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The full list of changes between the 6.15 and 6.16 releases can be found by reading the 6.16 release notes. The full list of changes between the 5.21 and 5.22 releases can be found by reading the 5.22 release notes.
Drupal 5.22 and 6.16 were released in response to the discovery of security vulnerabilities. Details can be found in the official security advisory:
To fix the security problem, you can either (1) upgrade Drupal or (2) patch Drupal.
More info: Drupal.org