A brute-force attack repeatedly tries candidate credentials to gain unauthorized access. The term is often used broadly, but password guessing, password spraying, and credential stuffing work differently—and recognizing the difference helps users and service operators choose the right defenses.
What is a brute-force attack?
In a password-guessing attack, an attacker repeatedly tests possible passwords against an account until one works or the service blocks further attempts. The guesses may come from common passwords, a wordlist, or other candidate combinations. OWASP’s Credential Stuffing Prevention Cheat Sheet describes related login attacks that are sometimes grouped under the wider brute-force label.
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The important distinction is whether the attacker is guessing passwords or testing credentials already known to have been exposed. Those approaches create different patterns and call for layered defenses.
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How brute force, password spraying, and credential stuffing differ
| Technique | What the attacker tries | How attempts are distributed |
|---|---|---|
| Password guessing | Multiple candidate passwords against an account. | Often many attempts against one account. |
| Password spraying | One or a few common passwords against many accounts. | Spread across accounts, which can avoid controls triggered by repeated failures on just one account. |
| Credential stuffing | Username-and-password pairs exposed in a separate breach. | Tested against another service in the hope that people reused credentials. |
| Distributed guessing | Candidate passwords, as in password guessing. | Attempts are spread over multiple IP addresses, potentially evading controls that count only by IP. |
Credential stuffing uses credentials that may already be valid somewhere; password guessing tests candidate passwords. OWASP discusses credential stuffing separately because its source of credentials and defensive considerations differ from ordinary guessing.
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What signs might indicate an attack?
For an individual, an unfamiliar sign-in alert, repeated failed-login notifications, or an unexpected account lockout can be a reason to investigate. None proves by itself that an attack occurred or that an account was compromised. Check activity through the service’s official website or app rather than following links in an unexpected message.
For operators, OWASP identifies signals that can help surface suspicious login activity, including:
- A new browser, device, IP address, or unusual location.
- One network address attempting to access multiple accounts.
- Scripted or unusually high-volume login attempts.
These indicators need context. A legitimate user may travel or replace a device, while distributed attacks may avoid obvious volume from any one address. Monitoring and investigation matter more than treating one alert as a definitive diagnosis.
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If an alert or account activity looks unfamiliar, visit the provider directly and review its account activity and security settings. If you suspect a successful unauthorized sign-in, use the provider’s account recovery process and take practical steps to regain control:
- Change the affected password to a long, unique password that you do not use on another service.
- Revoke sessions or devices you do not recognize, if the provider offers that option.
- Enable multifactor authentication (MFA) if available.
- Review recovery email addresses, phone numbers, and other account recovery settings for changes you did not make.
How to prevent brute-force and related login attacks
For individuals: use unique passwords and MFA
Use a unique, long password for each account. A password manager can generate and store different passwords so that a password exposed in one breach is not reusable to access other services. NIST’s SP 800-63B-4 implementation FAQs say that verifiers must allow password managers and autofill. They also specify a 15-character minimum for a single-factor password at Authentication Assurance Level 1 (AAL1). These are NIST requirements for the stated scope, not a guarantee that every consumer website follows them.
Add MFA to important accounts. It requires another form of proof beyond the password, so a guessed or reused password alone may not be enough to sign in. MFA methods differ in how well they resist phishing; CISA recommends phishing-resistant MFA, including FIDO/WebAuthn-based options. See CISA’s guidance on implementing phishing-resistant MFA and its overview, More Than a Password. Where a service supports a security key, check the service’s enrollment options, supported standards, and your device’s connector before choosing one.
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For service operators: layer account-aware controls
Blocking repeated attempts is useful, but a control based only on IP address can miss distributed activity. OWASP’s guidance on weak lockout mechanisms highlights the need to consider how login protections behave, including the risk that overly aggressive lockouts can prevent legitimate users from accessing their accounts.
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- Apply risk-based challenges or step-up authentication. A CAPTCHA or additional verification can help when activity looks suspicious, but neither should be treated as a complete defense on its own.
- Monitor and alert on patterns. Track failed and successful logins, new devices or locations, high-volume activity, and attempts against multiple accounts. Review alerts in context.
- Design lockouts to resist abuse. A policy that lets an attacker repeatedly lock out other people can itself become a denial-of-service problem for those users.
OWASP’s credential-stuffing guidance repeats an analysis attributed to Microsoft that MFA would have stopped 99.9% of account compromises. The year and original basis are not established in the cited passage, so this should not be read as a universal or independently verified rate.
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