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# lab-7

## Developing a Custom Gadget Chain for PHP Deserialization

**Difficulty:** Expert

### Lab Description

This lab uses a serialization-based session mechanism. By deploying a custom gadget chain, you can exploit insecure deserialization to achieve remote code execution.

**Objective:** Delete the `morale.txt` file from Carlos's home directory.

### Root Cause

The application stores user session data as a PHP serialized object and restores it using `unserialize()` on client-controlled data. During deserialization, PHP automatically invokes the `__wakeup()` magic method of the `CustomTemplate` class.

The `__wakeup()` method calls `build_product()`, which creates a new `Product` object using the `default_desc_type` and `desc` properties from the deserialized object. Because these properties are attacker-controllable, unvalidated data is trusted during object reconstruction.

The vulnerability becomes exploitable because the `Product` constructor performs dynamic property access (`$desc->$default_desc_type`). If the supplied object does not contain the requested property, PHP automatically invokes the `__get()` magic method. In the `DefaultMap` class, `__get()` forwards the requested property name to a callback using `call_user_func()`. This combination of magic methods and dynamic property access forms a custom gadget chain, allowing attacker-controlled data to reach a dangerous function.

### Exploitation

1. Log in as a normal user and observe that the session cookie contains a PHP serialized object.
2. In Burp's site map, find `CustomTemplate.php` and append `~` to the URL to retrieve the editor backup file containing the source code. Analyse the deserialization logic.
3. Identify that `CustomTemplate::__wakeup()` automatically invokes `build_product()` during deserialization.
4. Identify that `DefaultMap::__get()` uses `call_user_func()` to invoke a callback whenever an undefined property is accessed.
5. Craft a malicious serialized object such that:
   * `CustomTemplate` references a `DefaultMap` object.
   * The dynamic property lookup triggers `DefaultMap::__get()`.
   * `call_user_func()` invokes a dangerous function (such as `exec` or `system`) using attacker-controlled input pointing to `/home/carlos/morale.txt`.
6. Encode the payload: Base64-encode → URL-encode, and replace the session cookie.
7. Send the request. When the application calls `unserialize()`, the gadget chain executes automatically, the dangerous callback is invoked, and `morale.txt` is deleted.

### Lesson Learned

* Never use `unserialize()` on data that originates from users or other untrusted sources.
* Avoid storing serialized PHP objects in client-controlled cookies or other user-modifiable locations. Use safer formats such as JSON for session data wherever possible.
* Minimise the use of PHP magic methods (`__wakeup()`, `__get()`, `__destruct()`, etc.) for security-sensitive operations — they can be invoked implicitly during deserialization without the developer's intent.
* Avoid dynamic function invocation mechanisms such as `call_user_func()` with attacker-influenced values.
* Validate all object properties before using them in dynamic property access or callback execution.
* Remove backup and editor-generated files (`.php~`, `.bak`, `.old`) from production servers to prevent source code disclosure — exposed source code makes gadget chain construction significantly easier.
* Apply the principle of least privilege so that, even if deserialization is abused, the application's OS permissions limit the impact of arbitrary code execution.
* Conduct regular code reviews to identify gadget chains where multiple seemingly harmless classes can be combined into a dangerous execution path.


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