Former HBGary Federal CEO Aaron Barr has backed out of a scheduled appearance at next weekend's  DEFCON hacker conference after attorneys from his former company threatened legal action.

Barr withdrew Wednesday from the Aug. 6 panel discussion after attorneys representing HBGary Federal threatened to file a legal injunction against him if he did not back out immediately, according to Kaspersky Lab's security blog, Threatpost.com, which first reported the news.

Barr was scheduled to participate in a presentation, titled “‘Whoever Fights Monsters...' Aaron Barr, Anonymous, and Ourselves," alongside Joshua Corman, research director at consultancy The 451 Group, and "Jericho" of Attrition.org.

The panel, scheduled to be moderated by Paul Roberts, editor for Threatpost.com, promises to offer a “heated” discussion about issues relating to recent attacks by hacktivist groups such as Anonymous.

In a legal written notice received on Wednesday, HBGary Federal lawyers informed Barr that they planned to file an injunction preventing him from speaking at the conference, citing his separation agreement with the company, according to Threatpost.com.

Corman on Wednesday called the situation “frustrating.”

“We actually got a few lawsuit threats in the past,” Corman told SCMagazineUS.com on Wednesday. “Apparently we already almost lost him once.”

Despite past legal threats, Barr was, up until today, planning to participate in the panel, Corman added. The session will move forward with or without Barr.

In addition, panelists are in discussion with several “viable options” to replace Barr, Corman said.

Tanya Forsheit, an attorney with The Information Law Group, who represents HBGary Federal, could not be reached for comment. 

DEFCON was to be the first time Barr has spoken publicly since Anonymous in February hacked into an HBGary Federal web server and used that foothold to crack into sister firm HBGary's email server to post tens of thousands of its emails online, the content of which revealed that HBGary Federal was engaged in shady, potentially illegal, activities.

Anonymous apparently orchestrated the attack after hearing Barr planned to release information about the activist collective during a talk at the Security B-Sides show, held in February in San Francisco. The talk was subsequently called off.

The leaked emails exposed that HBGary Federal and two other security firms were in negotiations with a major law firm, believed to represent Bank of America, to launch an offensive against the whistleblower site WikiLeaks and its supporters.

Barr subsequently stepped down following the attack.