BetterLinks < 3.1.1 - Missing Authorization to Authenticated (Subscriber+) Arbitrary Short URL Creation via create_fbs_link AJAX Action

Description

The BetterLinks – Link Shortener, Link Cloaking, Redirects, Affiliate Link Manager & MCP plugin for WordPress is vulnerable to authorization bypass in all versions up to, and including, 3.1.0. This is due to the plugin not properly verifying that a user is authorized to perform an action. This makes it possible for authenticated attackers, with subscriber-level access and above, to create arbitrary BetterLinks short URLs with attacker-controlled slugs and redirect destinations, enabling phishing and SEO abuse. Exploitation requires the Fluent Boards companion plugin to be installed and active (so that the FLUENT_BOARDS constant is defined), and is further facilitated by the betterlinks_admin_nonce nonce being emitted on every frontend page via wp_localize_script, making it accessible to any authenticated user.