The case hinges on Trend's patent for antivirus scanning at the network gateway, which was awarded in September 1995.
However, an appeal by Barracuda to the open source community has turned up a product which used the technique in January 1995 and had hundreds of paying customers.
Swedish developer Goran Fransson has testified in the case that his then employer, TenFour Sweden, had already developed gateway antivirus scanning and produced sales logs, software and instruction manuals to prove it.
"If there is anything I can do to establish the truth in the matter, I'd be happy to do so," Fransson told Linux.com.
"I think it's time for the security industry to come together in common causes and stop fighting each other. That's why I'm glad to see companies like Barracuda trying to do something good for the internet."
Fransson also found that 380 companies in the US, where the patent was issued, were registered users of the TenFour software before the issue date.
"It's ridiculous to say that Trend Micro were the first ones [with virus scanning]," he said.
Trend brought its case against Barracuda after the company open sourced the ClamAV antivirus engine and made it free to all users.
Trend already collects royalty fees from other security companies such as Symantec and McAfee based on ownership of the patent, but Barracuda decided to fight the case in the courts.
"Trend Micro's actions illustrate that ClamAV and other open source projects remain vulnerable to commercial patent holders attempting to unjustly hinder the free and open source community," said Dean Drako, president of Barracuda Networks.
"Trend Micro appears to be seeking an interpretation of its '600 patent such that it would have exclusive control of gateway antivirus scanning.
"Scanning for viruses at the gateway is an obvious and common technique that is used by most businesses worldwide."
Breakthrough in Trend Micro/Barracuda showdown
By Iain Thomson on Jun 26, 2008 7:40AM