Over 60,000 sites impacted by WordPress Anti-Spam Plugin Vulnerability

A WordPress anti-spam plugin with over 60,000 installations patched a PHP Object injection vulnerability emerged from improper sanitization of inputs. The vulnerability discovered in the anti-spam plugin allowed encoded input (base64 encoded) which can then trigger a type of vulnerability called PHP Object injection vulnerability.

A vulnerability was discovered in the popular Stop Spammers Security, Block Spam Users, Comments, Forms WordPress plugin. The purpose of the plugin is to stop spam in comments, forms, and sign-up registrations. It can stop spam bots and has the ability for users to input IP addresses to block.

WPScan website described the issue as: “The plugin passes base64 encoded user input to the unserialize() PHP function when CAPTCHA are used as second challenge, which could lead to PHP Object injection if a plugin installed on the blog has a suitable gadget chain…” The classification of the vulnerability is Insecure Deserialization.

The non-profit Open Web Application Security Project (OWASP) described it as “The impact of deserialization flaws cannot be overstated. These flaws can lead to remote code execution attacks, one of the most serious attacks possible.

OWASP also notes that exploiting this kind of vulnerability tends to be difficult. It said, “Exploitation of deserialization is somewhat difficult, as off the shelf exploits rarely work without changes or tweaks to the underlying exploit code.”