In early 2022, security experts identified a bug impacting IBM AIX—one of the most trusted UNIX operating systems—and specifically, the "nimsh" daemon, a core component supporting remote system management through NIM (Network Installation Manager). This vulnerability, tracked as CVE-2022-22351 (IBM X-Force ID: 220396), could let a non-privileged user on a trusted host crash nimsh on another trusted system, causing a Denial of Service (DoS).

Let’s break down what this means, how it works, and look at some example exploit logic.

What Is Nimsh?

nimsh is a daemon process on IBM AIX systems, part of the Network Installation Manager (NIM), that allows secure and remote communication between management servers and client systems. It enables tasks like software deployment, patching, and general administration.

Explaining CVE-2022-22351

The flaw in nimsh relates to how it handles network traffic from trusted hosts. Normally, only privileged operations are allowed against nimsh, but due to improper validation, any non-privileged user on a trusted host could send special messages causing nimsh to crash repeatedly on another host.

That means someone with minimal access on a single trusted system could potentially disrupt management and automation processes across your AIX network.

IBM’s official advisory is here:
- Security Bulletin: Vulnerabilities in NIM and nimsh Daemon

The Exploit: How Does It Work?

The specifics of the bug haven’t been fully disclosed by IBM (to prevent widespread misuse), but the technique centers on sending malformed packets to the nimsh daemon over the network. If the sending host is listed as "trusted" in the NIM configuration, nimsh will accept the connection and process the malformed input, triggering a crash.

Attacker has a user account on a trusted AIX host in the NIM network.

2. Attacker crafts and sends a specially-formed message or request to the nimsh port (default 3901/TCP) on another NIM machine.

The target’s nimsh daemon receives the message, fails to process it properly, and crashes.

4. Repeat: The attack can be repeated, continually disrupting NIM-based management across the network.

Example Exploit Logic (Python Pseudocode)

*This is a safe, illustrative example, since the actual exploit details (i.e. message format) are undisclosed for safety reasons. Do not use against any system without authorization.*

import socket

def send_nimsh_packet(target_ip, payload):
    port = 3901  # Default nimsh port
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        try:
            s.connect((target_ip, port))
            s.sendall(payload)
            print("Payload sent to nimsh daemon.")
        except Exception as e:
            print(f"Connection failed: {e}")

# Example of a malformed packet; 
# actual payload would depend on the bug specifics.
payload = b'\xff\xff\xde\xad\xbe\xef' * 100  

target_ip = "192.168.1.100"  # IP of a trusted NIM host to attack
send_nimsh_packet(target_ip, payload)

The above Python snippet shows the concept of repeatedly sending a malformed payload to the nimsh service, causing repeated crashes (DoS).

How to Check if You’re Affected

1. Check AIX/VIOS Version

Mitigation: Protect Your Systems

IBM released fixes for this issue. Apply the interim fixes provided by IBM or upgrade to the latest level of AIX/VIOS.

Alternatively, as a temporary measure

- Restrict network access to port 3901/TCP.

References

- IBM Security Bulletin for CVE-2022-22351 / X-Force ID: 220396
- CVE-2022-22351, NIST, National Vulnerability Database
- IBM X-Force Exchange Report

Summary

CVE-2022-22351 demonstrates how even trusted, privileged backend processes can be vulnerable if message validation isn’t robust. With this flaw, someone with low-level access on a trusted host could crash nimsh on other AIX systems, potentially halting important admin workflows.

Patch now, restrict access, review NIM trust relationships—and always monitor your critical daemons.

Have further questions about AIX security or need help hardening your infrastructure? Leave a comment or reach out!

Timeline

Published on: 03/07/2022 17:15:00 UTC
Last modified on: 03/18/2022 13:40:00 UTC