Open-source security software maker Sourcefire has rejected a US$186 million unsolicited takeover offer from Barracuda Networks.

Late Thursday, Barracuda offered to pay US$7.50 per share, a 13 percent premium on the company's current stock price, but about half of what Sourcefire shares fetched a year ago. Barracuda, a maker of security appliances that include some of Sourcefire's open-source code, said that there was a synergy between the two companies.

In a Friday statement, Sourcefire flatly rejected that idea, saying that "the proposal is not in the best interests of Sourcefire and its stockholders" because it "substantially undervalues Sourcefire."

Sourcefire is best known for making the open-source Snort intrusion-detection software. Last year the Columbia, Maryland, company acquired ClamAV, a maker of open-source antivirus products.

Barracuda ships ClamAV with its appliances, and the code has been at the center of a high-profile legal dispute between Barracuda and competitor Trend Micro, which claims that ClamAV violates one of its patents.

Because it is already fighting a lawsuit with Trend Micro, Barracuda feels it is already addressing what could turn into a legal problem for Sourcefire, Barracuda President and CEO Dean Drako said in a Tuesday letter to Sourcefire's board of directors, which Barracuda made public Thursday.

"We also feel that the company's inaction in dealing with the looming threat of litigation from Trend Micro has had an effect on the stock price," he wrote.