From the PHPBB Group:

We are sorry to announce the immediate release of phpBB 3.0.7-PL1 to address a security issue which was introduced in 3.0.7, unfortunately the issue wasn't noticed during testing and has only surfaced a week after the release of 3.0.7.

We promised working feeds for phpBB 3.0.7. Sadly, we were not able to deliver on that promise - a critical bug in the permission handling for feeds slipped past. To all people who already have updated to 3.0.7, it is of critical importance to update to 3.0.7-PL1. Otherwise, it is possible for users to bypass permission settings under the following circumstances:

- Feeds are enabled
- Any of the posts or topics feeds are enabled
- The unauthorised user - or one of the groups they are a member of - has forum permissions set on a private forum
- If you have excluded a forum from the list of forums that provide feeds, it is unaffected

The fix for the issue is a single line change inside of feed.php, line 525 has changed from:

$forum_ids = array_keys($auth->acl_getf('f_read'));

to:

$forum_ids = array_keys($auth->acl_getf('f_read', true));