A security researcher says he bagged $10k after discovering and reporting a serious flaw in Yahoo! Mail that could have been exploited by crooks to read victims' messages.

Jouko Pynnönen says he reported the vulnerability in Yahoo! Mail via bug-bounty organizers HackerOne. "The impact of the bug is similar to the one I reported last year, which also brought a $10k bounty," Pynnönen told El Reg. "It allowed an attacker to, for example, read a victim's email."

The flaw – fixed in production late last month – could be exploited simply by tricking your target into opening a booby-trapped mail. The same vulnerability could also be abuse to spread malware, as a blog post by Pynnönen explains:

The flaw allowed an attacker to read a victim's email or create a virus infecting Yahoo Mail accounts, among other things. The attack required the victim to view an email sent by the attacker. No further interaction (such as clicking on a link or opening an attachment) was required.

The root cause of the problem was a failure to sanitize user-supplied values in dynamic content. It's pretty similar to the stored cross-site scripting vulnerability Pynnönen discovered in the webmail service last year, we're told.

For this latest programming blunder, Pynnönen supplied proof-of-concept exploit code to Yahoo!'s security team in the form of an email that, when viewed, would use AJAX to read the user's inbox contents and send it to an attacker's server.

A spokesperson for Yahoo! was not available for immediate comment. ®