Tumblr this week released a "very important" security update for iOS users after identifying a breach that compromised their passwords.
The company has urged users to download the latest version of the Tumblr app, which is available in the Apple iTunes Store.
"If you've been using these apps [iPhone and iPad], you should also update your password on Tumblr and anywhere else you may have been using the same password," Derek Gottfrid, vice president of product, wrote in a blog post.
According to a Tumblr spokeswoman, the security vulnerability was brought to light on Tuesday.
"Please know that we take your security very seriously and are tremendously sorry for this lapse and inconvenience," Gottfrid said.
The company did not provide further details on the breach; there was no mention of it on its Twitter account. (So far this week, tweets include only a "Happy Peach Ice Cream Day!" message and a reminder that founder David Karp made an appearance on this week's Colbert Report.)
The breach comes just a week after Tumblr's latest mobile update, which boosted iOS search and discovery options, and rolled out a new user interface for Android users.
The popular blogging service was purchased by web giant Yahoo earlier this summer for $1.1 billion (£724 million). Since the acquisition, Yahoo has remained fairly hands-off, allowing Tumblr to continue operating as a separate business.