Security experts were befuddled Thursday as to why a company as large as Yahoo would fail to cryptographically store the passwords in its database. Instead, they were left in plain text, which means a hacker could easily read them.

"It is definitely poor security," Marcus Carey, a security researcher at Rapid7, said. "It's not even security 101. It's basic application development 101."

Yahoo declined a request for an interview, and only emailed a statement confirming the breach that occurred Wednesday. The company said that an "older file" containing roughly 450,000 user names and passwords was stolen from , a subset of Yahoo's massive network of Web sites.

Membership in the Contributor Network consists of freelance journalists who write content for Yahoo Voices. The network was established following Yahoo's 2010 acquisition of Associated Content.

Less than 5 percent of the stolen data had valid passwords, Yahoo said. "We are taking immediate action by fixing the vulnerability that led to the disclosure of this data, changing the passwords of the affected Yahoo! users and notifying the companies whose users accounts may have been compromised," the statement said.