Two Things Fintech Backends Get Wrong

Short summary

A payment infrastructure engineer outlines two common fintech backend mistakes: using unauthenticated encryption modes like CBC instead of AES-GCM, and relying on database constraints alone for webhook idempotency. The article explains padding oracle attacks, key rotation, and race conditions in duplicate webhook handling with concrete code examples.

• •AES-CBC without HMAC is vulnerable to padding oracle attacks; use AES-256-GCM for reversible encryption
• •Key management with versioned keys is the hard part, not the encryption call
• •Unique constraints on event IDs are necessary but not sufficient for webhook idempotency due to race conditions

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