PSD2 Strong Customer Authentication and Mobile RASP

The short answer

PSD2 strong customer authentication and its regulatory technical standards define authentication and transaction-related requirements for payment services. Mobile RASP may help protect app integrity and authentication workflows but is not an authentication factor.

The strongest implementations of PSD2 Strong Customer Authentication and Mobile RASP make evidence, ownership, and failure behavior explicit. Map runtime controls to threats against possession, knowledge, inherence, dynamic linking, credentials, and transaction execution with legal and payments specialists.

Where it fits in a Mobile RASP program

Mobile RASP can support control objectives and supply technical evidence, but it does not make an application compliant by itself. A defensible mapping states the requirement, the runtime contribution, the complementary controls, the test evidence, and the residual gap.

Runtime evidence is strongest when it is fresh, bound to an authenticated request, corroborated by a different source, and evaluated with the value of the action in view.

• Use authoritative requirement text and current versions
• Separate mandatory requirements from implementation guidance
• Connect each claim to retained build, test, policy, and operational evidence

A practical design model

1. Define the protected outcome

Start with the secret, privilege, transaction, content, or user action at risk. Describe what an attacker must achieve and which part of that path occurs inside the mobile process. Map runtime controls to threats against possession, knowledge, inherence, dynamic linking, credentials, and transaction execution with legal and payments specialists.

2. Collect evidence without overclaiming

A detector reports a condition; it does not report intent. Record where the evidence came from, how fresh it is, how easily it can be suppressed or forged, which other signals share the same underlying assumption, and how legitimate configurations can produce the same observation.

3. Separate local containment from remote authority

The app can hide a screen, withhold a local secret, request new evidence, or pause a workflow immediately. A trusted backend should still decide whether an account changes, money moves, a credential is issued, content is unlocked, or durable state changes. This separation limits the value of patching a local branch.

4. Respond in proportion to confidence and consequence

Useful response states include observe, refresh evidence, step up authentication, lower a transaction limit, disable one high-risk feature, revoke a session, route to review, or deny. Safe recovery and support paths are part of the security design, not an afterthought.

Evidence to document

Evidence discipline: retain the application build, protection version, policy version, platform, device class, request identifier, timestamps, and final backend outcome. Without that context, results cannot be reproduced or compared after the next release.

Implementation workflow

1. 01
   Confirm applicability and version

   Define the failure and recovery behavior here; leaving it implicit transfers the decision to an outage or incident.

2. 02
   Parse the actual requirement

   Give this step a named owner, an observable output, and a review date. Link it to the protected workflow rather than a generic security objective.

3. 03
   Map the RASP contribution

   Record the decision in a form that mobile, backend, product, security, and operations teams can verify independently.

4. 04
   List complementary controls

   Use release-equivalent conditions and retain enough context to repeat the work after an app, platform, policy, or protection update.

5. 05
   Collect operated evidence

   Define the failure and recovery behavior here; leaving it implicit transfers the decision to an outage or incident.

6. 06
   Review with the accountable assessor

   Give this step a named owner, an observable output, and a review date. Link it to the protected workflow rather than a generic security objective.

How to validate it

Validation should prove more than the presence of code. Use the final protected artifact, a written authorization boundary, and a test environment that can observe both the application and server. Attempt the expected technique, vary the method, and record whether the valuable outcome remained possible.

• Have control owners and assessors review the mapping.
• Sample evidence from a released build and production policy.
• Track version changes in standards and platform services.
• Map runtime controls to threats against possession, knowledge, inherence, dynamic linking, credentials, and transaction execution with legal and payments specialists.

Where server enforcement is expected, call the backend directly or alter the client flow. Otherwise a local success can mask an unchanged business exposure.

Common failure modes

Production checklist

• The PSD2 scope and protected business outcome are written down.
• Client evidence is treated as potentially observable, suppressible, and forgeable.
• Critical authorization remains on a trusted backend where connectivity and workflow allow.
• Signal provenance, confidence, freshness, and policy version are retained.
• Responses include a proportionate path between silent allow and permanent denial.
• Release-equivalent Android and iOS artifacts are tested on a representative matrix.
• False positives, accessibility, privacy, performance, and availability have owners and budgets.
• A monitored rollout, rollback path, incident procedure, and retest trigger are approved.

Questions the team should settle

• Which business loss does PSD2 reduce, and how will that reduction be measured?
• Which properties can the app observe, which can the platform attest, and which must the backend decide?
• What happens when evidence is missing, stale, contradictory, or produced during a provider outage?
• Which legitimate devices, users, accessibility tools, or distribution channels could look unusual?
• How quickly can the team change policy or protection after a bypass or platform update?

PSD2 Strong Customer Authentication and Mobile RASP: frequently asked questions