The vulnerability exists due to failure in the "/_layouts/help.aspx" script to properly sanitize user-supplied input in "cid0" variable. Successful exploitation of this vulnerability could result in a compromise of the application, theft of cookie-based authentication credentials, disclosure or modification of sensitive data.
An attacker can use browser to exploit this vulnerability.
On Twitter, Microsoft said it was aware of the issue and promised to issue guidance for affected customers:
This isn't Microsoft SharePoint's first brush with XSS security problems. Back in 2007, the company shipped an "important" security patch to fix a flaw that allowed attackers to run arbitrary script that could result in elevation of privilege within a SharePoint site.