Drupal 7.21 was released this week.
This is a maintenance release, so upgrading immediately is recommended but not absolutely essential.
However, this is a good opportunity to check up on all your sites.
Drupal 7 releases have been coming rapidly over the last few months. Drupal 7 had no releases in its first 5 months, but has now had 6 releases in the last 6 months. 4 of those releases contained security fixes.
We’ve moved rapidly from 7.16 in October to 7.21 in March, so please make sure that all your sites are up-to-date.
Important Drupal 7.21 Information
7.21 is designed to fix a bug with image uploads. To cut a long story short, Drupal 7.20 fixed a security flaw in the Drupal core Image module. However, the fix introduced incompatibilities with a number of contributed modules. To take one example, we’ve had several people report the issue in the comments on our Plupload multiple image upload tutorial.