While investigating an entirely separate issue with one of my WordPress installations, I came across this:
“Beginning with WordPress Version 2.6, three (3) security keys, AUTH_KEY, SECURE_AUTH_KEY, and LOGGED_IN_KEY, are used to insure better encryption of information stored in the user’s cookies. Beginning with Version 2.7 a fourth key, NONCE_KEY, was added to this group.
If you don’t find the keys in your
wp-config.phpfile, add the keys definitions with reference to Editing wp-config.php – Security Keys, and upload to your server.”
Step 13: Add security key definitions to the wp-config.php file
WP Codex
Since most of my WordPress installations are pretty ancient, and upgrading them has long since become a routine job, this was the first time I heard about this feature. So my blogs have been lacking in this security feature, until now (I set up the aforementioned keys in each of my wp-config.php files).