An image resizing utility named timthumb is widely used by many WordPress themes and plugins. Some versions of this utility contain a security vulnerability that allows hackers to upload and execute arbitrary PHP code in your timthumb cache directory. Acunetix identified that your blog contains a vulnerable version of this utility. It's recommended to immediately upgrade to the latest version.
Upgrade to the latest version of timthumb.php. Version 1.34 is the first version that contains the fixed code.