The gen_rand_string function in phpBB 2.0.19 uses insufficiently random data (small value space) to create the activation key ("validation ID") that is sent by e-mail when establishing a password, which makes it easier for remote attackers to obtain the key and modify passwords for existing accounts or create new accounts.
phpBB 2.0.11, and possibly other versions, with remote avatars and avatar uploading enabled, allows local users to read arbitrary files by providing both a local and remote location for an avatar, then modifying the "Upload Avatar from a URL:" field to reference the target file.