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Overview

Improper Authentication occurs when an application incorrectly verifies, or fails to verify, a user’s identity. This is a broad category covering various flaws in the login mechanism itself, distinct from missing authentication entirely (CWE-306) or authorization issues. Examples include:
  • Accepting incorrect passwords or credentials under certain conditions.
  • Vulnerabilities related to type juggling or comparison errors (e.g., '0' == 0 being true in PHP).
  • Allowing authentication bypass through alternate channels or error conditions.
  • Incorrectly implementing cryptographic signature checks for token-based authentication (related to CWE-347).
  • Flaws in multi-factor authentication logic. 🔑❓

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Business Impact

Flaws in the authentication mechanism itself can allow attackers to bypass login procedures entirely:
  • Account Takeover: Attackers gain access to arbitrary user accounts without needing the correct credentials.
  • Privilege Escalation: Attackers might bypass authentication for administrative accounts.
  • Complete System Compromise: If authentication is bypassed for critical system functions.

Reference Details

CWE ID: CWE-287 Related CWEs: CWE-288 (Auth Bypass Alt Path), CWE-290 (Spoofing), CWE-304 (Missing Critical Step) OWASP Top 10 (2021): A07:2021 - Identification and Authentication Failures Severity: Critical

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Framework-Specific Analysis and Remediation

While modern frameworks provide robust authentication libraries (Django Auth, Spring Security, ASP.NET Core Identity, Passport.js, Devise), vulnerabilities often arise from:
  1. Custom Implementations: Developers building their own authentication logic introduce subtle flaws.
  2. Misconfiguration: Incorrectly configuring framework authentication (e.g., allowing null passwords, misconfiguring providers).
  3. Integration Errors: Flaws in how different authentication systems (e.g., LDAP, OAuth, SAML) are integrated.
Key Remediation Principles:
  1. Use Framework Defaults: Rely on the built-in, vetted authentication mechanisms of your framework whenever possible.
  2. Strong Credential Comparison: Use constant-time comparison functions for passwords and tokens to prevent timing attacks. Frameworks usually handle this internally.
  3. Type Safety: Use strict comparisons (e.g., === in PHP/JS, .equals() in Java, strong typing) when checking credentials or states.
  4. Fail Securely: Ensure error conditions during login do not accidentally grant access.
  5. Multi-Factor Authentication (MFA): Implement MFA for enhanced security.

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Framework Context

Custom authentication backends in Django or manual password checking in Flask that contain logical errors.

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Vulnerable Scenario 1: Custom Django Backend Error

A custom backend tries to handle multiple user types but allows bypass if an error occurs.
# myapp/auth_backends.py
from django.contrib.auth.backends import BaseBackend
# Assume StandardUser, AdminUser models exist and inherit from AbstractBaseUser or similar
from .models import StandardUser, AdminUser
from django.contrib.auth import get_user_model # Use get_user_model if applicable

class CustomAuthBackend(BaseBackend):
    def authenticate(self, request, username=None, password=None):
        user = None
        try:
            # Try finding as admin first (example logic)
            user = AdminUser.objects.get(username=username)
        except AdminUser.DoesNotExist:
            try:
                user = StandardUser.objects.get(username=username)
            except StandardUser.DoesNotExist:
                return None # No user found
        except Exception as e:
            # DANGEROUS: If DB error occurs finding AdminUser,
            # it might skip password check and return None, OR worse,
            # flawed logic might proceed assuming a user was found.
            print(f"Error finding user: {e}")
            # return None # Correct behavior is to return None on error

        # DANGEROUS: If an exception occurred above and user is None,
        # this check might be skipped or fail unexpectedly.
        # Also, check_password might be flawed if custom implemented.
        # Needs null check for user
        if user and user.check_password(password):
             return user # Return the authenticated user object
        return None # Password incorrect

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Vulnerable Scenario 2: Flask Manual Check with Null Password Issue

# app.py (Flask) - Assume User model, find_user_by_username, session, redirect, flash exist
from werkzeug.security import check_password_hash # Import for check

@app.route('/login', methods=['POST'])
def login():
    username = request.form.get('username')
    password = request.form.get('password')
    user = find_user_by_username(username) # Assume this finds user object/dict

    # DANGEROUS: If user exists but has a NULL or empty password stored in DB,
    # and password input is also empty, this check might pass incorrectly.
    # Depends heavily on how check_password_hash handles empty/null values.
    # Also vulnerable if check_password_hash is not used at all.
    # Need to ensure user.password_hash is not null/empty before check.
    if user and user.password_hash and check_password_hash(user.password_hash, password):
         session['user_id'] = user.id
         return redirect('/dashboard')
    else:
         flash('Invalid credentials')
         return redirect('/login')

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Mitigation and Best Practices

  • Django: Use the default ModelBackend or inherit from it carefully. Ensure custom authenticate methods handle all exceptions securely (fail closed by returning None). Rely on user.check_password().
  • Flask: Always use secure comparison functions like werkzeug.security.check_password_hash. Ensure users cannot register or exist with null/empty passwords. Handle exceptions properly.

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Secure Code Example

# myapp/auth_backends.py (Secure Custom Backend)
from django.contrib.auth.backends import ModelBackend # Inherit for safety
# from .models import StandardUser, AdminUser # If using custom models
from django.contrib.auth import get_user_model
from django.core.exceptions import MultipleObjectsReturned # Handle non-unique usernames

class SecureCustomAuthBackend(ModelBackend): # Inherit default checks
    def authenticate(self, request, username=None, password=None, **kwargs):
        User = get_user_model() # Allows flexibility
        if username is None:
            username = kwargs.get(User.USERNAME_FIELD) # Handle different username fields
        try:
            # Prefer finding user case-insensitively if needed and DB supports it
            user = User.objects.get(username__iexact=username)
        except User.DoesNotExist:
             # Run the default password hasher once to reduce timing leaks
             User().set_password(password)
             return None
        except MultipleObjectsReturned:
             # Should not happen if username is unique, but handle defensively
             return None
        except Exception as e:
            print(f"DB Error during user lookup: {e}")
            return None # Fail closed on error

        # SECURE: Rely on Django's built-in check_password and active check
        if user.check_password(password) and self.user_can_authenticate(user):
            return user
        return None # Password incorrect or user inactive

# app.py (Flask - Secure Check)
from werkzeug.security import check_password_hash # Import correctly

@app.route('/login-secure', methods=['POST'])
def login_secure():
    username = request.form.get('username')
    password = request.form.get('password')
    if not username or not password: # Basic check for empty submission
         flash('Username and password required')
         return redirect('/login')

    user = find_user_by_username(username) # Assume returns object with passwordHash

    # SECURE: check_password_hash handles hash comparison safely.
    # Ensure user.passwordHash exists and is not empty/null.
    if user and hasattr(user, 'passwordHash') and user.passwordHash and check_password_hash(user.passwordHash, password):
         # Regenerate session ID after login (Session Fixation defense)
         # This requires a session interface supporting regeneration, e.g., Flask-Session
         # session.regenerate() # Example name - check your session library
         session.permanent = True # Example session setting
         session['user_id'] = user.id
         return redirect('/dashboard')
    else:
         flash('Invalid credentials')
         # Add rate limiting here (CWE-307)
         return redirect('/login')

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Testing Strategy

Test login functionality thoroughly:
  • Invalid Credentials: Ensure incorrect usernames/passwords fail.
  • Empty/Null Credentials: Try logging in with empty username or password. Does it bypass login? (Should fail).
  • Error Conditions: Try to cause database errors during login (e.g., overly long username). Does the application fail securely (deny access)?
  • Case Sensitivity: Check if username comparisons are case-sensitive or insensitive as intended.
  • Timing Attacks: If using custom comparison logic (not recommended), check if comparing incorrect passwords takes significantly less time than correct ones. Standard library functions usually prevent this.