that the passwords were stored entirely unencrypted,” the security firm said in its blog.
Meanwhile social network Formspring has disabled nearly 30 million passwords following a separate attack.
It said it was a precautionary move after 420,000 passwords showed up on a security forum.
Formspring, which launched in 2009 as a crowd-powered question-and-answer site, has asked users to reset their passwords.
In a blog post it confirmed that a breach had occurred after someone hacked into one of the San Francisco-based company’s servers.
A spokeswoman said it had been alerted on Monday that some 420,000 encrypted passwords had shown up on a security forum which she refused to name because she did not want to draw attention to it.
Encrypted passwords aren’t immediately useable, although they can sometimes be decoded by a clever attacker.
Source: BBC NEWS