LinkedIn Patches Clickjacking Flaw Exploitable via CSS

LinkedIn has disclosed the details of a recently patched clickjacking vulnerability that could have been exploited using CSS (Cascading Style Sheets) and style attributes.

The security hole was reported to the business-oriented social networking service by Ruben van Vreeland, CEO and co-founder of Netherlands-based security firm BitSensor. The flaw could have been exploited by an attacker to trick users into accessing potentially malicious websites.

LinkedIn’s publishing platform is designed to allow users to customize the look and feel of their blog posts by using CSS and some HTML tags. The HTML tags that can be used are limited to a few basic ones to prevent cross-site scripting (XSS) attacks.

However, Van Vreeland discovered that existing CSS classes could have been leveraged for malicious attacks. By using trusted CSS class selectors, an attacker could have made changes to the user interface (UI) in an effort to conduct UI redress, or clickjacking, attacks.

In an attack scenario described by LinkedIn, the attacker uses the li_style CSS style to force an element containing a link to stretch across the entire height and width of the page. By doing so, the victim would get redirected to an arbitrary website regardless of where they clicked on the page.

For instance, malicious actors could have leveraged this technique to trick users into visiting websites set up to serve malware or ones hosting phishing pages.

“We believe that this attack is applicable to many sites, as many allow members to create and share rich media content. This is an interesting technique that uses existing resources to facilitate UI-redressing attacks by chaining together CSS class selectors, and has similarities to Return Oriented Programming (ROP),” said LinkedIn information security engineer Jovon Itwaru.

LinkedIn has been running a private bug bounty program since October 2014 and as of June 2015 it had paid out a total of more than $65,000 to participants. The company doesn’t usually disclose the details of vulnerabilities reported by external researchers, but in the case of the issue reported by Van Vreeland it made an exception, arguing that the technique “is unique and exemplifies the creativity needed to produce high-quality research.”

As a result of his work, Van Vreeland has been invited to join LinkedIn’s private bug bounty program.

In mid-November, a researcher reported finding a persistent XSS vulnerability in LinkedIn’s Help Center, which the company patched within three hours after learning of its existence.

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