Passwords are one of the most common ways to protect online accounts.
Users enter a username and password, and the application verifies whether the credentials are correct. However, attackers may repeatedly try different password combinations until they find one that works.
This type of attack is known as a brute-force attack.
Brute-force attacks can target login pages, admin panels, APIs, SSH services, and other authentication systems. Without proper security controls, repeated login attempts can eventually lead to account compromise.
Fortunately, applications can significantly reduce this risk by combining several security measures.
In this article, we’ll explain what brute-force attacks are, how they work, and how to prevent brute-force attacks using rate limiting, account protection, multi-factor authentication, secure passwords, monitoring, and other best practices.
What Is a Brute-Force Attack?
A brute-force attack is an attempt to gain access to an account or system by repeatedly trying different passwords or credentials.
The basic idea looks like this:
Attacker
↓
Try Password
↓
Incorrect
↓
Try Another Password
↓
Incorrect
↓
Repeat
An attacker may use automated tools to make a large number of login attempts.
The goal is to eventually find valid credentials.
For example, an attacker may repeatedly attempt to log in to:
example.com/login
using a known username and many possible passwords.
How Does a Brute-Force Attack Work?
A typical brute-force attack involves repeated authentication attempts.
Login Attempt
↓
Password Correct?
↙ ↘
No Yes
↓ ↓
Try Again Access Account
Attackers often automate this process.
Instead of manually entering passwords, automated software can send many authentication requests.
This makes it important for applications to detect and limit repeated failed attempts.
Common Types of Brute-Force Attacks
Brute-force attacks can take different forms.
1. Traditional Brute-Force Attacks
An attacker repeatedly tries many possible password combinations.
For example:
password1
password2
password3
password4
The attacker continues until a valid password is found.
2. Dictionary Attacks
A dictionary attack uses a list of common passwords and words.
For example:
123456
password
qwerty
welcome
Attackers often start with commonly used passwords because they have a higher chance of success.
3. Credential Stuffing
Credential stuffing uses username and password combinations exposed in previous data breaches.
For example:
Email + Password
↓
Try on Another Website
This attack can succeed when users reuse the same password across multiple services.
4. Password Spraying
Password spraying works differently from repeatedly attacking one account.
Instead, an attacker may try a small number of common passwords across many accounts.
For example:
Common Password
↓
User 1
User 2
User 3
User 4
This approach can sometimes avoid simple account-based lockout rules.
Why Are Brute-Force Attacks Dangerous?
A successful brute-force attack can allow an attacker to access an account.
Depending on the system, this may lead to:
- Account takeover
- Data theft
- Unauthorized transactions
- Identity misuse
- Access to internal systems
- Further attacks against an organization
Therefore, authentication endpoints should always be protected.
1. Implement Rate Limiting
Rate limiting is one of the most effective protections against repeated login attempts.
It limits how many requests can be made within a specific period.
For example:
5 Login Attempts
Within 1 Minute
The flow might look like this:
Login Request
↓
Rate Limit Check
↓
Limit Reached?
↙ ↘
No Yes
↓ ↓
Continue Temporarily Block
Rate limits can be applied based on several signals, such as:
- Account identifier
- IP address
- Device or client characteristics
- Endpoint
- Combination of multiple signals
Using multiple signals can help reduce simple bypasses.
2. Add Progressive Delays
Instead of immediately locking an account, an application can gradually increase the delay after repeated failed attempts.
For example:
First Failure
↓
No Delay
Second Failure
↓
Short Delay
More Failures
↓
Longer Delay
This slows down automated attacks while avoiding unnecessary disruption for users who make an occasional mistake.
A progressive delay can make large-scale password guessing significantly more expensive.
3. Use Multi-Factor Authentication
Multi-factor authentication adds another verification step.
Even if an attacker discovers a user’s password, they may still need another factor.
For example:
Password
+
Authentication App
The process may look like:
User Enters Password
↓
Password Valid?
↓
Second Verification Required
↓
Access Granted
Multi-factor authentication can significantly reduce the impact of stolen or guessed passwords.
4. Encourage Strong and Unique Passwords
Weak passwords are easier to guess.
For example:
123456
password
admin
Users should use strong and unique passwords.
A good password should:
- Be difficult to guess
- Not reuse common passwords
- Be unique for important accounts
- Be stored using a trusted password manager when possible
Applications should also avoid requiring predictable password patterns that encourage users to make only small changes to old passwords.
Long passphrases can be easier for users to remember while still providing strong protection.
5. Block Known Compromised Passwords
Applications can check whether a password is known to have appeared in public data breaches.
If a password has already been widely exposed, it should not be accepted for a new account.
The process can be:
User Creates Password
↓
Password Safety Check
↓
Known Compromised?
↙ ↘
Yes No
↓ ↓
Reject Accept
This helps protect users who may otherwise choose commonly exposed passwords.
6. Use Secure Password Hashing
Applications should never store passwords as plain text.
For example, this is unsafe:
Password: myPassword123
Instead, passwords should be stored using a strong password hashing algorithm designed for password storage.
The process is:
Password
↓
Password Hashing Algorithm
↓
Stored Hash
During login:
Entered Password
↓
Verify Against Stored Hash
↓
Match?
Modern password hashing functions are intentionally designed to make large-scale password guessing more expensive.
7. Add CAPTCHA Carefully
A CAPTCHA can help distinguish automated requests from legitimate users.
For example:
Multiple Failed Attempts
↓
Additional Verification
↓
Continue Login
However, CAPTCHA should usually be introduced based on suspicious behavior rather than shown unnecessarily to every user.
This approach can improve security while reducing friction for legitimate users.
8. Use Account Lockout Carefully
After multiple failed attempts, an application may temporarily restrict further login attempts.
For example:
Too Many Failed Attempts
↓
Temporary Restriction
↓
Wait Before Trying Again
However, permanent or overly aggressive account lockouts can create another problem.
An attacker could intentionally trigger lockouts against many legitimate users.
Therefore, temporary and carefully designed restrictions are often safer than simple permanent lockouts.
9. Protect Login APIs
Brute-force attacks do not only target traditional login pages.
Attackers can also target authentication APIs.
For example:
POST /api/login
The same security controls should be applied to APIs.
These include:
- Rate limiting
- Input validation
- Monitoring
- Progressive delays
- MFA where appropriate
API authentication endpoints should be treated as high-risk endpoints.
10. Monitor Failed Login Attempts
Monitoring can help identify brute-force attacks.
For example, an application may detect:
Normal User
↓
1–2 Failed Attempts
compared with:
Suspicious Activity
↓
Hundreds of Failed Attempts
Useful signals can include:
- Repeated failed logins
- Attempts against many accounts
- Attempts from unusual sources
- Sudden traffic spikes
- Repeated password failures
Monitoring helps security teams respond before an attack causes significant damage.
11. Use Generic Login Error Messages
Login responses should not reveal unnecessary information.
For example, avoid telling users:
This email does not exist.
or:
The password is incorrect.
These messages can help attackers discover valid usernames or email addresses.
Instead, use a more generic message:
Invalid email or password.
This reduces the risk of account enumeration.
12. Protect Against Account Enumeration
Account enumeration occurs when attackers try to discover which usernames or email addresses exist.
For example:
Email A
↓
Account Exists
Email B
↓
Account Does Not Exist
Different responses can reveal information.
A safer approach is to provide consistent responses and similar processing behavior where practical.
For example:
Invalid login credentials.
Whether the account exists or not.
13. Use IP-Based Controls, but Do Not Rely Only on IP Addresses
Blocking an IP address after repeated failures can be useful.
However, IP addresses alone are not enough.
Attackers may:
- Use distributed infrastructure
- Change networks
- Use large numbers of IP addresses
At the same time, legitimate users may share an IP address.
Therefore, combine IP-based controls with:
- Account-based limits
- Request behavior analysis
- Device and client signals
- Risk-based authentication
A layered approach is stronger than relying on one signal.
14. Use HTTPS Everywhere
Authentication credentials should always be transmitted over HTTPS.
Without encryption, login information could be exposed during transmission.
The secure flow is:
User
↓ HTTPS
Application
HTTPS protects data while it travels between the client and the server.
However, HTTPS does not replace rate limiting or other authentication protections.
15. Secure Password Reset Endpoints
Password reset functionality can also become a target.
For example, an attacker may repeatedly request password reset emails or attempt to guess reset codes.
Therefore, password reset systems should include:
- Rate limits
- Expiring reset tokens
- One-time use tokens
- Secure token generation
- Monitoring
- Generic responses where appropriate
A reset token should also expire quickly and become invalid after use.
16. Require Reauthentication for Sensitive Actions
Logging in once should not always be enough for every action.
For highly sensitive operations, applications can require additional verification.
For example:
User Logged In
↓
Attempts Sensitive Action
↓
Reauthentication Required
Sensitive actions may include:
- Changing a password
- Changing an email address
- Adding a new payment method
- Viewing highly sensitive information
This reduces the impact of a stolen session or compromised account.
17. Use Security Alerts
Users can sometimes help detect account compromise.
Applications can send alerts when suspicious events occur.
For example:
New Login Detected
↓
Notify User
Other events may include:
- Password changes
- MFA changes
- Recovery email changes
- New device sign-ins
Users should have a clear way to report suspicious activity.
18. Use Risk-Based Authentication
Modern applications can evaluate the context of a login attempt.
For example:
Login Attempt
↓
Evaluate Risk
↓
Low Risk → Continue
Higher Risk → Additional Verification
Risk signals may include unusual behavior or unexpected changes in login patterns.
This allows applications to add stronger verification only when necessary.
A Secure Login Flow
A modern authentication flow can look like this:
User Login Request
↓
Rate Limit Check
↓
Validate Credentials
↓
Too Many Failures?
↙ ↘
Yes No
↓ ↓
Temporary Limit Continue
↓
Credentials Valid?
↙ ↘
No Yes
↓ ↓
Record Failure MFA Check
↓
Create Session
↓
User Authenticated
This combines several security layers.
Brute-Force Attack Prevention Checklist
Before launching an authentication system, review this checklist:
- Implement rate limiting.
- Add progressive delays for repeated failures.
- Use multi-factor authentication.
- Encourage strong, unique passwords.
- Reject known compromised passwords.
- Use strong password hashing.
- Add CAPTCHA when suspicious behavior is detected.
- Design account lockouts carefully.
- Protect login APIs.
- Monitor failed login attempts.
- Use generic login error messages.
- Protect against account enumeration.
- Combine IP and account-based controls.
- Use HTTPS.
- Secure password reset functionality.
- Require reauthentication for sensitive actions.
- Notify users about important account events.
Common Mistakes to Avoid
Several mistakes can weaken brute-force protection.
Relying Only on Password Complexity
Strong passwords are important, but they are not enough by themselves.
Use additional protections such as MFA and rate limiting.
Using Only IP Blocking
Attackers can use many different IP addresses.
Combine IP controls with account-based and behavior-based protections.
Permanent Account Lockouts
An attacker may intentionally lock legitimate users out of their accounts.
Temporary restrictions and progressive delays are often better.
Revealing Too Much in Error Messages
Detailed login errors can help attackers discover valid accounts.
Use generic messages where possible.
Storing Passwords Incorrectly
Never store passwords in plain text or using outdated password storage practices.
Use a modern password hashing algorithm designed for password storage.
Final Thoughts: How Do You Prevent Brute-Force Attacks?
There is no single protection that can stop every brute-force attack.
The strongest approach is to use multiple layers.
A good strategy is:
Rate Limiting → Progressive Delays → Strong Password Protection → MFA → Monitoring → Risk-Based Controls
Each layer makes automated attacks more difficult.
For example, even if an attacker can send repeated login requests, rate limiting slows them down. If credentials are stolen, MFA can provide another barrier. Monitoring can then help identify suspicious behavior.
Conclusion
Brute-force attacks target one of the most important parts of an application: authentication.
Without proper protection, repeated automated login attempts can lead to account compromise.
Fortunately, developers can significantly reduce this risk by implementing rate limiting, progressive delays, MFA, secure password storage, compromised password checks, monitoring, and carefully designed authentication controls.
The most important principle is simple:
Do not rely on one security measure.
A layered authentication strategy provides stronger protection against brute-force attacks and helps create a safer experience for users.
