MyBB is a free and open source forum software. The default list of disallowed remote hosts does not contain the 127.0.0.0/8 block, which may result in a Server-Side Request Forgery (SSRF) vulnerability. The Configuration Files Disallowed Remote Addresses list ($config[disallowed_remote_addresses]) contains the address 127.0.0.1, but does not include the complete block 127.0.0.0/8. MyBB 1.8.38 resolves this issue in default installations. Administrators of installed boards should update the existing configuration (inc/config.php) to include all addresses blocked by default. Additionally, users are advised to verify that it includes any other IPv4 addresses resolving to the server and other internal resources. Users unable to upgrade may manually add 127.0.0.0/8 to their disallowed address list.
The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete, leading to resultant weaknesses.