CVE-2022-41706: Browsershot 3.57.2 Arbitrary Local File Obtaining via Incorrect URL Protocol Validation

CVE-2022-41706 highlights a critical vulnerability in Browsershot version 3.57.2 that potentially allows an external attacker to remotely obtain local files stored on the server. All exploitation occurs due to improper validation of the URL protocol passed to the Browsershot::url method. In this post, we will dive deep into the details and technical aspects of this vulnerability and provide information about potential attack scenarios and mitigation strategies.

Technical Details

Browsershot is a popular open-source package widely used to convert webpages or HTML content into images or PDFs using the headless browsers like Google Chrome, Chromium, and Microsoft Edge. In its version 3.57.2, the package has a critical vulnerability that can result in the potential exposure of sensitive local files.

The core of the issue lies in the Browsershot::url method where the URL protocol remains unchecked. This lack of validation means the application can process any URL with an arbitrary protocol, thus making it possible for an attacker to abuse the system by remotely obtaining local files on the server.

Here's a simple code snippet showcasing the vulnerability

<?php
// ...
$browsershot = new Browsershot();
$content = $browsershot->url('file:///etc/passwd')->getBody(); // Will return the content of a local file
// ...
?>

As demonstrated above, the file:// protocol allows unintended access to the local files on the server by simply instructing Browsershot to process the URL starting with file://.

Exploit Scenario

Imagine a web application that allows users to generate PDF thumbnails of websites by providing a URL to Browsershot. An attacker can exploit this vulnerability by sending a specially crafted request containing a URL with the file:// protocol, pointing to the local file they wish to obtain like this:

https://example.com/generate_thumbnail?website=file:///etc/passwd

Once processed, the attacker can extract the contents of the local file, such as the /etc/passwd file that contains user account information. This can lead to further attacks and data breaches.

Mitigation

To address this vulnerability, developers must ensure proper validation of the input URL protocol. Enforcing strict conditions and only allowing valid URL protocols, such as 'http://' and 'https://', can prevent attackers from exploiting this specific issue.

For instance, the following code snippet can be used to validate protocols before passing the URL to the Browsershot:

<?php
// ...
$url = 'file:///etc/passwd'; // Any user input or request
if (preg_match('/^(https?):\/\//', $url)) {
    $browsershot = new Browsershot();
    $content = $browsershot->url($url)->getBody();
} else {
    // Invalid protocol or malformed URL, handle the error...
}
// ...
?>

Furthermore, it is highly recommended to keep the Browsershot package up-to-date and apply security patches promptly as they are released.

References

1. Browsershot Official Repository
2. CVE-2022-41706 - Official CVE Details
3. Common Vulnerabilities and Exposures (CVE®)

Conclusion

CVE-2022-41706 is a critical vulnerability in Browsershot 3.57.2, allowing attackers to remotely access local files on the server by exploiting the improper validation of URL protocols. To mitigate the risk, developers must validate URL protocols when passing them to the Browsershot::url method, ensuring only valid URL protocols are accepted. Staying informed about potential vulnerabilities and applying patches in a timely manner will significantly reduce the likelihood of security breaches.

Timeline

Published on: 11/25/2022 18:15:00 UTC
Last modified on: 12/01/2022 16:46:00 UTC