Allegedly the data contained therein is account ID, realm tme and realm IP. These non-random repeating patterns don't harbour passwords, user IPs or real names but some are still unhappy.
The reason the WoW community is still prickly over the issue is because no such measure is mentioned in the Terms of Service agreement.
It's believed to have been kept a secret in order to catch out would be NDA breakers but it seems it has been in place from as early as 2008. The author of the piece which detailed the discovery warns these watermarks "can be used by hackers to link alt. characters to accounts and target specific spam or scam attacks," they said.
It can also "used by Blizzard to track down private WoW servers." Sneaky. For more information on this secret watermarking check out the over at OwnedCore.
Report: Blizzard bury account IDs in screenshot watermarks
12 September 2012 | By Simon Priest