Warning: Tumblr announced an important security update for iPhone/iPad users.

Announced yesterday, this Important security update , is intended for iPhone and iPad apps.

 According to derekg, it addresses an issue that allowed passwords
to be compromised in certain circumstances ("Sniffed" in transit on certain versions of the app).

Therefore, if you’ve been using these apps, Tumblr, recommends you to update your password on Tumblr and anywhere else you may have been using the same password. 
You can download the new version of Tumblr for iOS here!