wp-admin/options.php in WordPress MU before 1.3.2, and WordPress 2.3.2 and earlier, does not properly validate requests to update an option, which allows remote authenticated users with manage_options and upload_files capabilities to execute arbitrary code by uploading a PHP script and adding this script's pathname to active_plugins.
Improper Input Validation The product receives input or data, but it does
not validate or incorrectly validates that the input has the
properties that are required to process the data safely and
correctly.