Senior Application Security Engineer in London

Flagstone is an online savings platform, reinventing how individuals, businesses, and charities manage, protect, and grow their cash. A diverse group of people, bound by a collaborative spirit, and shared purpose. Each definition shares a common thread: our unique culture. It’s our pride and joy. And our competitive advantage.

A feel for our culture: To revolutionise the savings market, we need to be at our best. But high performance takes more than talent – it takes a culture of kindness, respect, and growth. That’s why we’re building a diverse, inclusive community, where your voice is heard and valued. Where, with close support and room to develop, you can surpass even your own expectations. And be rewarded for it.

This is a new AppSec-focused addition to Security Engineering, working closely with product engineering teams across the SDLC. The role sits alongside, not inside, GRC/Compliance — customer security questionnaires and RFP responses are owned by GRC, with this role providing technical input as needed rather than end-to-end ownership.

We're currently hiring a Senior Application Security Engineer to own AppSec end-to-end across our development lifecycle: from design-stage threat modelling through to code review, tooling, and remediation of findings. This is a hands-on engineering role, not a compliance or customer-facing one — you'll work directly with product engineering teams to build security in rather than bolt it on afterwards.

What you’ll do:

• Own the AppSec programme for Flagstone and report back on its success to relevant stakeholders including leadership.
• Formalise a secure development lifecycle which includes owning threat modelling and secure design review for new features and systems, working directly with engineering teams during design rather than after the fact, introducing security gates and guardrails and ensuring applications are secure even post deployment.
• Run and continuously improve secure code review practices, including SAST/DAST/SCA tooling integrated into CI/CD pipelines and code review (manual and/or AI assisted).
• Coordinate external and execute internal penetration test engagements — scope, logistics, findings triage — and drive remediation to closure with engineering teams.
• Maintain and evolve secure coding standards, and run a security champions programme to scale AppSec practice across engineering.
• Track and report on vulnerability management across the application estate, prioritising by exploitability and business impact.
• Contribute AppSec expertise to incident response where application-layer issues are involved.
• Run and own a security champions programme.
• Manage application cyber risk according to the internal Flagstone Risk Framework.

What we’re looking for:

• 5+ years in application security or a security engineering role with a strong AppSec component.
• Practical experience embedding security in the SDLC: threat modelling frameworks (e.g. STRIDE), secure design review, and working directly with engineering teams.
• Hands-on experience with SAST/DAST/SCA tooling and CI/CD pipeline integration.
• Strong grasp of OWASP Top 10 and secure coding practices across at least one major language/framework used in a production environment.
• Experience coordinating and running penetration testing programmes, including remediation tracking.
• Comfortable communicating security risk clearly to engineering and product audiences and influencing senior leadership.
• Demonstrable experience with adversarial security testing.
• Demonstrated experience in reviewing, threat modelling and securing agentic and AI systems.
• Great communication skills and stakeholder management, which includes influencing stakeholders and creating relationships.
• Strong critical thinking skills and a curiosity for learning new technologies and frameworks.
• Great at troubleshooting and thinking out of the box.

Nice to Have:

• A strong link to the AppSec community and industry which includes conferences, OWASP or other Open-Source contributions, public speaking and engagement in thought leadership.
• OSCP, CSSLP, or equivalent hands-on offensive/AppSec certification.
• Experience in a regulated financial services environment.
• Prior experience running or scaling a security champions programme.
• Exposure to cloud-native application security (containers, serverless, API security).

How we reward you:

• Competitive bonus scheme - designed to reward and recognise high performance.
• Flexible benefits budget - a pot to fund meaningful benefits for you, whether it's hormone or fertility testing, cancer screening, neuro-diversity coaching or something that matters for you.
• A range of salary sacrifice options to help you make tax efficient savings on electric cars, nursery schemes, home and tech goods.
• Around the World scheme - 3 months work from anywhere scheme (some exclusions do apply).
• Mental wellbeing support – Access therapy and mental health sessions through Spill.
• Learning and development – £1,000 personal development budget to help you grow in your role.
• Private health care - Enjoy all the benefits AXA has to offer, including reduced gym memberships and medical history disregarded.
• Medical cash plan - To help you with the costs of dental and optical expenses.
• Life insurance and Income Protection- four times your annual salary for peace of mind.
• Matched pension contributions up to 5%.
• 25 days holiday - plus bank holidays, well-being days and volunteering days.
• Enhanced Parental Leave – enhanced maternity, paternity and adoption pay. All are welcome.

At Flagstone, we’re assembling a diverse team that defies our industry’s norms. Think this role could suit you? We encourage you to apply, no matter your background.