Most SharePoint Installations Vulnerable to XSS Attacks

One of the vulnerabilities patched by Microsoft this week with its monthly security updates is a potentially serious cross-site scripting (XSS) flaw believed to affect most SharePoint 2016 installations.

Microsoft SharePoint is a collaboration platform that allows users to share and manage content and applications with other people in their organization. Microsoft says the product is used by more than 200,000 organizations and 190 million people.

The security hole, discovered by researcher Ashar Javed and tracked as CVE-2017-8514, affects SharePoint’s Follow feature, which allows users to follow sites that interest them.

Users can follow a site by clicking on the “Follow” button in the top right corner of a page. The feature is also available in the emails users receive when someone shares a site with them via SharePoint.

Javed noticed that the request associated with the “Follow” link included in emails has a parameter named “SiteName” whose value is not properly sanitized, allowing malicious actors to abuse it for XSS attacks.

In order to exploit the vulnerability, an attacker needs to convince the targeted user to click on a link that looks something like this, where the part after “SiteName=” is the XSS payload:

http[s]://<SHAREPOINT URL>?FollowSite=0&SiteName=’-confirm(document.domain)-‘

In its advisory for CVE-2017-8514, Microsoft said this reflected XSS flaw can be exploited to run a malicious script in the context of the current user.

“These attacks could allow the attacker to read content that the attacker is not authorized to read, use the victim’s identity to take actions on the SharePoint site on behalf of the victim, such as change permissions, delete content, steal sensitive information (such as browser cookies) and inject malicious content in the browser of the victim,” Microsoft said.

A Google search conducted by Javed showed more than 19,000 vulnerable SharePoint sites, including ones belonging to government organizations and universities from around the world. The expert believes nearly all SharePoint installations – both on-premises and online – are affected.

Microsoft was informed about this vulnerability on February 20 and confirmed it roughly one week later. The company has paid out a $2,500 reward for the security hole.

This is not the only SharePoint XSS flaw found by Javed and patched by Microsoft this month. The researcher also discovered a stored XSS vulnerability (CVE-2017-8551) affecting the Project Web App component of SharePoint 2013. Javed told SecurityWeek that he received a $1,500 bounty for the second XSS he reported.

Microsoft informed customers that this month’s security updates also include defense-in-depth measures for SharePoint Enterprise Server 2013 and 2016. These measures are designed to prevent remote code execution.

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