Unvalidated input and lack of output encoding within the Related Posts for WordPress plugin before 2.0.4 lead to a Reflected Cross-Site Scripting (XSS) vulnerability within the 'lang' GET parameter while editing a post, triggered when users with the capability of editing posts access a malicious URL.
Cross-Site Scripting (XSS)
An adversary embeds malicious scripts in content that will be served to web browsers. The goal of the attack is for the target software, the client-side browser, to execute the script with the users' privilege level. An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute code and scripts. Web browsers, for example, have some simple security controls in place, but if a remote attacker is allowed to execute scripts (through injecting them in to user-generated content like bulletin boards) then these controls may be bypassed. Further, these attacks are very difficult for an end user to detect.