Credential Stuffing Attacks: How to Detect and Block Them with IP and Network Signals

Introduction

Credential stuffing is one of the quietest threats a login page faces. Attackers take usernames and passwords leaked in old breaches and try them on your site, betting that some of your users reused the same password somewhere else. Each attempt looks like a normal login, so the attack often hides in plain sight.

The scale is bigger than most teams expect. Verizon's research team looked at real authentication logs and found that a large share of everyday login traffic is not coming from real users at all.

"In analyzing SSO provider logs, we found that the median daily percentage of credential stuffing accounted for 19% of all authentication attempts." Source: Verizon 2025 DBIR credential stuffing research

What Credential Stuffing Is and Why It Works

A credential stuffing attack uses lists of stolen username and password pairs, often called combo lists, and replays them against your login form at scale. Attackers do not guess passwords. They test real ones that already worked somewhere else.

It works for two simple reasons. People reuse passwords across services, and many accounts still have no second factor. Verizon's same research found that, in the median case, only 49% of a user's passwords across different services were distinct from each other. That leaves attackers plenty of matches to find.

How Credential Stuffing Differs From Brute Force

Brute force attacks hammer one account with many password guesses. That pattern is noisy and easy to catch with account lockouts.

Credential stuffing flips the pattern. Each account usually gets one or two attempts with a password that was valid at some point. Because the volume is spread across thousands of accounts, simple per account lockout rules rarely fire. That is why you need signals that look at where the traffic comes from, not only at how many times one account failed.

The Real Cost of a Successful Attack for Your Business

A single successful login can lead to account takeover. From there, attackers drain stored balances, redeem loyalty points, place orders with saved cards, or harvest personal data.

The damage spreads beyond the stolen value:

• Chargebacks and refunds when attackers use saved payment methods
• Support costs from locked out and angry customers
• Reputation harm when users believe your site was breached
• Infrastructure costs from absorbing waves of fake login traffic

For a deeper look at what happens after a login is compromised, see how Greip supports account takeover prevention.

Warning Signs That Your Login Page Is Under Attack

You can often spot credential stuffing in your own logs before it turns into account takeover. Watch for:

• A sudden rise in failed logins across many different accounts
• Login success rates that drop sharply while total attempts climb
• Many attempts with usernames that do not exist in your system
• Bursts of traffic from hosting providers instead of home or mobile networks
• Logins from countries where you have few or no customers
• The same small group of IP addresses or networks touching many accounts

None of these signs proves an attack on its own. Together, they tell a clear story.

Use IP Reputation to Spot Anonymized Traffic

Attackers rarely log in from their own home connection. They route traffic through VPNs, open proxies, Tor exit nodes, and rented servers to hide their origin and rotate addresses quickly.

Greip VPN and proxy detection shows when a login comes from a VPN, proxy, Tor, hosting network, or relay service. You can then decide how to respond. A privacy minded user on a VPN is not automatically a threat, so many teams use this signal to add a verification step rather than to block outright.

Check the Network Behind Each Login With ASN Data

Every IP address belongs to a network, identified by an Autonomous System Number (ASN). This tells you whether the traffic comes from a consumer internet provider, a mobile carrier, or a data center.

Real customers mostly log in from home broadband and mobile networks. Credential stuffing traffic often comes from cloud and hosting networks. Using Greip ASN network intelligence, you can see the network owner behind each login and apply stricter rules to networks that rarely host real users of your service.

ASN data is also useful during an incident. When many failed logins share one network, you can slow down that network without affecting everyone else.

Compare Login Location With Normal User Behavior

Location is another strong signal. If a customer always logs in from one city and suddenly appears on another continent, that deserves attention.

Greip IP location intelligence returns country, city, and network details for each IP address. You can compare this with the user's usual locations, their billing country, or the markets you serve. Large jumps in distance between logins in a short time, sometimes called impossible travel, are a classic sign of a stolen session or stolen credentials.

Add Velocity Limits Across IPs, Networks, and Accounts

Rate limiting per account is not enough against credential stuffing. Add limits at several levels:

• Per IP address, for failed logins over a short window
• Per network or ASN, to catch attacks spread over many IPs in one provider
• Per account, to slow down repeated attempts on the same user
• Globally, to alert your team when total failed logins jump

Velocity limits work best when they are adaptive. Instead of a hard block, you can add delays, require a challenge, or ask for a second factor once a threshold is crossed.

Build a Risk Based Login Flow That Respects Real Users

The goal is not to block every unusual login. It is to match friction to risk. A practical flow looks like this:

1. Low risk login from a known network and location: let it through.
2. Medium risk, such as a VPN or a new country: ask for a second factor or an email confirmation.
3. High risk, such as a hosting network plus many failed attempts nearby: block or require a reset.

This approach protects accounts while keeping the experience smooth for most customers. It also gives your support team clear reasons behind every decision.

Strengthen Accounts So Stolen Passwords Matter Less

Network signals help you catch attacks. Account hardening makes the stolen passwords less useful in the first place:

• Offer and encourage multi factor authentication, or passkeys where possible
• Check new passwords against known breached password lists
• Notify users of logins from new devices or locations
• Force a password reset when an account shows signs of compromise
• Avoid revealing whether a username exists in login error messages

Each step reduces the payoff of a combo list, which makes your site a less attractive target.

How to Respond When an Attack Is Underway

When you see a spike, act in stages:

1. Confirm the pattern in your logs using the warning signs above.
2. Tighten rules for anonymized traffic and hosting networks first.
3. Add challenges or second factor prompts for risky logins.
4. Identify accounts that logged in successfully during the attack window.
5. Reset passwords and notify affected users.
6. Review what worked and turn temporary rules into lasting ones.

Keep a record of the networks and patterns you saw. Attackers often come back with the same tools.

Frequently Asked Questions

What is credential stuffing?

Credential stuffing is an attack where criminals take username and password pairs leaked in earlier breaches and test them on other websites. It succeeds when people reuse the same password across several services.

How is credential stuffing different from brute force?

Brute force tries many passwords against one account. Credential stuffing tries real leaked passwords across many accounts, usually with only one or two attempts each, which makes it harder to catch with simple lockouts.

How can I detect a credential stuffing attack?

Look for a rise in failed logins across many accounts, falling success rates, attempts on usernames that do not exist, and traffic from VPNs, proxies, or hosting networks. Combining these signals gives a reliable picture.

How do I prevent credential stuffing?

Use layered defenses: multi factor authentication, breached password checks, adaptive rate limits, and IP risk signals such as VPN and proxy detection, ASN data, and location checks to add friction to risky logins.

Does blocking VPNs stop credential stuffing?

Not fully. Attackers also use residential proxies and rotating IPs, and some real users rely on VPNs. Treat anonymizer detection as one risk signal and respond with extra verification rather than blocking everyone.

Conclusion

Credential stuffing thrives on reused passwords and on login pages that judge every attempt in isolation. Verizon's finding that a median of 19% of daily authentication attempts were credential stuffing shows this is everyday traffic, not a rare event.

The strongest defense layers account hardening with network context. IP reputation, ASN data, and location checks help you see where each login really comes from, so you can add friction only where it is needed. Explore how Greip can feed these signals into your login flow with VPN and proxy detection, and keep real customers moving while attackers hit a wall.