A bug bounty hunter has earned $7,000 from Yahoo for finding vulnerabilities in the company’s image and video hosting service Flickr.
Michael Reizelman, aka mishre, said he uncovered three minor flaws that could have been chained together to take control of Flickr accounts.
According to the researcher, when users log in to their Flickr.com account, they are redirected to the login.yahoo.com domain, where their credentials are entered and verified. If the credentials are valid, the user is taken back to Flickr.com and authenticated. The redirect to login.yahoo.com also occurs if the user is already logged in, but it takes place in the background.
The request to login.yahoo.com is used to obtain an access token for the user. Reizelman noticed that a parameter named .done, which controls where the login token is sent, could have been manipulated. However, Yahoo made sure that the token could only be sent to the flickr.com domain.
While finding an open redirect vulnerability on flickr.com could have allowed him to exploit the vulnerability, the expert could not find such weaknesses. He did however discover another method that involved embedding an image from an attacker-controlled server into a Flickr.com page using the <img> tag.
Yahoo had been manipulating this type of code to prevent abuse, but the expert discovered a bypass method.
Reizelman noticed that he could embed an external image into comments posted on flickr.com/help/forum/en-us/, which did not have a Content Security Policy (CSP). By pointing the .done parameter to a malicious image embedded into a post on the Flickr help forum, an attacker could have ensured that the access tokens were sent to their server.
An attacker simply needed to trick the targeted user into clicking on a specially crafted link. Once the token was in their possession, they could have easily accessed the victim’s Flickr account.
The researcher reported the vulnerabilities to Yahoo on April 2 via the tech giant’s HackerOne bug bounty program. The vendor addressed the issue roughly one week later and awarded the expert a $7,000 bounty.
Yahoo addressed the security hole by only allowing the .done parameter to point to flickr.com/signin/yahoo, adding CSP to the Flickr forum, and ensuring that the expert’s image embedding bypass method no longer works.
Reizelman has also earned bounties for reporting vulnerabilities to Uber, GitHub, Dropbox, Twitter, Google, Slack, Badoo and Imgur.
Related: Yahoo Pays Out $10,000 Bounty for Critical Mail Flaw
Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.
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