Stop Credential Stuffing with Prosopo

What is Credential Stuffing?

For the background, with real incidents and how stuffing compares with password spraying and brute force, read what is credential stuffing.

Credential stuffing is a type of cyberattack in which automated bots try to log in to user accounts using stolen username and password combinations. These credentials often come from earlier data breaches and are sold or shared on the dark web. Because many users reuse passwords across multiple platforms, even a breach on a single site can put accounts on other services at risk.

The attacks are automated: bots take large lists of credentials and try them against many websites in quick succession, which makes them efficient and hard to detect without specialised security measures. According to Have I Been Pwned, credential breaches are widespread, so protection needs to be in place before an attack starts.

Why Credential Stuffing is Dangerous

Credential stuffing is particularly dangerous because it exploits common user habits rather than technical vulnerabilities:

• Password Reuse: Even platforms that have never been breached are vulnerable if users recycle passwords from other sites.
• Financial Loss: Attackers can use access to perform fraudulent transactions, transfer funds, or make purchases.
• Data Theft: Sensitive information such as email addresses, personal details, and business data can be extracted.
• Trust Erosion: Users lose confidence in platforms that fail to protect their accounts, which can harm the brand's reputation.
• Stealthy Attacks: Bots often operate in ways that mimic normal user behaviour, which makes them harder to detect.

Credential stuffing attacks often go unnoticed until significant damage has been done.

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. Bot protection 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

• 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.