Big Bird, Oscar the Grouch and Elmo briefly were replaced on Sunday by guests not welcome on Sesame Street.
The YouTube channel for the long-running television series was back operating normally on Monday after hackers briefly gained access on Sunday to swap out educational videos with X-rated pornography.
The erotic content appeared on the channel for about 20 minutes before YouTube suspended the channel for violating its guidelines, Graham Cluley, senior technology consultant of security firm Sophos, said on his blog.
It's not clear how many explicit videos the hackers uploaded, but they also changed the description of the channel to ask: "Who doesn't love porn, kids?"
A spokesman for Google, which owns YouTube, could not immediately be reached for comment to explain what happened. But Cluley said he thinks those responsible for the channel "were sloppy with their password."
It's not uncommon for intruders to access social networking accounts that don't belong to them, but it is more common on Twitter than YouTube.
In a statement on its page, "Sesame Street" apologized.
"Our channel was temporarily compromised, but we have since restored our original line-up of the best classic 'Sesame Street' video clips," it said.
