Rein Security has raised $25 million in a Series A, bringing its total funding to $35 million. Glilot Capital and Sienna Venture Capital co-led the round, with Corner Ventures, Atlacle and RNP Capital Advisors also participating.
The startup, founded in 2024 and based in New York and Tel Aviv, is building security software for AI agents that can call APIs, read documents and make changes across business systems. The new financing arrives as companies give those agents more autonomy, creating a different problem from scanning code or reviewing a system's configuration before it runs.
Watching agents at runtime
Rein's platform is designed to operate at runtime, when an agent takes an action. The company describes its approach as "sidecar" technology that lets security teams see the code an agent executes and the resources it touches, then apply rules intended to stop harmful behavior.
That approach targets risks that appear only after an agent begins working. A malicious instruction hidden in a document, for example, could steer an agent outside its assigned role even if the agent's own code has not changed. Rein says its software detected and blocked such a prompt-injection attempt against a customer's onboarding agent. The account comes from the company and is not an independent test of the platform.
Rein also says the product can secure the software supply chain around AI agents without routing company or customer data through a gateway or proxy. SecurityWeek reports that Rein began with runtime application protection before extending its technology to AI agents.
Growth claims and next steps
Since launching the product in January 2026, Rein says revenue has grown eightfold and its customer base has increased fivefold. The company says the platform now secures thousands of agents that collectively execute millions of actions. Its announcement names Flex, Swimlane, Dun & Bradstreet and Lemonade among its customers, though those scale and performance figures have not been independently verified.
Rein plans to put the new capital toward product development, research by its Agent Breakers team and hiring in New York and Tel Aviv.
