More than 170,000 users of online forums operated by cybersecurity company Comodo Group reportedly had their data stolen by a malicious actor who exploited a recently disclosed vulnerability in vBulletin’s internet forum software.

The Clifton, N.J.-based Comodo learned of the attack on September 29, and responded by taking its forums offline and applying patches, the company announced yesterday on its main Comodo forum.

Although Comodo said that its forums database was only “potentially” accessed, BleepingComputer reported today that an unnamed website is selling a data dump containing information pertaining to over 170,000 Comodo Forums users. The seller reportedly claims the data was lifted from Comodo’s main discussion website, and that the passwords contained within were hashed with an insecure MD5 algorithm. (Comodo stated in its security notice that all user passwords are stored encrypted, but the algorithm was not specified.)

There are roughly 245,000 registered Comodo forum accounts. Although the primary forum actually runs on Simple Machine Forum software, Comodo uses vBulletin on a second forum called ITarian, where users discuss product updates. BleepingComputer theorizes that the attackers first breached the ITarian via the vBulletin flaw and then used that as a jumping-off point to compromise the other forum, possibly by stealing credentials.

Depending on the user, affected data includes forum usernames, names, e-mail addresses, IP addresses from the most recent login, social media usernames, passwords and corresponding salt, security questions and hashed security answers, registration dates, messenger usernames and total time logged in.

Comodo has recommended that forum users change their passwords.

Last week, a researcher used vBulletin’s Full Disclosure security mailing list to publish a remote code execution exploit for a critical zero-day bug he had found in the forum software. The vBulletin team patched the flaw on Sept. 25.