Hunt Fintech Graphql

Why Fintech GraphQL Is a Different Risk Class

Generic GraphQL bugs (IDOR, mass assignment, introspection, batching abuse — see hunt-graphql) still apply here, but the blast radius changes completely: a resolver bug in a SaaS app leaks data, the same class of bug in a ledger mutation moves money. Three properties make fintech GraphQL backends a distinct hunting surface:

• Money-movement mutations are almost always resolvers over a double-entry ledger. A single GraphQL mutation (transferFunds, redeemRewards, withdrawToBank) can trigger multiple ledger writes (debit + credit + fee) that must be atomic. GraphQL's flexible input shape and alias batching make it easy to desynchronize those writes.
• Decimals are attacker-controlled input, not display formatting. Amounts, exchange rates, interest, and rewards points are usually passed as GraphQL scalars (Float, String, custom Decimal/Money scalar). How the resolver parses and rounds that value is exploitable surface in its own right — this barely exists in non-financial GraphQL APIs.
• KYC/PII fields sit next to routine account fields in the same type. User or Account types commonly expose ssnLast4, routingNumber, kycStatus, governmentIdUrl, or linkedBankAccount alongside displayName and email — one missing field-level authorization check on a type used everywhere in the schema fans out to every query that touches it.

Attack Surface Signals

URL / schema naming patterns (in addition to hunt-graphql's generic /graphql list):

/graphql/ledger
/graphql/payments
/api/wallet/graphql
/internal/ledger-graphql
/banking/graphql

Field/type names worth grepping schema introspection or JS bundles for:

balance, availableBalance, pendingBalance, ledgerEntry, ledgerEntries
transferFunds, withdraw, redeem, topUp, reverseTransaction, adjustBalance
kycStatus, ssnLast4, routingNumber, accountNumber, governmentIdUrl
quoteExchangeRate, interestAccrued, rewardsPoints, portfolioValue
idempotencyKey, clientMutationId

Tech-stack tells specific to this vertical:

• Plaid/Stripe/Dwolla/Marqeta wrapped behind an internal GraphQL gateway (bankLink, plaidLinkToken mutations)
• Apollo Federation with a dedicated ledger or payments subgraph — check for the subgraph's own introspection being reachable directly, bypassing the gateway's stitched-down schema
• Custom Money/Decimal/BigDecimal GraphQL scalar in the schema (scalar Money) — the parser for this scalar is worth fuzzing directly

Run hunt-graphql's discovery + introspection methodology first to get the schema; everything below assumes you already have (or have partially enumerated) a schema with money-movement types.

Step-by-Step Hunting Methodology

1. Map every mutation that touches balance, whether directly or as a side effect. Not just transfer*/withdraw* — also redeemRewards, applyCoupon, upgradeTier, closeAccount (often refunds a balance), disputeTransaction (often provisionally credits).

2. For each money-movement mutation, identify the ledger write shape. Does one mutation call produce one ledger entry or several (debit sender, credit receiver, fee entry)? Multi-entry writes are the ones worth racing — see Stage 4.

3. Test idempotency-key handling. Send the identical mutation (same idempotencyKey / clientMutationId) twice, back-to-back and with a delay. A ledger write on the second call means idempotency isn't enforced server-side — replay = double-execute.

4. Test decimal/precision edge cases on every amount-accepting argument — see Payload section. Confirm server-side rounding matches client-displayed rounding; a mismatch is directly monetizable.

5. Probe cross-account IDOR on account/portfolio node IDs, same as hunt-idor/hunt-graphql, but specifically test whether a transferFunds-style mutation validates that the source account belongs to the authenticated caller — not just that some account with that ID exists. This is the fintech-specific IDOR: authz on the source of a debit is easy to forget when authz on the destination of a credit was correctly implemented (crediting an arbitrary account "looks safe" to a developer; debiting one clearly isn't, so it gets checked — but sometimes only one direction does).

6. Check field-level authorization on KYC/PII fields by querying the shared User/Account type from every context that returns it — not just the profile screen. A transaction type that embeds counterparty { ssnLast4 } is a common place for the check to be missing, because the developer authorized the top-level transaction query but didn't re-check field access on the nested counterparty.

7. Look for admin-tier mutations reachable via mass assignment, not just a missing auth check — e.g. an input object with a client-settable status or override field that a normal user's mutation shouldn't expose but that the resolver accepts anyway (updateTransaction(input: {id, status: "COMPLETED", amount: "..."})).

8. Test currency-argument consistency. Send a transfer/quote mutation with mismatched sourceCurrency/targetCurrency combinations the UI never generates (e.g. self-transfer with a currency conversion) and check whether the resolver's FX-rate lookup and the ledger write use the same rate — a TOCTOU window here is a direct arbitrage bug.

9. Combine alias batching with money-movement mutations to test for double-spend — see hunt-race-condition for the parallel-HTTP escalation once alias batching alone confirms the resolver isn't serializing writes per-account.

Payload & Detection Patterns

Idempotency-key replay test:

mutation {
  transferFunds(input: {
    idempotencyKey: "test-key-001"
    sourceAccountId: "acc_1"
    destAccountId: "acc_2"
    amount: "10.00"
  }) { transactionId status }
}

Send twice with the identical idempotencyKey. Two successful, distinct transactionId values = idempotency not enforced.

Decimal-precision / rounding probes:

mutation { transferFunds(input: {sourceAccountId:"acc_1", destAccountId:"acc_2", amount: "0.001"}) { transactionId } }
mutation { transferFunds(input: {sourceAccountId:"acc_1", destAccountId:"acc_2", amount: "9999999999999999.99"}) { transactionId } }
mutation { transferFunds(input: {sourceAccountId:"acc_1", destAccountId:"acc_2", amount: "1e2"}) { transactionId } }
mutation { transferFunds(input: {sourceAccountId:"acc_1", destAccountId:"acc_2", amount: "-50.00"}) { transactionId } }

Sub-cent amounts test truncate-vs-round handling (repeat N times to accumulate a rounding-error balance drift); scientific notation and oversized values test whether the Money/Decimal scalar parser falls back to a native float/int with overflow or precision-loss behavior; negative amounts test whether the resolver assumes sign server-side or trusts the client's.

Alias-batched double-spend probe (confirm before escalating to parallel HTTP):

mutation {
  r1: redeemRewards(input: {rewardId: "rwd_1", accountId: "acc_1"}) { success }
  r2: redeemRewards(input: {rewardId: "rwd_1", accountId: "acc_1"}) { success }
  r3: redeemRewards(input: {rewardId: "rwd_1", accountId: "acc_1"}) { success }
}

If more than one alias succeeds against a single-use reward/coupon, the resolver doesn't serialize per-account/per-resource writes within a batched request — see hunt-race-condition for combining this with parallel HTTP POSTs to confirm real double-spend impact.

Source-account authorization probe (asymmetric IDOR check):

mutation {
  transferFunds(input: {
    sourceAccountId: "VICTIM_ACCOUNT_ID"
    destAccountId: "ATTACKER_CONTROLLED_ACCOUNT_ID"
    amount: "1.00"
  }) { transactionId status }
}

Run as the attacker's own session/token. Success = the resolver validated the destination is attacker-controlled (obviously required) but never validated that the source belongs to the caller.

Nested field-level PII probe:

query {
  transaction(id: "txn_123") {
    amount
    counterparty { displayName ssnLast4 routingNumber kycStatus }
  }
}

Query as a user with no relationship to the counterparty beyond a shared transaction; success on the nested PII fields is the finding even if the top-level transaction query correctly scoped the transaction itself.

Mass-assignment probe on admin-shaped input fields:

mutation {
  updateTransaction(input: {id: "txn_123", status: "COMPLETED", amount: "0.01"}) { id status }
}

Send as a non-admin user against a mutation the client UI never exposes these fields for; a schema that accepts them anyway is mass assignment onto ledger state.