Senior Software Security Engineer in London

AI is being run everywhere, by humans and agents. Harmonic Security governs all of it. The AI Governance and Control (AIGC) platform understands user/agent intent and data context in real time, giving security teams visibility and control across every tool and workflow so companies can move faster with AI while minimizing risk. We operate in one of the fastest-moving spaces in tech: the emerging intersection of AI and cybersecurity. It's a market still being defined, and we're among the first to build natively for it - protecting sensitive data in real time, at the endpoint, with minimal friction for the teams using it. Enterprises get full visibility and control. Their people get to innovate without guardrails slowing them down.

As AI adoption inside the enterprise accelerates, the ability to safely observe, control, and enforce policy in real time isn't a nice-to-have. It's mission-critical infrastructure. That's what we build.

How We Work: AI-First by Design

Harmonic exists to help enterprises adopt AI safely and at scale. We hold ourselves to that same standard. Everyone here actively leverages AI tools to perform their best work — from deep research and writing to building robust processes and automating complex workflows. We expect every new hire to bring a genuine curiosity for AI and a commitment to using it to work smarter, faster, and with greater creativity.

About the Team: We sell security to some of the most demanding enterprise security teams in the world, and we've held ourselves to that bar from day one. Our engineers have already built a platform architecture we're genuinely proud of - strong foundations across authentication, data isolation, and secrets handling - but it's been built by generalist engineers who split their time across security and everything else on the roadmap. Now we want to bring in someone whose full-time focus is security architecture, to take that foundation and push it further than a generalist team has the bandwidth to.

About the Role

We're looking for a Senior Product Security Engineer to become the authority on our platform's security architecture - and to write the code that embodies it. You'll inherit a platform where the fundamentals of authentication and authorization, secrets and key management, encryption, and multi-tenant data isolation are already in decent shape, and your job is to sharpen, harden, and evolve them with the depth a dedicated specialist brings that a generalist team can't. Once you set a standard, the rest of engineering builds to it - that's a formal mandate, not a suggestion.

What You'll Do

• Take formal ownership of Harmonic's platform security architecture - authentication and authorization, secrets and key management, encryption at rest and in transit, and multi-tenant data isolation - and evolve it beyond what a generalist engineering team has had the focus to do.
• Write production code that directly hardens and extends these patterns in our core platform.
• Set security engineering standards for the wider engineering team, and have the mandate to enforce them.
• Manage your own backlog of platform security improvements, prioritized independently of the product roadmap.
• Run threat modeling and architecture reviews on major new systems and integrations before they ship.
• Partner directly with engineering leads to embed the right patterns at design time.
• Keep our own security posture ahead of what our most demanding enterprise customers and their security teams expect from us.
• Stay current on the threat landscape relevant to AI agent security, sensitive data handling, and multi-tenant SaaS platforms.

What You Bring

• 6+ years of hands-on software engineering experience, with deep specialization in security architecture.
• Proven track record designing and personally implementing platform-level security controls in production.
• Strong grounding in threat modeling and secure-by-design principles.
• Comfort setting technical standards that other engineers are expected to follow.
• Experience with cloud security fundamentals (AWS, GCP, or Azure).
• A track record of shipping secure, reliable, production-grade code.

You Might Be a Fit if You

• Have built or hardened platform-level security in a SaaS company that faces serious enterprise scrutiny.
• Would rather build the fix into the architecture than find the next hole in someone else's code.
• Enjoy being the go-to authority on a topic and setting the standard.
• Are comfortable being the first specialist in this seat, inheriting a solid foundation and having the autonomy to take it further.
• Understand not just how to build secure systems, but why specific architectural decisions matter for real attackers.
• Thrive in fast-paced startup environments where ambiguity is the norm.
• Leverage AI tools as an engineer to help you build smarter, faster and better.

Why Join Us

• This isn't just a job - it's a chance to help define a category.
• Recognized momentum - We made the Rising in Cyber 2026 list.
• Rare market opportunity - Join early in a category-defining company.
• Competitive pay and meaningful equity.
• Comprehensive benefits - 401K, pension plan, generous PTO, and flexible hybrid work.
• A small, high-trust team that values transparency, creativity, and continuous learning.
• Thoughtful leadership that cares deeply about your growth.
• Annual global off-sites.
• Real ownership - you'll shape the platform, the culture, and the direction of the company from day one.

Harmonic's Core Values

• Flourish in the Unknown: We embrace new, unfamiliar situations that require initiative and rapid decision-making.
• Never Full: We raise our hands, take on challenges, and assist others whenever possible.
• Perfect Harmony: We support one another to create cohesion and unity.