Skip to main content

​
Overview

This vulnerability occurs when an application fails to limit the number of repeated authentication attempts from a single user or IP address within a short period. Without such restrictions, attackers can use automated tools to rapidly submit thousands or millions of password guesses (brute-force attack) or common passwords (dictionary attack) against valid usernames. This significantly increases the likelihood of an attacker guessing a correct password and gaining unauthorized access. 🔑➡️💥

​
Business Impact

Failure to restrict excessive authentication attempts directly leads to:
  • Account Takeover: Attackers can systematically guess weak or common passwords, compromising user accounts.
  • Denial of Service (Account Lockout): If the only defense is permanent account lockout after a few tries, attackers can intentionally lock out legitimate users. A better approach often involves temporary lockouts or CAPTCHAs.
  • Resource Consumption: Brute-force attacks can consume server CPU and network bandwidth.
  • Detection Evasion: Slow, distributed brute-force attacks might go unnoticed if basic attempt logging is the only defense.

Reference Details

CWE ID: CWE-307 Related CWEs: CWE-799 (Improper Control of Interaction Frequency), CWE-1216 (Lockout Errors) OWASP Top 10 (2021): A07:2021 - Identification and Authentication Failures Severity: High

​
Framework-Specific Analysis and Remediation

Brute-force protection is usually implemented via rate limiting (temporary blocking) or account lockout (temporary or requiring admin intervention). This often involves tracking failed login attempts per username and/or IP address in a cache (like Redis) or database. Many frameworks have plugins or built-in features, but custom logic is also common. Key Remediation Principles:
  1. Limit Attempts per IP: Track failed login attempts by IP address and temporarily block IPs with excessive failures (e.g., block for 5 minutes after 20 attempts).
  2. Limit Attempts per Username: Track failed login attempts per username and implement stronger protections like temporary account lockout (e.g., lock for 30 minutes after 5 failed attempts) or requiring a CAPTCHA.
  3. Use Exponential Backoff: Increase the lockout duration after repeated lockout events for the same account or IP.
  4. Secure Lockout Mechanism: Ensure the lockout mechanism itself cannot be abused for Denial of Service (e.g., avoid permanent lockouts triggered solely by failed attempts).
  5. Logging and Monitoring: Log failed login attempts and lockout events to detect attacks. Alert administrators to high rates of failures.
  • Python
  • Java
  • .NET(C#)
  • PHP
  • Node.js
  • Ruby

​
Framework Context

Using libraries like django-ratelimit, django-axes, Flask-Limiter to apply limits specifically to login views.

​
Vulnerable Scenario 1: No Login Attempt Limit (Django)

# accounts/views.py
from django.contrib.auth.views import LoginView

class UserLoginView(LoginView):
    template_name = 'accounts/login.html'
    # DANGEROUS: No mechanism to prevent an attacker from submitting
    # thousands of password guesses per minute for a given username.

​
Vulnerable Scenario 2: Unprotected API Endpoint (Flask)

# app.py (Flask)
@app.route('/api/search', methods=['GET'])
def api_search():
    query = request.args.get('q')
    # DANGEROUS: This search might be resource-intensive.
    # An attacker can flood this endpoint, causing DoS.
    # No rate limiting is applied.
    results = perform_complex_search(query)
    return jsonify(results)

​
Mitigation and Best Practices

Integrate a rate-limiting library. Apply decorators or middleware to the specific views/routes that need protection. Use Redis or Memcached for distributed rate limiting in multi-server environments.

​
Secure Code Example

# views.py (Django with django-axes - Configuration in settings.py)
# settings.py:
# INSTALLED_APPS = [..., 'axes', ...]
# AUTHENTICATION_BACKENDS = ['axes.backends.AxesStandaloneBackend', ...] # Or AxesModelBackend
# AXES_FAILURE_LIMIT = 5 # Lockout after 5 attempts
# AXES_COOLOFF_TIME = timedelta(minutes=15) # Lockout duration
# AXES_LOCKOUT_TEMPLATE = 'accounts/lockout.html' # Custom lockout page
# AXES_HANDLER = 'axes.handlers.database.AxesDatabaseHandler' # Store attempts in DB
# --- views.py needs no specific axes code if using backend ---
# LoginView will automatically use the Axes backend if configured.

# views.py (Django with django-ratelimit)
from django.contrib.auth.views import LoginView
from ratelimit.decorators import ratelimit
from django.utils.decorators import method_decorator

class UserLoginViewSecureRL(LoginView):
    template_name = 'accounts/login.html'
    # SECURE: Rate limit based on username in POST data + IP
    @method_decorator(ratelimit(key='post:username', rate='5/m', block=True, method='POST'), name='dispatch')
    def dispatch(self, *args, **kwargs):
        return super().dispatch(*args, **kwargs)

# app.py (Flask with Flask-Limiter)
from flask import Flask, request, jsonify # Added request, jsonify
from flask_limiter import Limiter
from flask_limiter.util import get_remote_address

app = Flask(__name__) # Assume app exists
limiter = Limiter(
    get_remote_address, # Use IP address for tracking
    app=app,
    default_limits=["200 per day", "50 per hour"],
    storage_uri="memory://" # Use "redis://localhost:6379" etc. for production
)

@app.route('/api/search-secure')
@limiter.limit("10 per minute") # SECURE: Specific limit for this endpoint
def api_search_secure():
    query = request.args.get('q')
    results = perform_complex_search(query) # Assume this exists
    return jsonify(results)

@app.route('/login-secure', methods=['GET', 'POST'])
# SECURE: Limit POST requests based on IP AND username form field
@limiter.limit("5 per minute", key_func=lambda: f"{get_remote_address()}:{request.form.get('username')}", methods=['POST'])
def login_secure():
    if request.method == 'POST':
        # ... login logic ...
        if login_successful: # Assume this variable is set
             limiter.reset() # Optional: Reset limit on success for this key
             # ...
        else:
             # Limit applied automatically by decorator
             flash('Invalid credentials') # Assume flash exists
    return render_template('login.html') # Assume render_template exists

​
Testing Strategy

Use automated tools (like wfuzz, ffuf, Burp Intruder) or simple scripts (e.g., curl in a loop) to send rapid, repeated requests to login endpoints, password reset forms, and resource-intensive API endpoints. Verify that after exceeding the configured limit, the server responds with an appropriate error (e.g., 429 Too Many Requests) and blocks further requests for a period. Test different keys (IP vs. user vs. form field).