Telnetd CVE-2026-24061 Test Script

A Python security testing tool to detect vulnerability to environment variable injection in telnetd servers via the NEW-ENVIRON telnet option.

Overview

This script tests telnetd servers for CVE-2026-24061, a vulnerability that allows attackers to inject environment variables through the NEW-ENVIRON telnet option. If a server improperly handles these environment variables, it may be vulnerable to command injection or authentication bypass attacks.

The script provides detailed, color-coded output with step-by-step progress, comprehensive error handling, and a final summary that includes the test result, risk assessment, and impact analysis.

CVE-2026-24061 Details

Vulnerability Summary

CVE-2026-24061 is a security vulnerability in telnetd servers that improperly handle environment variables sent via the NEW-ENVIRON telnet option (RFC 1572).

Impact

• Severity: High
• Attack Vector: Network
• Authentication Required: No
• Impact:
  • Potential command injection
  • Authentication bypass
  • Unauthorized access to the system

Technical Details

The vulnerability occurs when telnetd servers:

1. Accept the NEW-ENVIRON telnet option
2. Process environment variables sent via NEW-ENVIRON without proper validation
3. Use these environment variables in unsafe ways (e.g., passing them directly to login processes)

The NEW-ENVIRON option (RFC 1572) allows telnet clients to send environment variables to the server. A vulnerable server may accept and process these variables without sanitization, potentially allowing an attacker to inject malicious values that could lead to command execution or authentication bypass.

Affected Systems

• Telnetd servers that support NEW-ENVIRON option
• Servers that process USER environment variable without validation
• Systems using vulnerable versions of telnetd implementations

Script Usage

Prerequisites

• Python 3.x
• Network access to target telnet server

Installation

No installation required. The script is standalone and uses only Python standard library modules.

Installing Python

Linux (Ubuntu/Debian):

sudo apt-get update
sudo apt-get install python3

macOS (using Homebrew):

# Install Homebrew if not already installed
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

# Install Python 3
brew install python3

# Verify installation
python3 --version

Windows:

1. Download Python from python.org
2. Run the installer and check "Add Python to PATH"
3. Verify installation:

   python --version
   

   Or using PowerShell:

   python --version
   

Basic Usage

Linux/macOS:

./telnetd_CVE‑2026‑24061_test.py <host> [port]

Windows (Command Prompt):

python telnetd_CVE‑2026‑24061_test.py <host> [port]

Windows (PowerShell):

python telnetd_CVE‑2026‑24061_test.py <host> [port]

Examples

Linux/macOS:

# Test default telnet port (23)
./telnetd_CVE‑2026‑24061_test.py 10.0.2.134

# Test custom port
./telnetd_CVE‑2026‑24061_test.py 10.0.2.134 2323

# Make script executable (if needed)
chmod +x telnetd_CVE‑2026‑24061_test.py

# Or run directly with python3
python3 telnetd_CVE‑2026‑24061_test.py 10.0.2.134 2323

Windows (Command Prompt):

REM Test default telnet port (23)
python telnetd_CVE‑2026‑24061_test.py 10.0.2.134

REM Test custom port
python telnetd_CVE‑2026‑24061_test.py 10.0.2.134 2323

Windows (PowerShell):

# Test default telnet port (23)
python telnetd_CVE‑2026‑24061_test.py 10.0.2.134

# Test custom port
python telnetd_CVE‑2026‑24061_test.py 10.0.2.134 2323

macOS (Homebrew Python):

# If using Homebrew-installed Python, use python3 explicitly
python3 telnetd_CVE‑2026‑24061_test.py 10.0.2.134

# Or make executable and run directly
chmod +x telnetd_CVE‑2026‑24061_test.py
./telnetd_CVE‑2026‑24061_test.py 10.0.2.134 2323

Features

• Color-coded Output: Automatic color support detection with ANSI colors for better readability
  • Green for success messages
  • Red for errors and high-risk results
  • Yellow for warnings and inconclusive results
  • Cyan for informational messages
  • Colors automatically disabled when piping output or if NO_COLOR environment variable is set
• Detailed Test Output: Step-by-step progress with detailed analysis
• Comprehensive Error Handling: Clear error messages with possible causes and recommendations
• Test Summary: Final summary includes result, risk level, risk severity, and impact assessment
• Connection Information: Displays remote and local addresses for debugging

Test Outcomes

The script provides four possible outcomes:

1. NOT IMPACTED

[-] NEW-ENVIRON not accepted
[RESULT] NOT IMPACTED

Meaning: The server does not support or accept the NEW-ENVIRON option. The server is not vulnerable to this specific attack vector.

Summary Information:

• Final Result: NOT IMPACTED
• Risk Level: NONE
• Risk Severity: N/A

Action Required:

• No immediate action required for this vulnerability
• Continue regular security maintenance and updates

2. LIKELY IMPACTED

[!] ENV accepted by server
[RESULT] LIKELY IMPACTED (CVE-2026-24061)

Meaning: The server accepted the NEW-ENVIRON option and appears to have processed the injected environment variable. The server is likely vulnerable to CVE-2026-24061.

Summary Information:

• Final Result: LIKELY IMPACTED (CVE-2026-24061)
• Risk Level: HIGH
• Risk Severity: CRITICAL
• Impact Assessment:
  • Command injection possible
  • Authentication bypass possible
  • Unauthorized system access possible

Action Required:

• Update telnetd to a patched version
• Disable NEW-ENVIRON support if not needed
• Implement proper input validation for environment variables
• Consider disabling telnet in favor of SSH
• Implement network access controls
• Review server logs for suspicious activity

3. INCONCLUSIVE

[+] No response to ENV injection
[RESULT] INCONCLUSIVE

Meaning: The server accepted NEW-ENVIRON but did not provide a clear response. Manual verification is recommended.

Summary Information:

• Final Result: INCONCLUSIVE
• Risk Level: UNKNOWN
• Risk Severity: MEDIUM
• Impact Assessment:
  • Vulnerability status unclear
  • Manual verification required
  • Further investigation recommended

Action Required:

• Perform manual testing
• Review server logs
• Check telnetd configuration
• Consult vendor security advisories
• Test with additional environment variables

4. TEST FAILED

[RESULT] TEST FAILED - Communication Error

Meaning: The test could not be completed due to a communication error.

Summary Information:

• Final Result: TEST FAILED - Communication Error
• Risk Level: UNKNOWN
• Risk Severity: N/A
• Impact Assessment:
  • Test could not be completed
  • Unable to determine vulnerability status

Action Required:

• Retry the test
• Check network stability
• Review server logs
• Verify network connectivity

How It Works