Credential stuffing prevention: stop breached-password replay at login

How to prevent credential stuffing: the short answer

Credential stuffing is bots replaying username and password pairs leaked from other sites against your login. To prevent it, check every request to login, password reset and the auth API for automation, and stop it before the password is compared. The attacker then never learns which pairs work on your site. Multi-factor authentication protects the accounts whose passwords were right, but it does not stop the traffic.

Per-IP rate limits and IP reputation are not enough on their own. Stuffing runs through residential proxy networks, so each address makes only a few attempts and arrives from an ordinary household connection. For the background, with real incidents and how stuffing compares with password spraying and brute force, read what is credential stuffing.

How Prosopo Protects Against Credential Stuffing

Prosopo sits on every login endpoint and scores each request as it arrives. Credential-stuffing infrastructure differs from legitimate sign-ins on several signals, and Prosopo reads all of them:

• How the visitor behaves. Mouse movement, scroll rhythm and typing cadence. Automation repeats itself in ways a person does not, and that holds even when every other detail about the session looks ordinary.
• Where the traffic really comes from. Attackers route through home broadband connections and racks of real phones so each request looks like a separate ordinary visitor. Residential proxy detection recognises those networks even when the individual address has no history.
• Sudden surges on the login page. A normal login page sees steady traffic from your usual markets. A burst from hosting networks or unfamiliar regions gets extra checks, without touching real sign-ins.
• Detection that moves during the attack. When an operator changes tactics mid-attack, the platform adjusts while it is still happening rather than in a write-up afterwards.
• Invisible for real users and trusted agents. Legitimate logins (including password-manager and AI-agent sessions) pass with zero friction; suspicious sessions get a CAPTCHA challenge-response — proof-of-work or image — served via Invisible CAPTCHA; known-bad traffic is blocked before it reaches your login.

As a result, real users sign in without interruption, agents acting on a user's behalf are recognised, and credential-stuffing infrastructure is stopped at the login form.

Credential stuffing stopped at a financial provider

A financial provider that protects customer balances at login had 2,644,299 sessions scored, 23.67% stopped as automated, and 22,091 blocks in thirty days where the attacker had paid a commercial service to solve the challenge. When the asset behind a login has a market price, the attacker has a budget, and a harder puzzle costs your customer more than it costs them. The full figures are in the financial provider case study.

What to use

• Enterprise (Prosopo Protect) — bot checks on login, password reset and the auth API: a script tag on your pages, with edge protection on Cloudflare Workers or AWS Lambda@Edge.
• Bot Protection — flag high-risk login attempts so your backend can require step-up authentication on borderline scores.
• Access Control — block hosting networks, abusive ASNs and TLS fingerprints associated with breached-credential replay.
• Invisible CAPTCHA — stop automation at login without adding visible friction for legitimate users.

Credential stuffing is the main route to account takeover, which also covers password reset and account recovery. Comparing vendors? The vendor comparison page sets out how Prosopo, DataDome, HUMAN, Kasada and Arkose Labs each handle it, and what each costs.