Gumtree classifieds site leaked personal info via the F12 key

British classifieds site Gumtree.com suffered a data leak after a security researcher revealed that he could access sensitive personally identifiable data of advertisers simply by pressing F12 on the keyboard.

When pressing the F12 key in a web browser, the application will open the developer tools console, which allows you to view a website's source code, monitor network requests, and view error messages produced by the website.

It is considered a primary security measure to make sensitive data not publicly viewable when using a website, even if you view its source code.

However, Pen Test Partners security researcher Alan Monie discovered that he could see the PII of sellers simply by viewing the HTML source code of the advertising shown on Gumtree's website.

"The site was super leaky. Every advert on the site included the seller's postcode or GPS coordinates – even if the seller requested the map of their location to be hidden. It leaked the sellers email address, and their full name was available via a simple IDOR vulnerability," explained a report by Monie.

Gumtree is one of the top 30 websites in the UK, receiving many millions of unique visitors every month. As such, this leak may have impacted a large number of advertisers on the site.

Monie found that the HTML source was leaking the following information for registered advertisers:

• full name
• username
• account registration date
• account type
• email address
• postcode or GPS coordinates

The consequences of having such data exposed are significant, as the leaked users could be targeted by phishing or social engineering attacks that use this information to try and harvest more sensitive information.

The site also features an API exclusively used by the Gumtree app on iOS. Unfortunately, one of that API's endpoints was vulnerable to an IDOR (insecure direct object references) attack, resulting in another leak of full names and other account info.

Upon finding this problem on November 11, 2021, Monie informed Gumtree of the issue, which partially fixed the problem on November 16, 2021. After multiple subsequent messages by the researcher, the platform addressed all problems on December 06, 2021.

As such, sellers on Gumtree had their PII exposed for almost a month, if not longer.

Bleeping Computer has reached out to Gumtree asking for a comment on what action has been taken concerning the incident, and we received the following response from a spokesperson.

“We were made aware by a user of a security issue affecting our website source code in November 2021. This was resolved within hours of it being brought to our attention. After becoming aware of the above, we were subsequently notified of a further issue with our API for iOS devices. This has also been resolved.

“In response to these issues, we reported the incident to the Information Commissioner’s Office (ICO) outlining our actions already taken, and planned, to monitor the issue. These included fixing the vulnerabilities, updating our safety messaging on site and mitigating against future issues.”

“We did not notify our users and are confident that our response to the reported issues was timely, appropriate and proportionate. We have communicated proactively with the regulator as these issues came to light and as we were taking remedial actions. We will take any appropriate further action should that be required.”

Even though it is possible that the researcher was the only person to have discovered this elementary data leak flaw, we would advise Gumtree users to remain vigilant and treat all incoming communications with caution.