Yahoo appears to have been the victim of a security breach that yielded more than 400,000 login credentials stored in plain text, CNET reports.

The hacked data, posted to hacker site D33D Company, contained more than 453,000 login credentials and appears to have originated from the Web pioneer's network. The hackers, which said they used a union-based SQL injection technique to penetrate the Yahoo subdomain, said the data dump was intended to be a "wake-up call."

"We hope that the parties responsible for managing the security of this subdomain will take this as a wake-up call, and not as a threat," the hackers said in a comment at the bottom of the data. "There have been many security holes exploited in webservers belonging to Yahoo! Inc. that have caused far greater damage than our disclosure. Please do not take them lightly. The subdomain and vulnerable parameters have not been posted to avoid further damage."

The hacked subdomain appears to belong to Yahoo Voices, according to a TrustedSec report. Hackers apparently neglected to remove the host name from the data. That host name - dbb1.ac.bf1.yahoo.com - appears to be associated with the Yahoo Voices platform, which was formerly known as Associated Content.