Uh oh. BFFs Katy Perry and Rihanna may be on the outs -- and Chris Brown may be to blame.

Perry and RiRi were inseparable for years, sitting side by side at awards shows and helping one another through breakups. But sources are whispering that Katy is really not happy with Rihanna for reuniting with Brown, who infamously assaulted her in 2009, and that the friendship is over because of it. C'mon, RiRi -- chicks before ... you know.

A source close to the pals told The New York Daily News (via Monsters and Critics), "They're barely talking and ever since the couple reunited, there has been tension between them. Katy has been spending a lot of time with pals Robert Pattinson and Kristen Stewart. Katy fears Rihanna is making a huge mistake and doesn't want to be part of it."

This report conflicts with one from March, in which Perry seemed on board with the pairing. "Katy and Rihanna are best friends," a source said at the time. "Katy will support anything that Rihanna does. Rihanna has been there for Katy through everything and they want the other to be happy.”

Oof. The site also notes that Perry and Rihanna attended different Halloween parties this year and that Rihanna skipped out on Perry's birthday bash, despite the pair being in the same city at the time. In fairness to both parties, they're busy gals, so sometimes these things happen. However, while we're pretty wary of Rihanna and Breezy reuniting -- especially after the way he unceremoniously kicked Karrueche Tran to the curb -- we gotta say that Perry has quite a bit of nerve to criticize anyone's taste in men. This is a woman who thinks she can change John Mayer. Additionally, if Perry truly fears for Rihanna's safety and happiness, she should know it's not wise for RiRi to be isolated from others (aside from Brown) who care about her.

Here's hoping the friends can work it out -- because chances are a good BFF will outlast either one of their BFs.

Watch Rihanna + Katy Perry at the 2012 MTV Video Music Awards