A researcher has earned $10,000 for finding a critical Yahoo! Mail vulnerability that could have been exploited simply by getting the targeted user to open a specially crafted email.
Nearly one year ago, Jouko Pynnönen of Finland-based software company Klikki Oy discovered a stored cross-site scripting (XSS) vulnerability in the web version of the Yahoo! Mail service that could have allowed an attacker to execute malicious JavaScript code hidden in emails.
The researcher said the flaw could have been used to forward the victim’s emails, change their account settings and even create an email worm that attached itself to all outgoing emails.
The vulnerability, which earned Pynnönen $10,000, existed due to Yahoo’s failure to properly sanitise code in HTML emails.
The expert recently decided to take another look at Yahoo’s Mail service and discovered a similar flaw, but this time it was related to code inserted when a certain type of content is attached to an email.
The “Share files from cloud providers” attachment option allows users to attach files from their cloud storage accounts, such as Dropbox or Google Drive. While analysing the code inserted into an email when this option was used, Pynnonen noticed some HTML attributes named data-* (e.g. data-URL, data-category, data-embed-URL). Further analysis showed that these attributes are also used when a YouTube video is added to an email.
The researcher found a way to abuse these attributes in YouTube video attachments to get arbitrary JavaScript code executed. There was no need for the user to click on a link or open an attachment – the code would get executed as soon as the email was opened.
Similar to the flaw discovered one year ago, this security hole could have been exploited to steal a user’s emails and create a worm that spreads by attaching itself to outgoing emails, the expert said.
Pynnönen told SecurityWeek that he did not get a chance to test the attack method using the Google Drive and Dropbox attachment options. Yahoo was informed about the vulnerability on November 12 and fixed it by the end of November.
Yahoo awarded the expert a $10,000 bounty for the flaw. The company reported in May that it had paid out more than $1.6 million since the launch of its public bug bounty program in 2013.
