Network security company Sourcefire is acquiring Immunet, a cloud-based anti-malware start-up, for $21 million in cash, the companies announced today.

The acquisition expands the cloud-based offerings for Sourcefire, creator of the open-source Snort intrusion detection technology.

Columbia, Md.-based Sourcefire said it will not lay off any of Immunet's full-time staff, which is based in Palo Alto, Calif.

Sourcefire paid $17 million at the closing of the deal and will pay $4 million during the next 18 months dependent on product delivery milestones, the companies said in a statement.

Immunet Chief Executive Oliver Friedrich co-founded SecurityFocus, which Symantec acquired in 2002, and Secure Networks, which McAfee bought in 1998.

The acquisition announcement comes on the heels of news yesterday that Dell is acquiring SecureWorks.