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Credential Stuffing Guide

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Combo lists and credential stuffing explained: the email:pass format, where aggregated combinations come from, how automated login attempts convert them into account access, and what the validity economics actually look like. Combos are raw ore - the checking and stuffing pipeline is the refinery.

TL;DR - Under 1% per attempt - scale and proxy cost decide profit, not cleverness.

THE FORMAT AND THE SOURCE

Standard combo syntax:

  • email:password - the base pair
  • email:password:source - which breach or stealer produced the row
  • token / session variants - extracted auth strings riding alongside credentials

Two source classes feed every list: breach aggregation (historic database dumps, years old, aggregated across incidents) and fresh stealer harvests (device-level captures, hours to days old). The freshness split is violent - old aggregations run 0.2-2% valid on modern services, fresh stealer-derived rows run 30-60% (material tiers cover the decay curves).

CREDENTIAL STUFFING MECHANICS

  • Input: combo list filtered to one target service
  • Automation: request workers replay login endpoints at volume, rotating residential exits per attempt (proxy hygiene rules apply directly)
  • Signal handling: responses bucket into success, invalid credential, rate-limited, MFA-required, CAPTCHA
  • Output: working session or validated credential, staged for monetization

Success rates sit low per attempt - commonly well under 1% against services with decent password hygiene - which is why scale and proxy cost, not cleverness, decide whether the economics work. Credential reuse does the rest: the same breached password rides on ten services, and one hit per thousand attempts is profit at scale.

THE CHECKING LAYER

Combo checkers pre-filter lists per service before stuffing runs (the checker anatomy applies - proxy layer, response parse, buckets). Checking costs less than stuffing attempts at full list size; sampling first (200-500 rows) tells you the working rate before you commit proxy budget to the full run.

WHERE COMBOS CONVERT

Account classMonetization path
Payment appsDirect balance - fastest and rarest (MFA density high)
Email (primary)Pivot point: resets unlock everything tied to the inbox
Streaming / SaaSResale of access, subscription pools sold in bulk
GamingInventory and wallet value, account resale markets
Loyalty / retailPoints and stored-card checkout at linked shops

Email access outranks every other class - one inbox converts to any service tied to it through reset flows (the ATO playbook walks the chain).

WHAT KILLS RUNS

  • Rate limiting and device-fingerprint challenges on the target - throughput dies, proxy burn climbs
  • Credential stuffing detected service-side ? password resets fire ? working rows die mid-run
  • MFA on the payment tier - stops the direct path, leaves only pivot routes
  • Stale lists - paying stuffing budget against dead rows is the classic way the economics invert

Freshness sampling first, per-service checking second, stuffing only what clears - sequence decides whether the campaign pays.

★ MEMBER BONUS — FIELD CHEAT SHEET

Hit reply to unlock the sheet - takes five seconds.

Post working-rate observations below - service class, list age, and success per thousand attempts.

— RELATED GUIDES —
 
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