Purple Team: Definition and Use in Compliance

What is Purple Team?

Purple Team is a cybersecurity testing methodology in which red team attackers and blue team defenders collaborate in real time during a security exercise, rather than operating as adversaries with findings delivered after the fact. The red team executes attack techniques against live systems. The blue team watches, detects, and responds. When detection fails, both sides stop, analyze the gap, and fix it before moving to the next technique.

That collaborative loop is the defining characteristic. Traditional penetration testing keeps attackers and defenders separate, which makes the exercise realistic but slows the feedback cycle considerably. A finding that emerges in week one of a pentest might not reach the detection engineering team until the report lands six weeks later. Purple team eliminates that lag. Detection engineers update rules during the exercise, sometimes within hours of the red team demonstrating a gap.

For financial institutions, the regulatory relevance is direct. The ECB's TIBER-EU framework mandates a purple team remediation phase as a required output of threat intelligence-based red team testing at systemically important banks. The Bank of England's CBEST programme carries the same expectation. In both frameworks, the purple team phase is the mechanism by which red team findings translate into documented control improvements rather than a PDF sitting in a shared drive.

The output of a well-run purple team exercise includes a control assurance matrix: each attack scenario tested, each detection gap identified, each remediation action taken, with owners, timelines, and evidence. That document is what examiners from the PRA, FCA, or ECB expect to see when they ask whether your controls have been validated against realistic threat scenarios.

Purple team is different from a tabletop exercise, which tests process and decision-making through structured discussion. Purple team tests live technical controls. Both belong in a complete operational resilience programme, but they address different risk questions.

How is Purple Team Used in Practice?

Most regulated financial institutions run purple team exercises two to four times per year. The trigger is usually one of three things: an upcoming supervisory examination, a significant change to critical infrastructure, or new threat intelligence identifying attack techniques relevant to the institution's sector.

The process starts with scenario selection. A threat intelligence team, internal or contracted, identifies which threat actor groups are active against institutions of similar size and profile, which techniques appear in recent incident reports, and which internal systems represent the highest-value targets. That analysis produces three to five specific attack chains to test, not an open-ended scope.

A UK retail bank running a 2023 purple team exercise focused on three scenarios: account takeover via credential stuffing against its mobile banking API, lateral movement from a compromised workstation to payment processing infrastructure, and data exfiltration using DNS tunneling to evade perimeter controls. Each scenario was drawn from threat intelligence tied to groups actively targeting UK retail banking.

The red team then executes each chain while the blue team monitors live. When a detection fires, both sides document what triggered it and whether the response was correct. When detection fails, the team pauses. Detection engineers examine why the technique bypassed existing rules and update the logic before resuming. By the exercise's end, that bank had identified 11 detection gaps and closed 9 of them before the engagement concluded.

Results feed directly into impact tolerance reviews. If the exercise shows that a simulated ransomware attack would take 68 hours to detect and contain against a stated tolerance of 24 hours, that gap requires a documented response: control improvement, a revised tolerance, or both. Examiners expect the link between exercise findings and operational resilience decisions to be explicit and traceable.

For institutions deploying AI-driven transaction monitoring or fraud detection, purple team exercises increasingly include AI-specific scenarios. Can an attacker manipulate input features to evade a machine learning model? Does the model's detection rate hold against adversarial inputs that differ from training data patterns? Those questions are moving from theoretical to regulatory expectation, particularly under emerging AI governance frameworks.

Purple Team in Regulatory Context

The regulatory basis for purple team testing in financial services has solidified over the past eight years, starting with TIBER-EU and extending across multiple jurisdictions.

The European Central Bank published TIBER-EU in May 2018 as a common framework for threat intelligence-based ethical red teaming across EU financial institutions. The framework has three mandatory phases: threat intelligence production, red team testing, and a purple team remediation phase. The remediation phase requires the institution to review all red team findings jointly with defensive staff, document what was tested and what failed, and demonstrate that control improvements are underway. National competent authorities across the EU, including De Nederlandsche Bank, the Bundesbank, and the Banque de France, have implemented TIBER-EU or aligned national variants.

In the UK, the Bank of England's CBEST framework applies to firms the Bank identifies as critical to UK financial stability. Updated in 2021, CBEST explicitly requires collaborative remediation as a documented output. Firms in scope include systemically important banks, payment systems operators, and financial market infrastructure providers.

US requirements are less prescriptive but moving in the same direction. The Federal Reserve, OCC, and FDIC's joint interagency guidance on operational resilience, published in 2023, expects institutions to test controls under realistic threat conditions without mandating a specific methodology. The FFIEC Cybersecurity Assessment Tool treats adversarial testing as a maturity indicator. Examiners at well-managed institutions increasingly expect evidence of purple team-style exercises rather than relying solely on point-in-time penetration test reports.

For institutions with AI-driven compliance systems, purple team methodology intersects with AI governance requirements. Regulators want to know whether AI detection models hold up against adversarial inputs. A purple team exercise that includes model evasion scenarios directly tests whether those systems perform as documented under realistic attack conditions, which is the standard model validation frameworks already require.

The three lines of defense model applies here. Security and compliance teams (first line) run the exercise. Internal audit (third line) reviews whether the methodology met regulatory expectations and whether findings were properly tracked to remediation.

Common Challenges and How to Address Them

The most common failure mode is scope that's too broad. When the red team has an open mandate and the blue team isn't tracking specific scenarios, the exercise reverts to a traditional pentest with marginally faster reporting. The fix is tight scenario definition before the engagement starts: three to five named attack chains based on current threat intelligence, not an instruction to "assess the perimeter."

Organizational resistance is the second challenge. Red teams sometimes resist sharing techniques in real time because they believe it compromises realism. That concern makes sense for a pure red team exercise. It misses the point of purple team. The goal is improving detection, not demonstrating that attackers can breach defenses. Getting explicit buy-in from both teams on the collaborative model before the exercise starts prevents this from derailing the engagement mid-exercise.

Staffing constraints affect mid-sized institutions disproportionately. A properly run purple team exercise requires detection engineers who can update SIEM rules and behavioral analytics configurations in real time. Many institutions don't have that capacity in-house. The practical answer is managed service providers or specialist firms that supply the red team capability while the institution's security operations center focuses on detection and response. That structure works, but it requires clear contractual terms on data handling, particularly for institutions with data residency obligations.

Documentation quality is often weak. A 60-page narrative report that arrives two weeks after the exercise is not what regulators or internal audit want. The output should be a control assurance matrix: scenario, technique, detection result, gap identified, remediation action, owner, deadline. That format answers examiner questions directly and tracks to the institution's incident management and audit trail requirements.

Frequency is the final issue. A single annual exercise is better than nothing. Attack techniques evolve faster than annual cycles. Institutions that treat purple team as a calendar event rather than a threat-intelligence-driven practice will consistently be testing yesterday's techniques against today's adversaries. Exercises should be triggered by new threat intelligence, not just by the approach of an examination.

Related Terms and Concepts

Purple team sits at the intersection of several disciplines that security and compliance teams at financial institutions manage together.

The red team is the offensive counterpart: a group tasked with simulating realistic attack scenarios using techniques drawn from current threat intelligence. The blue team is the defensive counterpart: security operations center analysts, detection engineers, and incident responders. Purple team is the structured collaboration between the two during a shared exercise rather than a sequential handoff.

Tabletop exercise is a related practice that works differently. A tabletop walks participants through a scenario via discussion to test process, communication, and decision-making. Purple team tests live technical controls against actual attack execution. Both belong in an operational resilience testing programme, but they answer different questions. A tabletop tells you whether people know what to do; purple team tells you whether the systems can detect it.

Operational resilience is the regulatory framework that motivates purple team testing in financial services. Regulators in the UK, EU, and US require institutions to demonstrate that they can withstand and recover from severe disruption to their critical business services. Purple team exercises produce the evidence that controls have been tested against realistic threats, which is what those frameworks require.

Impact tolerance connects directly to purple team outputs. When an exercise reveals that a specific attack chain would cause disruption exceeding stated tolerances, the institution has a documented gap that requires a formal response.

For institutions using AI agents in security or compliance workflows, human-in-the-loop controls and kill switch capabilities are increasingly in scope for purple team exercises. Can an attacker manipulate an AI agent's inputs to produce incorrect outputs? Can the kill switch be triggered fast enough to limit harm? These are operational resilience questions, and purple team is the right tool for testing them.