United States citizens should know that their social media accounts are off limits to Customs and Border Protection agents.

The rules surrounding checks of phones and social data has been murky at best, and still would be if it weren't for a letter authored by CBP acting commissioner Kevin McAleenan that NBC recently obtained and published.

"...border searches conducted by CBP do not extend to information that is located solely on remote servers," McAleenan wrote. The acting commissioner also wrote that this limitation was made clear to CBP officials in guidelines issued in April of this year.

President Donald Trump's administration has pushed for more stringent social media vetting, so the timing of this announcement seems a little strange until you realize it wasn't an announcement. McAleenan didn't plan to publicize the guidelines, but he did write the letter to Oregon Sen. Ron Wyden after the senator sent a list of questions to the the Department of Homeland Security in February. The DHS oversees the CBP, and Wyden inquired about CBP agents searching the phones and social data of U.S. citizens.

Though the admission is revelatory, McAleenan didn't provide data on just how many of those phones have been searched.

And CBP can still search a phone for whatever's stored on the actual device, which means texts and photos. The phone's owner doesn't have to give an agent the password, but that comes with the risk of having their phone taken to some black hole of government.

For non-U.S. citizens visiting the country, anything accessible on a phone appears to be "fair" game.