Senior Security Engineer - Contract

Flagstone; Greater London; On-site; GBP 111,000 - 166,000; Full time

Job summary

Flagstone is seeking a senior security engineer for a contract role in the UK. You will own critical detection and response capabilities across cloud security, incident response, and security tooling, with hands-on work on Azure Defender for Cloud, Sentinel, and AI security considerations.

You will write infrastructure-as-code, coordinate pen tests, and partner with engineering to drive rapid remediation. The role rewards practical impact and visible results.

Qualifications

• Hands-on SIEM experience with modern tooling (Microsoft Sentinel preferred).
• Strong Azure security knowledge across Defender for Cloud and Entra ID.
• Experience writing IaC with Terraform or Bicep in security projects.
• Proven ability to drive vulnerability remediation with engineering teams.
• Awareness of data-loss-prevention controls and AI security considerations.

Responsibilities

• Own and operate alerting and monitoring during transitions with steady detection and response.
• Maintain and improve Microsoft Sentinel deployment: rules, connectors, noise reduction.
• Operate Defender XDR and Defender for Cloud; manage policies and posture.
• Extend automated security checks in delivery pipelines (shift-left).
• Lead remediation coordination with engineering squads to reduce Down Time.
• Support data-loss-prevention controls and access safeguards for AI workloads.
• Investigate Sentinel/Defender alerts: triage, escalate, contain as needed.
• Support incident response activities: containment, evidence gathering, post-incident review.
• Contribute to detection-as-code and IaC tooling (Terraform/Bicep).
• Coordinate penetration testing engagements and prioritize findings with teams.

Skills

SIEM

Azure security

Threat modeling

Incident response

KQL proficiency

Terraform/Bicep

Vulnerability remediation

AI security

Fintech experience

Security operations

Tools

Microsoft Sentinel

Azure Defender for Cloud

Entra ID

Terraform

Bicep

Job description

What is Flagstone?

Flagstone is many things. 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. And lastly, a thriving, profitable business – where smart people do their best work.

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.

We may not change the world, but we can change the world of financial technology. And all it takes is a winning mix of drive, talent, and empathy. Our culture celebrates all three.

But enough about us. Let’s talk about you.

About the team

Security Engineering is a team of five covering cloud security, detection, and security operations. They work directly in the Azure estate and are close to the infrastructure, not abstracted behind a policy layer. The team runs Microsoft Sentinel, Defender XDR, and Defender for Cloud, and manages tooling through infrastructure-as-code. They run a quarterly penetration test programme and are continuously building out our detection and response capability. It's a small team with broad scope, which means your work is visible, your opinions are heard, and there are meaningful problems for our engineers to work on.

This is a contract role, so we're looking for someone who can hit the ground running. You'll bring immediate impact and add value from day one, working independently without a long ramp-up. Your work will be visible, and the problems you solve will matter.

Does this sound like you?

You're a senior security engineer who operates credibly across cloud security, detection tooling, and incident response, without being narrowly specialised. You own meaningful parts of the security stack, contribute hands-on to Azure cloud hardening, and show up reliably when incidents need investigating or pen test cycles need coordinating. You're energised by visible impact, your work matters, and your voice is heard.

What you’ll do:

• Own and operate our alerting and monitoring capability through a transition period, keeping detection and response steady.
• Maintain and improve our Microsoft Sentinel deployment: writing and tuning detection rules, managing data connectors, and reducing alert noise.
• Operate and optimise Defender XDR and Defender for Cloud, including policy management and posture recommendations.
• Maintain and extend automated security checks in our delivery pipelines (shift-left).
• Drive down time-to-fix on known vulnerabilities, coordinating remediation with engineering squads.
• Support data-loss-prevention controls that reduce the risk of sensitive data leaving the business.
• Support safeguards around how the business accesses and uses AI, including securing AI workloads and their data exposure.
• Investigate suspicious activity surfaced through Sentinel and Defender: triage, elevate, or contain as appropriate.
• Support incident response, including containment, evidence gathering, and post-incident review.
• Contribute to detection-as-code and infrastructure-as-code (Terraform or Bicep) for security tooling.
• Coordinate penetration test engagements (scope, logistics, findings review) and work with engineering teams to prioritise remediation.

What you’ll bring:

• Hands-on SIEM experience, ideally Microsoft Sentinel; equivalent platforms (Splunk, Chronicle, QRadar) considered.
• Practical Azure security experience across Defender for Cloud, Entra ID, Azure networking, and cloud security posture management.
• Experience writing infrastructure-as-code using Terraform or Bicep in a security engineering context.
• Experience driving vulnerability remediation cycles with engineering squads.
• Familiarity with data-loss-prevention controls.
• Familiarity with AI security considerations (securing AI workloads, data exposure risks) and/or using AI tooling to augment security engineering workflows.
• Ability to contribute to threat modelling and communicate security risk clearly to engineering and product audiences.
• A growth mindset and genuine curiosity to keep learning.
• SC-200 (Microsoft Security Operations Analyst) certification.
• KQL proficiency for detection rule authoring and threat hunting.
• Experience working in a similar fintech or financial services environment

All are welcome:

At Flagstone, we’re assembling a diverse team that defies our industry’s norms. Think this role could suit you?