Zscaler Inc. moved $100 million closer to its long-anticipated public offering this morning after announcing the completion of an oversubscribed round led by private equity giant TPG Capital. The investment is the latest sign of the venture capital community’s unprecedented interest in cloud security amid the accelerating migration of corporate data beyond the firewall.

That trend has helped more than half a dozen startups land funding over the past few quarters, many with similar value propositions to Zscaler.  Bitglass Inc. raised $25 million on occasion of its launch from stealth late last year for a proxy that watermarks files on their way outside the data center, while CipherCloud Inc. nabbed $50 million shortly thereafter for a rivaling security filter.

Zscaler applies the same approach to the outgoing browser traffic of an organization in order to protect users against malware and all the other threats that can pose a threat to corporate security on the open web. Its cloud-based proxy comes integrated with a firewall that enables administrators to centrally enforce policies on the packets passing through and monitor authorized activity.

That’s a pitch makers of on-premise security appliances have offered CIOs long before the rise of cloud computing, with one major difference. The fact that Zscaler’s technology is delivered as a service from a global network of data centers rather than through a local machine means that its reach can extend beyond a single office to the entire organization.

That encompasses not only the workplace but also the mobile devices on which employees perform more and more of their work, often outside the confines the corporate network. The sweeping administrator control afforded by Zscaler’s model has attracted more than 5,000 customers over its seven years in operation, including the likes of United Airlines, Inc. and the UK’s National Health Services.

The $100 million in additional funding gained through today’s round will help fend off all the emerging startups clamoring for those customers and fuel operations until the company is ready to hit the stock exchange. In addition to TPG, the round also saw the participation of existing backers Lightspeed Ventures and storage giant EMC Corp., a major player in enterprise security through encryption subsidiary RSA Software.

TPG partner Nehal Raj is joining Zscaler’s board as part of the investment, which brings its total raised to a hefty $138 million.

Photo via Geralt