Networking giant Cisco Systems has taken legal action to stop a researcher talking about a flaw in its software.
Cisco software is used in many of the internet's routers
The company filed for a restraining order against Michael Lynn after he gave a presentation on the flaw at a security conference in Las Vegas.
Mr Lynn said the flaw in the Cisco software could expose the internet to a crippling attack.
The hole has already been fixed by software updates to Cisco routers, which are widely used for net traffic.
Black Hat talk
The legal filing was made jointly by Cisco and Mr Lynn's former employee, Internet Security Systems.
Cisco said it was trying to protect its intellectual property by stopping any further revelations about the software flaw.
Mr Lynn said he left his job so that he could deliver his presentation to the Black Hat conference of computer security professionals on Wednesday 27 July.
In his talk, he showed how an attacker could exploit a known vulnerability in Cisco's router software, explaining how this could severely affect the internet.
Cisco is aware of the flaw and patched it in recent software updates to its routers.
The company is the world's largest maker of equipment used to connect computers and power the internet.
Last year, the core software for its routers was stolen and released on the internet.