Cybersecurity

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====== BBC Ch 14: Insecure Deserialization ======

//Source: Bug Bounty Bootcamp by Vickie Li//

===== Mechanisms =====

Serialization converts a program object into a format (byte stream or string) suitable for storage or network transfer. Deserialization reconstructs the object. Many languages support this: Java, PHP, Python, Ruby.

Developers often trust serialized data because it looks opaque or unreadable. Insecure deserialization occurs when user-supplied serialized data is deserialized without verification, letting attackers manipulate object properties or trigger dangerous magic methods.

===== PHP Object Injection =====

PHP serialize format example:
<code>
O:4:"User":2:{s:8:"username";s:6:"vickie";s:6:"status";s:9:"not admin";}
</code>

Format: ''O:CLASS_NAME_LENGTH:"CLASS_NAME":PROPERTY_COUNT:{PROPERTIES}''

**Manipulating variables:** if a serialized object is used for authentication and isn't signed or encrypted, modify the status field:
<code>
O:4:"User":2:{s:8:"username";s:6:"vickie";s:6:"status";s:5:"admin";}
</code>

Note: update the string length marker (''s:9'' -> ''s:5'') when changing values.

**Magic methods:** PHP auto-invokes certain methods during object lifecycle:
  * ''__wakeup()'' -- called when an object is deserialized (via ''unserialize()'')
  * ''__destruct()'' -- called when no references to the object remain
  * ''__toString()'' -- called when the object is treated as a string
  * ''__call()'' -- called when an undefined method is invoked

If a magic method passes object properties into dangerous functions like ''eval()'', an attacker can achieve RCE by controlling those properties in the serialized payload.

**Classic PHP RCE via __wakeup:**
<code php>
class Example2 {
  private $hook;
  function __wakeup() {
    if (isset($this->hook)) eval($this->hook);
  }
}
$user_data = unserialize($_COOKIE['data']);
</code>

Set ''$hook = "phpinfo();"'' in a crafted serialized object, URL-encode it, and pass it as the ''data'' cookie. The server calls ''eval("phpinfo();")'' on deserialization.

Generate the payload:
<code php>
class Example2 { private $hook = "phpinfo();"; }
print urlencode(serialize(new Example2));
</code>

===== POP Chains =====

When magic methods don't directly contain exploitable code, chain method calls across multiple classes (property-oriented programming). Each "gadget" is a code snippet from the existing codebase. Magic methods serve as the entry point; they call methods on other classes that eventually reach dangerous code like ''eval()'' or ''exec()''.

Example flow: ''__wakeup()'' calls ''$obj->evaluate()'' -> ''CodeSnippet::evaluate()'' calls ''eval($this->code)'' -> attacker controls ''$code''.

===== Java Deserialization =====

Java serialized objects implement ''java.io.Serializable''. Signatures that identify them:
  * Hex: ''AC ED 00 05'' at the start
  * Base64: starts with ''rO0''
  * Content-Type header: ''application/x-java-serialized-object''

Java deserialization exploits use gadget chains from libraries (Apache Commons-Collections, Spring Framework, Apache Groovy, Apache Commons FileUpload) to reach code execution.

**Ysoserial** automates gadget chain generation:
<code>
java -jar ysoserial.jar CommonsCollections1 'id'
</code>

Feed the output as a serialized object in the vulnerable parameter.

===== Prevention =====

  * Never deserialize data tainted by user input without verification
  * Use an allowlist to restrict which classes can be deserialized
  * Prefer simple data types (strings, arrays) over serialized objects for transport
  * Keep session state server-side instead of in serialized cookies
  * Keep dependencies patched; many deserialization vulns come from third-party libraries

===== Hunting for Insecure Deserialization =====

**With source code:** search for dangerous deserialization functions:
  * PHP: ''unserialize()''
  * Java: ''readObject()''
  * Python: ''pickle.loads()''
  * Ruby: ''Marshal.load()''

Confirm that user-supplied data flows into these calls.

**Without source code:**
  * Find large blobs of data in HTTP requests (cookies, POST body, headers) -- these may be serialized objects
  * Decode them: base64 blocks starting with ''Tzo'' or ''rO0'' are likely PHP or Java objects
  * Look for ''Content-Type: application/x-java-serialized-object''
  * Tamper with the object: change usernames, status flags, or role names; re-serialize and replay
  * Try to achieve RCE using known gadget chains for the detected language

===== 6-Step Checklist =====

  - Find entry points where user-supplied data is deserialized (cookies, POST params, headers).
  - Identify the serialization format (PHP, Java, Python, Ruby) from signatures or Content-Type.
  - Tamper with variable values in the serialized object (status, role, username) and check for privilege changes.
  - If variable tampering achieves authentication bypass, document and report.
  - See if magic methods or POP chains can escalate to RCE (use Ysoserial for Java).
  - Draft report; be careful not to cause damage when testing RCE payloads.