EXCLUSIVE

Kristen Stewart and Stella Maxwell have lawyered up, demanding that several websites take down nude photos of them that were illegally hacked.

Kristen's lawyer, Scott Whitehead, fired off a letter challenging the websites that have plastered pics of Kristen and Stella, along with pics of Tiger Woods, Lindsey Vonn, Miley Cyrus, and Katharine McPhee.

The pics in question show Stella in the bathroom, taking a selfie with Kristen posing naked behind her.

Whitehead claims posting the pics is a flagrant violation of copyright laws ... i.e., Kristen and Stella own their images and no one can use them without permission.

The letter follows a similar demand by Tiger Woods' lawyer. As we reported, the hacker got hold of nude selfies of Lindsey Vonn and in the set there was a naked pic of Tiger that he sent while he was dating her.