Senior Security Engineer

| London, hybrid (1 to 2 days on site)The Rolenumi Partners is partnering with a specialized engineering firm that builds mission-critical infrastructure for clients in law enforcement and government, operating in environments where records are examined adversarially and the stakes are high.You will join a small engineering team as a working engineer with deep security expertise. This is not an advisory role and it is not a governance role. You will ship production code alongside the rest of the team (Go services, Postgres, React, all on AWS) and you will be the person who makes sure that what is shipped stands up to adversarial scrutiny.Security here is not a separate function reviewing the team's work from the outside. It is a discipline embedded in how the team designs, builds, and operates the product. You will set the standard for that.ResponsibilitiesContribute to day-to-day product development as a senior engineer: designing, writing, reviewing, and shipping production code in Go and React.Own the security dimension of engineering work: lead threat modelling on new features and architectural changes, drive secure design decisions, and review high-risk changes.Own and improve the security of the AWS estate: IAM design, network segmentation, secrets management, and data isolation in a multi-tenant system.Build security into the development lifecycle: dependency and vulnerability management, CI/CD security controls, static and dynamic analysis, and the tooling that makes the secure path the easy path.Design and build product security features where the domain demands them: audit integrity, tamper evidence, access controls, and data isolation.Lead the technical side of incident response: detection, investigation, and remediation, plus the exercises that keep the team ready.Support client-facing security work with technical depth: contribute to security questionnaires, due diligence responses, and client assessments where engineering answers are needed.Raise the security literacy of the wider team through design reviews, pairing, and pragmatic internal guidance, rather than policy documents.Essential ExperienceFive or more years as a software engineer, with substantial time spent in security-focused work, whether as a security engineer or a product engineer who owned security. You are comfortable being measured as an engineer first.Strong backend development experience. Go is strongly preferred; deep experience in a comparable language with a willingness to work in Go is acceptable.Demonstrable, hands-on ownership of cloud security in a production AWS environment. You should be able to reason about IAM design, network segmentation, secrets management, and data isolation in a multi-tenant system without reaching for a reference.Threat modelling as a practised discipline rather than an occasional exercise, along with the ability to run it with engineers who haven't done it before.Practical experience with application security: you can find, exploit, and fix the vulnerabilities that matter in a web application backed by a relational database, and you know which ones don't matter.Comfort working in a small team where you will be the security depth, without a larger security function behind you.DesirableExperience in a domain with irreversible, high-value transactions: digital asset custody, payments infrastructure, finance, or similar.Frontend experience with React, or enough to be dangerous in code review.Experience supporting a formal security or compliance regime from the engineering side, such as SOC 2 Type II, ISO 27001, or a government assurance framework.Detection and response tooling experience, particularly building or operating it rather than just buying it.Experience as the first or only security-focused engineer somewhere.Postgres depth: schema design, row-level security, and performance under audit-heavy workloads.