Comisión Nacional para la Protección y Defensa de los Usuarios de Servicios Financieros — Mexico's financial-consumer watchdog. It handles complaints against banks, SOFOMes and insurers, publishes the CAT Promedio and the RECO (the public registry of fees), and fines misleading advertising. If a bank charges you a fee that wasn't in your contract, a CONDUSEF complaint usually gets resolved within weeks.
CONDUSEF is your defense as a user. That's where you file a complaint when a bank or SOFOM overcharged you, didn't honor a contract or reported you wrong to the Buró. It's free and you don't need a lawyer; the process starts online or at their offices and the institution is required to respond.
Its real strength is the public registry: before signing with a SOFOM you don't know, look it up in CONDUSEF's SIPRES. If it isn't there, be suspicious. It also publishes the Average CAT and fee comparisons. CONDUSEF handles consumer protection; CNBV handles the bank's solvency — they don't overlap, they target different things.