Tumblr has been removed from Apple's app store because it let some users post images of child sexual abuse.
The social network's app was removed on 16 November but the reason for it being unavailable has only just come to light.
In a statement it said the illegal images got through because its filters failed to spot them.
It said getting the app re-listed was a "priority" but has given no date for when it might be available.
Tumblr gave more details about why it was taken off Apple's store after being approached by tech news site Cnet with a claim that indecent images of children were the cause.
In its statement, it said that all images uploaded to Tumblr pass through a database of "known child sexual abuse material". It added that any matches to the database are detected and deleted - and would never appear on its platform.
In this case, it said, the images being uploaded were not in the industry database so its filters did not catch them. However, it said, the illegal content was discovered during a routine audit.
"We immediately removed this content," it said, and added: "Content safeguards are a challenging aspect of operating scaled platforms."
Tumblr has a reputation for allowing sexually-themed material to be shared on its service. This led to it being banned for a day in Indonesia over the mature content. South Korea has also asked it to do a better job of moderating adult content on the service.