Checkout and Payment Flow Penetration Testing
CREST-accredited penetration testing for your cart and checkout, your payment page or hosted checkout integration, and the payment or payout platform behind them. We test your checkout code, your server-side payment handling and the webhooks you receive, covering amount and currency integrity, payment state, callbacks and third-party scripts, never the payment provider’s own platform. Fixed quote in 24 hours.
- Unlimited retesting
- Unlimited pre-retesting
- No hidden fees
“I would highly recommend EJN Labs to any organisation seeking reliable, detailed, and well-managed penetration testing services, particularly for government or enterprise-level projects.”
“There wasn’t another company we could find that could deliver what we needed in the timeframe we needed. The client loved it, and we got instant ROI from the engagement.”
layers separate what we test from what your payment provider tests: your checkout page, your integration code and your server-side payment handling. The provider’s own platform is out of scope.
Your Checkout Is In Scope. Your Payment Provider Is Not
What we test. Your cart and checkout pages, your payment page or hosted checkout integration, the order and payment state your application keeps, the callbacks and webhooks your server receives, and the scripts that run on those pages. We do not test the payment provider’s own platform, its card network connections or its banking infrastructure: that sits inside the provider’s own security programme, not yours.
PCI DSS. Where card data or the payment page sits inside your cardholder data environment, PCI DSS Requirement 11.4.1 requires a documented penetration testing methodology. Requirements 11.4.2 and 11.4.3 require internal and external testing at least every 12 months and after significant change. Requirement 11.6.1 requires a mechanism to detect and alert on unauthorised changes to payment-page scripts and content at least weekly. Not every checkout sits in PCI scope; your QSA or acquiring bank confirms that. See our PCI DSS penetration testing page and the PCI Security Standards Council.
Business logic and webhooks. Whether a legitimate sequence of cart, discount and shipping steps can be replayed to change price or currency, whether a success callback or webhook from your payment provider can be forged or replayed to mark an order paid, and whether refund amount, recipient or approval state can be tampered with once a payment leaves your system.
Who we test for. Online retailers and D2C brands, SaaS platforms with in-app billing, subscription commerce, and marketplaces where buyers pay sellers directly, see our online marketplace and retail and e-commerce pages. If your checkout runs on Shopify, Magento or WooCommerce, we scope the platform’s checkout extensions and plugins alongside your own code.
SCOPE
Checkout and Payment Flow Security Testing
Cart & Checkout Business Logic
Business-logic testing across the cart, address and payment steps: price, quantity, discount-code and shipping tampering, and step-skipping or promo codes that stack or replay after use.
Amount & Currency Integrity
Server-side re-validation of amount, currency and quantity at every step from cart to charge, so a manipulated client-side total, downgraded currency or altered quantity cannot reach the payment provider.
Payment State & Order Status
Whether an order can be marked paid without a genuine payment event, whether a success redirect or callback can be replayed, and whether a cancelled or failed payment can still release goods or services.
Webhook Security Testing
Signature verification, origin validation, replay protection and idempotency for the webhooks your application receives from your payment provider, so a forged or replayed event cannot change order or subscription state.
Stripe & Adyen Integration
Hosted checkout embeds, Stripe Elements and Payment Intents, Adyen Drop-in and Components. We test what runs in your code against what the provider hosts, not the provider’s own platform.
Worldpay & GoCardless
Worldpay hosted payment pages and direct API integrations, and GoCardless Direct Debit mandate creation, collection and cancellation.
Third-Party Scripts on Payment Pages
Which analytics, tag-manager, chat and marketing scripts load on the cart and payment pages, whether they can read card fields or redirect the submitted form, and whether an unexpected change to those scripts would be noticed.
OUR PROCESS
From Scope to Attestation in 4-6 Weeks
Scoping Call
30-minute call to map your checkout flow, payment methods and provider integrations. Fixed-price quote within 24 hours.
Test Environment
You provide a staging or sandbox checkout with test card numbers and API credentials for each payment provider in scope.
Active Testing
3-15 days of hands-on testing by CREST-certified pen testers, covering the checkout, the integration code and your server-side payment handling. Live findings in your client portal.
Report & Retest
CVSS-scored report with reproduction steps for every finding, walkthrough call, free retest and a letter of attestation.
CREDENTIALS
Verified Accreditations Auditors Accept
Every credential below is independently verifiable. UK procurement teams, FCA supervisors, ISO 27001 / SOC 2 auditors, and cyber insurance underwriters all recognise these standards.
GET YOUR QUOTE
Get a CREST checkout and payment pen test quote in 24 hours
A fixed-price quote back in one business day, from a named CREST assessor. No sales pipeline, no chasing.
- CREST and IASME accredited. Testing your auditors and clients already recognise.
- Fast-track testing within 24 hours where required. Free retest of every fix included.
- Live findings via your client portal, not a four-week PDF.
- Fixed price from £3,500 for a single-role, single-app scope, agreed up front. Most engagements run £5,000 and up. No day-rate surprises.
Dan WilcocksonFounder, CelloriUnder NDA Further named references available on a scoping call.
- We reply within one business day with a fixed-price quote from a named CREST assessor.
- You approve the scope and we book a start date, usually within 24 hours.
- Live findings land in your client portal as we test, with a free retest of every fix.
Get your fixed pen test quote in 24 hours
Quote request received
We will reply within one business day with your fixed-price quote from a named CREST assessor.
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or book a 20-min scoping call first
We reply within one business day. Your data stays with us. No newsletter signup.
COMPLIANCE READY
Reports Aligned to Every Framework
Findings map to specific control references in each framework, so your audit team submits the report directly without translation work.
PCI DSS 11.4
Documented methodology, internal and external penetration testing at least annually, where card data or the payment page is in scope.
PCI DSS 11.6.1
Weekly detection of unauthorised changes to payment-page scripts and content.
PCI DSS 11.3
Quarterly internal vulnerability scans and external ASV scans alongside penetration testing.
UK GDPR
Article 32(1)(d) evidence that you regularly test the effectiveness of your security measures.
ISO 27001
Annex A 8.8 technical vulnerability management, once you declare that control applicable.
Cyber Insurance
Findings and remediation documented against the questions on your proposal form. Requirements vary by insurer and policy.
PRICING
Transparent Checkout & Payment Flow Pen Testing Pricing
Pricing depends on the number of payment methods, hosted or custom checkout steps, and payment-provider integrations in scope. The day count flexes; the included deliverables stay the same across all engagements.
Depends on app complexity
Single user role, basic CRUD application, marketing website with auth. Around 5 working days from kickoff to report.
Get a fixed quoteDepends on app complexity
Multi-role SaaS, business application with payment integration. Around 8 to 12 working days from kickoff to report.
Get a fixed quoteDepends on app complexity
Multi-tenant platform, complex authorisation matrix, integration-heavy applications. Around 15 to 20 working days from kickoff to report.
Get a fixed quoteWHY EJN LABS
What You Get From Checkout and Payment Flow Penetration Testing
Six concrete differentiators competitors don’t all match.
CREST-Certified Testers, Verifiable
Every test by a CREST-certified pen tester (CRT, CCT APP, CCT INF where applicable). Verify our company status at crest-approved.org.
24-Hour Startup, Where Required
From signed scope to active testing in a single business day for incident response, audit deadlines, or regulator-driven timelines.
Live Findings, Not 4-Week PDFs
Critical issues reported during testing through your client portal. Your team remediates while testing continues.
Audit-Ready Reports
Executive summary plus full technical report with CVSS scores and explicit framework mappings (ISO 27001, SOC 2, PCI DSS, FCA SYSC).
Free Retests, Standard
Verify remediation of every finding before close-out. Letter of attestation for audit submission included. Most competitors charge £1,500-£3,000 per retest.
UK-Based CREST Testers
Every engagement performed by vetted, UK-based CREST-certified testers, matched to your needs, security clearance, and compliance scope.
FAQ
Frequently Asked
What’s the difference between testing our checkout and testing our payment provider’s platform?
We test your cart and checkout pages, your payment page or hosted checkout integration, your server-side payment logic, and the webhooks your application receives. We do not test the payment provider’s own platform, for example Stripe’s, Adyen’s or Worldpay’s infrastructure, that is covered by the provider’s own security programme and is out of scope for this engagement.
Do you need real card data or a PCI-scoped test environment?
No. We test using sandbox or test-mode card numbers and API credentials provided by your payment provider. Real cardholder data is not required, and we avoid testing with live payment data unless you specifically ask for a narrowly scoped exception.
Does every checkout need a penetration test for PCI DSS?
No, not automatically. PCI DSS applies pen testing requirements where card data or the payment page sits inside your cardholder data environment. PCI DSS Requirements 11.4.2 and 11.4.3 require internal and external testing at least every 12 months once that scope applies, and Requirement 11.6.1 requires weekly detection of unauthorised changes to the payment page. Whether your checkout is in scope depends on how you take payment; ask your QSA or acquiring bank, or see our PCI DSS penetration testing page.
Can you test hosted checkout pages, like Stripe Checkout or a PayPal redirect?
Yes. Where card entry happens on the provider’s own hosted page, we test the surface you control: how the checkout is initiated, how success, cancel and failure states are handled, and whether your order is created or fulfilled before payment is actually confirmed.
Can you test our Shopify checkout before a retailer launch?
Yes. For a Shopify checkout we test your checkout UI extensions, discount and shipping function logic, and the order webhook your app receives once Shopify’s own checkout completes, alongside the usual amount, currency and payment-state checks.
Do you test webhooks from Stripe, Adyen, Worldpay or GoCardless?
Yes. We test webhook signature verification, origin validation, replay protection and idempotency for Stripe, Adyen, Worldpay and GoCardless integrations, and whether a forged or replayed event can change order or subscription state.
Can you test refunds and payout redirection?
Yes. We test whether refund amount, recipient or approval state can be tampered with, and for marketplaces, whether a seller’s payout account or payee can be changed without proper authorisation. See our online marketplace page for the payout-specific angles.
What’s in the report?
Executive summary (board-ready), technical report with CVSS 3.1 scores, reproduction steps, screenshots, specific remediation guidance, and a 60-minute walkthrough call. Letter of attestation issued after free retest.
Do you sign NDAs?
Yes. We sign client-supplied NDAs as standard. Engagement data is protected under our ISO 27001 (BSI-audited) information security management system.
How quickly can you start?
From signed scope to active testing in 24 hours where required. Standard pipeline is 3-5 business days from initial scoping call to test start.
Are your testers UK-based and what certifications do they hold?
Every engagement is performed by vetted UK-based CREST-certified testers matched to your engagement based on security clearance, compliance scope, and sector specialism. Testers hold CREST certifications relevant to their discipline (CRT, CCT APP, CCT INF, CCSAM).
20+ CREST-accredited testing services in one place
Web, mobile, API, cloud, AI, infrastructure, red team. Pick the test that fits your environment.
Get my checkout and payment pen test scope
Tell us which payment providers, checkout steps and payment methods you run. A CREST-certified pen tester will contact you within one business day with a fixed price.
