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A new type of spam is emerging on popular photo-sharing app Snapchat: porn bots.

Snapchat users are increasingly receiving spam messages with nude photos, according to security researchers at Symantec. The spam messages are automated, but they are designed to look like they are coming from a real person. The nude photos don't appear until a Snapchat user accepts a porn bot as a friend, but that would be an easy mistake for some to make.

The spam messages appear with photo of a nude woman. A caption below asks the recipient to add "her" as a friend on messaging app Kik to receive more nude pictures. If the user follows through with the request, he or she will be prompted to download a mobile application. The mobile applications vary -- many are games. The porn bot then promises more nude pictures in exchange for a screenshot proving the game was installed.

"The idea of using naked pictures or a secret admirer -- that technique has been around forever," said Kevin Haley, director of Symantec's security response team. "How it's going to be used in this particular case is specific to this application, so we're seeing a new twist on an old trick."

Related: What is Snapchat?

Haley says the mobile app developers aren't likely behind the scam. Instead, the perpetrators are the firms the app makers hire to help increase downloads.

"They're getting paid to get users to download this application," Haley speculates. "The vendor of the application doesn't really care how they do it, so they can come up with some creative ways... in essence they fool users or trick them into downloading this application and now the mobile application vendor has a lot of new users."

It's a common tactic, according to Tom Ritter, a researcher at security company iSEC. Though some are aware of the tactics, many app makers don't even know they're partnering with spammers.

"The shady people get a commission for each install they generate, and they turn to spam to do it," he said.

The spam problem is just the latest on a long list of Snapchat's growing pains. The app has come under fire for a security flaw that exposed usernames and phone numbers. Last year, security researchers also found ways for hackers to recover supposedly deleted photos.

But Snapchat usage has exploded. It turned down a reported $3 billion dollar buyout offer from Facebook(FB).

Apps like Snapchat that quickly grow in popularity are often in a rush to gain market share without having proper security in place, Haley says.

"Part of their growing pains will be to experience this kind of thing," he said. "Whatever the next application that has a lot of users you're going to have a lot of people trying to figure out how to take advantage."

CNNMoney (New York) First published January 16, 2014: 12:00 PM ET