Some iPhone users are getting unpleasant surprises from a sharing feature they may not realize allows anyone to send pictures to their phones.

According to the New York Post, female subway riders in New York received messages alerting them that someone wanted to share a picture with them over AirDrop. The feature is designed to make it easy to share photos and files on your iPhone and Mac with anyone in your immediate vicinity. If the women accepted the request, they saw an unsolicited picture of a man's private parts.

"It never even crossed my mind that someone may use it to send stuff like that," Britta Carlson, who received a picture, told the New York Post.

New York City's Metropolitan Transit Authority did not immediately respond to a request for comment on the incidents.

AirDrop, which uses WiFi and Bluetooth, is unique because it doesn't require you to share your number or a username with someone to send or receive a file. People turn on AirDrop for work to quickly share files, and teens have used it to share photos and get around social media bans in class, Business Insider reported. Some people have also used AirDrop to send funny pictures to unsuspecting strangers.

For those who don't want any surprises, however, there are some steps that you can take to better control the feature.

To change your AirDrop settings, head into the Control Center - the menu that appears when you swipe up from the bottom of your screen. Just look for "AirDrop," and it should tell you what your settings are. Tap the icon to turn off receiving completely, or you can choose to receive files from just your contacts or from everyone. (Those using Apple's iOS 11 in beta can change these settings from the "General" menu of the settings app or by using 3D Touch on the Control Center panel that includes the switches for airplane mode and WiFi.)

This is not the first time that people, particularly women, have been harassed by others using the feature. One woman reported a similar incident on the London Underground in 2015, when such a request popped up on her phone along with a preview image, meaning she didn't even have to accept the request to find herself digitally flashed.

AirDrop harassment has also taken other forms. In June, writer Renee Bracey Sherman said that someone had surprised her by sending her an image of Pepe, the much-memed cartoon frog that's become an alt-right symbol, at Reagan National Airport in Washington, D.C.

"It was quite unsettling and jarring to not only see this racist meme, but it was scary that it was showing up on my computer," Bracey Sherman told the Daily Dot.

Security researchers have also raised concerns that AirDrop could be used to spread computer viruses, though Apple addressed those concerns with the release of iOS 9.

Just remember that if you want to keep the feature turned on it's generally not a good idea to accept files from people you don't know.