Is It True? Did Eddie Cibrian Cheat on LeAnn Rimes With Ex-Wife?

Cibrian may have stepped out on LeAnn, but supposedly she already knows about it

Seems like LeAnn Rimes might be the latest celeb in Hollywood to be on the receiving end of "what goes around comes around."

Friends of Brandi Glanville, ex-wife of Rimes' BF Eddie Cibrian, tell E! News that the Star magazine report of Cibrian sleeping with his estranged wife after he and Rimes were officially a couple are true.

Remember, Cibrian cheated on Glanville with his Northern Lights costar Rimes in July 2009. The couple filed for divorce that August and are still in the process of negotiating a settlement.

But there are a few things you may wanna know about Eddie supposedly stepping out on LeAnn...

First of all, Cibrian took to his Twitter today to vehemently shoot down the reports: "Once again my ex has stooped to a new low attempting to sabotage my beautiful relationship with LeAnne."

Second, a rep for Rimes call's bull on the story: "The Star magazine story is absolutely untrue and completely made up of lies fabricated by people who clearly have no lives of their own. LeAnn refuses to stoop to their level by acknowledging or responding to these ridiculous stories."

However, the source close to Brandi insists the hookup happened...just not recently. Supposedly the late-night rendezvous was right after E and L went public with their relationship, but after the divorce papers.

Sleazy, yes, but reportedly, LeAnn already knew about it, so she probably wasn't exactly stunned when she saw the cover of this week's Star.

Supposedly Cibrian was the one who came clean about his extracurricular activities to Rimes.

"LeAnn knew about it 100 percent ," says our source. "Obviously, she was upset. Eddie and Brandi were intimate after he and LeAnn were together, but it only happened twice" confesses the insider.

Our source adds that once Brandi began dating her current boyfriend, real-estate mogul Mike Meldman, she stopped hooking up with Cibrian.

Glanville and Meldman are still going strong says our source, and are currently on vacation in Idaho together.

"Eddie wanted her back, but she was not interested," says the source close to Brandi. "There is no way she would ever take him back."

 We put in a call to Cibrian's rep but have yet to hear back.

Either way, is it just us, or are guys the only ones who comes out of love triangles unscathed? Totally not fair.