The acquisition is aimed at adding a cloud platform for delivery of malware protection and extending Sourcefire's real-time detection and prevention capabilities to the cloud.
According to Sourcefire, the deal enables the company to combine its network intrusion prevention system with reputation services, data loss prevention and forensics to provide comprehensive security.
"Immunet is a disruptive innovator in delivering cloud-based security solutions with over 750,000 users in 192 countries," said John Burris, chief executive of Sourcefire.
"This acquisition enables Sourcefire to accelerate our cloud initiatives and provides significant value to customers by protecting them against advanced client-side attacks," he said.
Immunet has historically been a freeware consumer product, but Sourcefire's goal is to build a premium enterprise version.
Sourcefire will retain all fulltime Immunet staff, including founders Oliver Friedrichs, Alfred Huger and Adam O'Donnell.
Friedrichs said the deal will allow Immunet to continue developing its technology with a large degree of autonomy, while exposing it to millions of Sourcefire's users.
"Sourcefire is a great match for us from a technological, cultural, and philosophical standpoint - that was one of the reasons why this acquisition made so much sense," he said in a blog post.
"They are a well-respected company with deep roots in security solid products such as Snort," he said.