Member of Technical Staff - Security Engineering in London

Fractile was founded in 2022 on the bet that, eventually, the world’s most capable AI systems would be limited in their impact by the time taken to produce useful outputs. We bet everything on the logical conclusion: that the only way to truly unlock this latent value, to make speed viable at scale, was to radically re-invent the hardware that we run our frontier AI models on. Ever since, we have been building chips and systems that tackle this problem: how to efficiently generate output at thousands of tokens per second, while handling the complexity and capacity challenges of operating large models at very long contexts.

The role involves building the control chain that powers on, monitors, updates, and protects devices and racks across bare metal, RTOS, and embedded Linux. Security runs through all of it. A chain of trust from the hardware up to the rack management plane: the root of trust each controller boots from, the signed updates that reach the fleet safely, the identity that lets a device prove what it is, the authenticated channels the controllers and devices talk over, and the runtime protections that keep the host Linux kernel and the on-die firmware trustworthy long after boot. You’d be deeply involved in that security across the chain, making it real in software.

You’ll work hand in hand with our architecture and hardware teams, bringing the software and system-level view: turning threat models into concrete mechanisms in bare metal, RTOS, and Linux, then designing, prototyping, and validating them. Some of the calls in this model are still open, and you’ll help shape them. We want engineers who understand security from the inside out. You know how the root of trust, boot chain, key hierarchy, and secure interconnects actually work under the hood, not just how to switch them on. In this role, the security of your software solutions depends entirely on how well you understand the hardware beneath it. Some of the protections we rely on live in silicon, some in software, and part of the skill is knowing which belongs where. We’re looking for people who know both sides well enough to build the mechanisms that close whole classes of bugs by design.

What you’ll do:

• Shape the security of the control chain (device, board, chassis, and rack controllers): threat models, trust boundaries, and a coherent secure-by-design approach across all platforms.
• Design and help build secure boot and the root of trust on each controller: verified/measured boot, signing and verification, anti-rollback, and locking down debug and privileged access in production.
• Secure the interconnects, between controllers and between devices and the host, with authentication, integrity, and additional protections where needed.
• Design and build the cryptographic mechanisms: key hierarchies, provisioning and storage, device identity, and remote attestation so the fleet can prove what it’s running.
• Make firmware updates safe end to end: signed images, rollback protection, staged rollout, and clean recovery, across every hop of the chain.
• Protect the running system, not just boot: harden the host Linux kernel and the on-die firmware.
• Work closely with hardware and the rest of the software team to build security into the product, running threat-model reviews, triaging vulnerabilities, and driving fixes to closure.

What we’re looking for:

• An adversarial mindset and sharp threat-modeling instincts: trust boundaries, assets, attacks, and honesty about residual risk.
• Hands-on experience with the core security primitives in software and in silicon: secure boot, root of trust, cryptography, and key management, including how they’re implemented and where they can fail.
• Secure protocol and channel design between components.
• Strong C and Rust, and the instinct for memory safety and defensive coding that low-level security work needs.
• Experience building, not just using, security mechanisms: a secure-boot chain, key hierarchy, or attestation flow you shaped.

We get especially curious about your application when you also bring some of the following:

• Attestation at boot and at runtime, and management-plane protocols (DICE, IETF RATS, SPDM, MCTP, PLDM; confidential-computing directions like TDISP).
• Hardware building blocks and firmware/Linux hardening (HSM, TPM, secure element, Caliptra, secure debug; open firmware/boot projects, image minimisation).
• Assurance and resistance (supply-chain security with SBOM and SLSA, fuzzing and testing, standards like NIST 800-series, Common Criteria, OCP incl. S.A.F.E., TCG/DMTF; physical and side-channel awareness).
• AI/LLM-assisted security tooling, and RISC-V / GPU / ML-accelerator experience.

What you’re signing up for:

This is a young company doing hard things, balanced across hardware and software. We’re building the software layer for a whole inference system as it takes shape, so you’ll often be securing systems ahead of the hardware: reasoning about a threat model against a specification first, then a simulator, then early boards. You’ll work with incomplete information and watch it get sharper with every iteration. That takes comfort with ambiguity and a bias for action. If that sounds daunting, this may not be the role. If it sounds like the fun part, we should talk.

Boundaries between areas are soft here. Security touches every layer, so you’ll work across bare metal, RTOS, and Linux, and alongside the architecture and hardware teams as much as the rest of the software team. That breadth is a good part of the job. You’ll be there for first bring-up, the first secure boot that verifies, the first device that attests itself, the first clean end-to-end run. A piece of what you built will live in silicon and in every rack we ship. We want a team that reflects the world it’s built for, and we welcome applicants from every background, including those whose path here hasn’t been a straight line. We care that you’re excited by this kind of work, that you bring security rigour, and that you make a team better. Come build it with us.

What We Offer:

• Competitive salary: A competitive salary reflective of your experience and the specialist nature of the role.
• Equity & Ownership: meaningful equity so everyone shares in the value creation.
• Benefits: Private Medical, Dental and Vision, Contributory Pension, 25 Days holiday plus bank holidays and Life/Critical Illness Insurance.
• Diverse & fun office: we believe the hardest problems get solved by the broadest range of minds. We are committed to Equal Employment Opportunity through attracting and retaining a diverse team and building an inclusive environment.

Fractile is seeking to increase the clock speed of global progress, one chip at a time. We’ve recently raised $220M from investors including Founders Fund and Accel and our most important work lies ahead. Join us!

Export controls: Our work involves technologies subject to UK, US and other international export control regulations. Certain roles may require additional eligibility checks to ensure compliance with applicable law. We'll be transparent about this throughout the hiring process.