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.