audit-payment-system — Money-Movement Correctness & Compliance Audit

Degree of freedom: MIXED — Scope, matrix, and severity [HIGH freedom]; Phase 0 detection searches [LOW freedom — run exactly]. Do not write exploit or payment-fraud PoCs — quote the missing control, never a replay or spoof recipe.

Read-only. Assess and prioritize; do not change code. Payment code is a STOP-and-confirm surface — findings feed a human-reviewed remediation run at high effort with a fresh-context reviewer. Delegations: per-call resilience → audit-resilience; PCI/secrets/authz → audit-security; ledger schema → audit-db-schema; outbox/saga structure → audit-backend-architecture; append-only integrity → plan-data-integrity; Stripe integration → the Stripe plugin skills. This skill owns payment-domain correctness.

Payment failures are silent: a retried charge is a double-charge, a lost ledger write is vanished money, a logged PAN is PCI liability, an unverified webhook is an untrusted "paid". Three pillars: idempotency, a double-entry ledger (when in scope), reconciliation — plus PCI DSS v4.0.1 and webhooks as source of truth (never trust the sync API response alone).

Core principle — earn each control by scope; every gap is money or liability

Not every app needs an in-house double-entry ledger. Stripe Checkout offloads ledger, settlement, and most PCI — flagging "no double-entry ledger" there is noise. Idempotency, webhook verification, state sync, and tokens-only apply to everyone who moves money. Gate depth by scope (Phase 0). Critical = a customer is charged twice, money is lost/unaccounted, or card data is exposed. There is no "low severity" for a double-charge.

How to reason — Observe → Interpret → Classify → Severity

1. Observe — quote the mutation/webhook/ledger file:line (or "searched, none found")
2. Interpret — what money or liability path breaks if that control is missing?
3. Classify — Implemented / Partial / Missing / N/A (tier reason); control id (A1–G4)
4. Severity — double-charge, lost money, or PAN/CVV exposure = Critical; P2-only rows are N/A on P0

Worked example

Observe: P0 Stripe Checkout. POST /api/checkout calls paymentIntents.create with no idempotency key and no unique business-intent constraint (app/api/checkout/route.ts). Webhook handler updates order status from the parsed body without constructEvent / signature verification (app/api/stripe/route.ts). Interpret: a network retry can create two PaymentIntents for one checkout; an unverified webhook is not a trusted state change. Classify: Missing A1 + Missing C2. Ledger rows B* are N/A (P0). Severity: Critical — double-charge and untrusted "paid" are in-scope. Finding: A1+C2 | checkout + webhook routes | Critical | add intent-scoped idempotency + verify-then-process. Do not demonstrate a replay or spoof.

Phase 0 — Detect payment surfaces & scope (gates every later finding) [LOW freedom — run exactly]

Find the money paths and the provider first. Never report an in-house-ledger control as "Missing" on a pure merchant-integrator (N/A with a reason).

# Provider / SDK
rg -n --hidden -g '!node_modules' -i "stripe|paypal|braintree|adyen|square|payjp|paypay|razorpay|checkout\.com|worldpay|mollie|@stripe/|payment_intent|paymentintent" -l
# Money-movement verbs
rg -n -i "\b(charge|capture|authoriz|refund|void|payout|settle|chargeback|dispute|reversal)\b" -l
# Webhook endpoints + signature
rg -n -i "webhook|/webhooks?|constructEvent|verifyHeader|Stripe-Signature|x-signature|hmac" -l
# Idempotency
rg -n -i "idempotenc|idempotency[-_]?key|Idempotency-Key" -l
# Ledger / accounting
rg -n -i "ledger|double[-_ ]entry|debit|credit|journal|balance|posting|book(keeping)?" -l
# Reconciliation / settlement
rg -n -i "reconcil|settlement|settle|payout report|balance_transaction|three[-_ ]way" -l
# Money type (float smell = red flag)
rg -n -i "amount|price|money|currency|minor[-_ ]unit|cents" -g '*.{ts,tsx,js,py,go,java,rb,cs,sql}' -l
# Fraud / risk / SCA
rg -n -i "fraud|risk|velocity|3ds|3-?d ?secure|sca|radar|device.?fingerprint" -l
# Card-data smell (should find NOTHING raw)
rg -n -i "card[-_ ]?number|\bpan\b|cvv|cvc|card\.number|primary_account" -l

Record a payment profile and pick the tier — apply only in-scope rows:

If there is no money movement (no PSP, no charge/ledger paths), stop and report reduced applicability. If card data appears in the last rg above, that is Critical, report immediately.

Phase 1 — Research (version-anchored, provider-aware) [HIGH freedom]

Follow /research. Anchor to the installed SDK version and the provider's current API (e.g. Stripe PaymentIntents, not the legacy Charges API). Confirm the current-year shape of the controls before judging the code.

When the provider is Stripe and the Stripe MCP is connected, use it as the authoritative source. This pack does not bundle it (mcp/ ships Firecrawl, Context7 and Supabase); connect Stripe's own MCP or plugin, or else follow /research against docs.stripe.com for the same checks:

• Concepts / best practice (idempotency keys, webhook signature verification, PaymentIntents lifecycle, SCA/3DS2, Radar) — search_stripe_documentation with search_only_api_ref: false.
• Exact API params the integration should be sending — stripe_api_search then stripe_api_details on the operation id (confirm PaymentIntent.create is called with an idempotency key and amounts in minor units).

For PayPal / Square / Adyen / PayPay / Braintree / others, the Stripe MCP does not apply — use /research against the provider's official docs. Never invent a param or endpoint the provider doesn't expose.

Phase 2 — Payment correctness matrix [HIGH freedom]

For each in-scope row, mark Implemented / Partial / Missing / N/A with file:line, a one-line "why it bites in prod", and the fix-delegate. Full detection commands, good-vs-red-flag signals, and fix targets are in references/checklist.md — load it and work the applicable groups.

Groups and the tier that owns them:

• A. Money-movement correctness (P0+) — A1 idempotency on every mutation, A2 dedup / stored result, A3 payment state machine, A4 money as integer minor units, A5 multi-currency & FX
• B. Ledger & data integrity (P1 internal balances · P2 full ledger) — B1 double-entry, B2 append-only / immutable, B3 balance derived & snapshotted separately, B4 auditability, B5 schema for scale
• C. Async orchestration & webhook delivery (P0+) — C1 sync-auth vs async-everything, C2 webhook signature verified, C3 event-id dedup + 200-then-process, C4 atomic state+ledger+outbox, C5 pull-based recovery for stuck payments, C6 refund/dispute/payout as saga
• D. Reconciliation & settlement (P1/P2) — D1 automated daily reconciliation, D2 3-way match + break report, D3 discrepancy handling, D4 safety brake
• E. Fraud, risk & SCA (P0 delegate · P1/P2 in-house) — E1 risk scoring pre-auth, E2 3DS2 / SCA step-up, E3 fraud-service failure policy, E4 chargeback / dispute monitoring, E5 AML / sanctions
• F. Compliance & security — PCI DSS v4.0.1 (all tiers) — F1 never store/log PAN or CVV, F2 tokenization, F3 key rotation & secret handling, F4 access audit
• G. Error handling & resilience (P0+) — G1 PSP/bank API timeout + retry + circuit breaker, G2 bulkhead / pool isolation, G3 partial-write safety, G4 graceful degradation Full matrix (control | bites in prod if missing | fix via): references/matrix.md.

Rules:

• Evidence or it didn't happen — every verdict cites file:line or "searched, none found".
• N/A is first-class — record why (scope tier), don't drop the row.
• No double-counting — link per-call resilience to audit-resilience, PCI to audit-security.
• No PoCs — do not write replay, spoof, or card-testing procedures.

Phase 3 — Prioritized report (read-only) [HIGH freedom]

One markdown report: header (provider(s), scope tier + evidence, in-scope groups, N/A rows + why); a Critical table (finding | control | file:line | why it bites | fix via) for money loss, double-charge, and card-data exposure; a High / Medium matrix summary per group (Implemented | Partial | Missing | N/A | fix via); and a lift-to-production roadmap ordered by blast radius — idempotency → verify + dedup webhooks → atomic state+ledger+outbox and pull-based recovery → [P1/P2] double-entry ledger + daily reconciliation → tokens-only + key rotation + access audit → breaker/bulkhead + fraud fail-policy. Report skeleton with example rows: references/report-template.md.

Forbidden: declaring "production-grade" from passing tests alone; flagging P2-only controls (in-house ledger, 3-way bank match, sharding) against a P0 merchant integrator; re-auditing per-call timeouts/retries audit-resilience owns; assigning any severity below Critical to a double-charge, lost-money, or PAN-exposure finding; recommending a refund/payout saga without compensation logic; writing exploit or payment-fraud PoCs; editing payment code — this skill reports; remediation is human-reviewed, runs at high effort, and keeps the approved-plan-execution rule's STOP-and-ask on payment code.

Self-critique before reporting [LOW freedom — do not skip]

1. Evidenced — file:line or "searched, none found", not "webhooks are probably signed"
2. No PoC — finding names the missing control; no replay, spoof, or card-testing steps
3. Tier respected — P2 ledger / 3-way match are N/A on P0, with why
4. Severity justified — double-charge, lost money, or PAN/CVV = Critical
5. Right owner — IAP / StoreKit → audit-monetization-iap; per-call retry → audit-resilience
6. Nothing edited — STOP-and-confirm; human-reviewed remediation only

Related

• audit-resilience — per-call idempotency keys, timeouts, retry+backoff+jitter, circuit breaker, cancellation
• audit-security — PCI/PAN handling, webhook auth, secrets, key rotation, injection, access logging
• audit-db-schema — double-entry ledger schema, append-only constraints, money types, partitioning
• audit-backend-architecture — outbox/saga/breaker structure and topology fit (this skill checks payment correctness on top)
• plan-data-integrity — append-only/immutability guarantees and destructive-op safety
• plan-secrets-audit — rotate vs relocate provider API/signing keys
• backend-patterns / backend-patterns/references/architecture-patterns.md — implement idempotency, outbox, saga, state machine
• data-pipeline — reconciliation/settlement ingestion jobs
• the Stripe plugin skills (stripe-best-practices, connect-recommend, upgrade-stripe) — Stripe-specific integration
• audit-monetization-iap — StoreKit / Play Billing / RevenueCat (not Stripe/web)
• complete-everything — close audited gaps to done with verification (human-reviewed for payment code)

Further reading

• Payment correctness matrix A1–G4
• Per-control detection commands and signals
• Prioritized report template