A newly discovered security flaw, CVE-2025-20138, has rocked the networking community. This is a privilege escalation vulnerability affecting the Command Line Interface (CLI) of Cisco IOS XR Software. In simple terms, it means a user with low privileges (someone who shouldn’t have much power) can become root (the most powerful user) by exploiting insufficient checks on the commands they enter.

In this post, we’ll explain what this vulnerability is, how it works, show a code example of exploitation, and provide references so you can learn more. Our goal is to make it easy to understand—even if you aren’t a security expert.

What is CVE-2025-20138?

CVE-2025-20138 is a vulnerability in the Cisco IOS XR CLI. If someone has any kind of user account on a vulnerable router (like a guest or technician), they can run special commands that the system doesn’t check carefully enough. This lets them take over the whole device—as root!

How Does the Exploit Work?

The root cause is insufficient input validation. When the device’s software runs certain CLI commands, it trust user input too much—especially input that is passed to underlying shell commands. If the attacker crafts their commands in a special way, they can trick the system into doing more than intended.

Exploit Example (Code Snippet)

Let’s say the CLI has a feature where a user can run “show” commands or manipulate files with a command like:

copy file <source> <destination>

Suppose the IOS XR CLI passed the user-supplied arguments directly to a shell command, like

os.system("cp %s %s" % (source, destination))

If source is something like

test.txt; whoami #

The actual command run (as root) becomes

cp test.txt; whoami # destination

This runs cp test.txt and then whoami (as root), showing the attacker that they have succeeded in privilege escalation:

root

Attackers can use this trick to run

; cat /etc/shadow ;
; rm -rf / ;
; set up a reverse shell ;

You get the idea—anything a root user can do.

A pseudo-code demonstration

# BAD: No input sanitization
def handle_cli_copy_command(source, destination):
    os.system(f"cp {source} {destination}")
    
# Attacker input:
# source = "file.txt; id; #"
# destination = "junk.txt"

# System executes:
# cp file.txt; id; # junk.txt
# (Which executes the Unix 'id' command as root)

1. Patch Fast!

Cisco released patches that fully fix this flaw. Upgrade to the latest IOS XR version ASAP.

- Official Cisco advisory: Cisco Security Advisory – CVE-2025-20138
- Cisco Downloads: Cisco Software Downloads

2. Restrict CLI Access

Limit user account creation, and restrict who can log into the device via console/SSH.

3. Monitor Logs

Watch for suspicious CLI commands or unfamiliar root activity.

4. Use Least Privilege

Grant users only the privileges required for their job—avoid shared or “all-in-one” accounts.

References

- Cisco Security Advisory for CVE-2025-20138 (official notice)
- NIST NVD: CVE-2025-20138
- How Shell Injection Attacks Work (Wikipedia)
- Cisco IOS XR CLI Documentation

Conclusion

CVE-2025-20138 is a classic example of why input validation matters—especially in system-level software like Cisco routers. Anyone running Cisco IOS XR should patch immediately, audit CLI access, and monitor for abuse.

The core lesson: Never trust user input, especially when you’re running commands as root! This time, a small mistake could let someone take total control of a critical device.

If you manage Cisco gear, check if you’re affected, update right away—and reach out to Cisco support with any questions.

Timeline

Published on: 03/12/2025 16:15:21 UTC