CVE-2023-6926

Executive Summary

CVE-2023-6926 is a critical OS command injection vulnerability in Crestron AM-300 firmware version 1.4499.00018. The flaw allows a user with limited SSH access to escalate privileges to root-level by injecting arbitrary OS commands. With a CVSS score of 8.4, this vulnerability poses a severe risk to confidentiality, integrity, and availability of affected devices. No public proof-of-concept or patch is currently available, but the vulnerability is exploitable locally via SSH.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in Crestron AM-300 hardware.

CVSS Breakdown & Threat Level

• CVSS Score: 8.4 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Local (requires SSH access)
• Attack Complexity: Low
• Privileges Required: Low (limited SSH user)
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

Threat Level: High. Exploitation allows full device compromise from a limited SSH session.

Deep Dive & Mechanics

The vulnerability exists in the Crestron AM-300 firmware version 1.4499.00018. The device exposes an SSH service that provides limited shell access for administrative tasks. However, input validation is insufficient in certain command handlers, allowing an authenticated user to inject arbitrary OS commands. By crafting malicious input within SSH commands, the attacker can bypass intended restrictions and execute commands with root privileges.

The injection likely occurs in a function that passes user-supplied arguments directly to a system call (e.g., system(), popen(), or exec()) without proper sanitization. For example, a command like ping or traceroute might accept an IP address parameter that is not filtered, enabling command chaining with ;, &&, or |.

Vulnerable vs. Patched Code

Vulnerable Code (Firmware 1.4499.00018):

// Example vulnerable function
void handle_ping(char *ip) {
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "ping -c 4 %s", ip);
    system(cmd); // No input sanitization
}

Patched Code (Hypothetical):

// Patched version with input validation
void handle_ping(char *ip) {
    // Validate IP address format
    if (!is_valid_ip(ip)) {
        printf("Invalid IP address\n");
        return;
    }
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "ping -c 4 %s", ip);
    system(cmd);
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• SSH access to Crestron AM-300 with limited user credentials.
• Device running firmware 1.4499.00018.

Exploitation Steps:

1. Connect via SSH: ssh limited_user@<target_ip>
2. Identify a vulnerable command (e.g., ping, traceroute, diagnostics).
3. Inject command: ping -c 4 127.0.0.1; id
4. Observe root-level output: uid=0(root) gid=0(root) groups=0(root)
5. Escalate to full root shell: ping -c 4 127.0.0.1; /bin/sh

Example Payload:

ssh limited_user@192.168.1.100 "ping -c 4 127.0.0.1; id"

Blue Team

Detection Steps:

1. Monitor SSH logs for unusual command patterns (e.g., semicolons, pipes, &&).
2. Enable command logging on the device if possible.
3. Use network monitoring to detect anomalous outbound connections from the device.
4. Restrict SSH access to trusted IPs only.
5. Apply principle of least privilege for SSH users.

Honeypot Setup:

• Deploy a decoy Crestron AM-300 with SSH exposed.
• Log all commands and alert on injection patterns.

Indicators of Compromise (IOCs)

• Network: SSH connections from unusual IPs to port 22.
• Process: Unexpected processes like /bin/sh or /bin/bash spawned from SSH sessions.
• File: Modified system binaries or scripts in /usr/bin or /sbin.
• Log: SSH logs containing commands with ;, &&, |, or backticks.
• Behavior: Device performing outbound connections to attacker-controlled IPs.

Detection Engineering

YARA Rule

rule CVE_2023_6926_OS_Command_Injection {
    meta:
        description = "Detects OS command injection attempts in Crestron AM-300 SSH logs"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2023-6926"
    strings:
        $cmd1 = /[;&|`$]/ 
        $cmd2 = /(ping|traceroute|diagnostics)\s+\S+\s*[;&|]/
    condition:
        any of them
}

Sigma Rule

title: Crestron AM-300 OS Command Injection Attempt
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential OS command injection in Crestron AM-300 SSH sessions
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        CommandLine|contains|all:
            - 'ping'
            - ';'
    condition: selection
falsepositives:
    - Legitimate use of semicolons in commands (rare)
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 22 (msg:"Crestron AM-300 OS Command Injection Attempt"; flow:to_server,established; content:"|3B|"; distance:0; within:100; content:"ping"; nocase; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable SSH access if not required.
2. Restrict SSH access to trusted IPs via firewall.
3. Monitor SSH logs for suspicious activity.
4. Implement command whitelisting on the device.

Long-term Fix:

• Apply firmware patch from Crestron when available.
• Upgrade to a non-vulnerable firmware version.
• Replace device with a supported model if no patch is released.

Workaround:

• Use SSH key-based authentication only.
• Limit SSH user permissions to read-only commands.

Valters IT
Securing Digital Frontiers