The Friends plugin for WordPress is vulnerable to PHP Object Injection in version 3.5.1 via deserialization of untrusted input of the query_vars parameter This makes it possible for authenticated attackers, with subscriber-level access and above, to inject a PHP Object. No known POP chain is present in the vulnerable software, which means this vulnerability has no impact unless another plugin or theme containing a POP chain is installed on the site. If a POP chain is present via an additional plugin or theme installed on the target system, it may allow the attacker to perform actions like delete arbitrary files, retrieve sensitive data, or execute code depending on the POP chain present. This requires access to the sites SALT_NONCE and and SALT_KEY to exploit.
The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.