Telnet

https://tryhackme.com/jr/telnetcve2026

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Task 1: Introduction

Welcome to the Telnet 2026 Vulnerability Room!

Telnet is an ancient protocol, dating back to the late 1960s, designed for remote terminal access over networks. It allows users to log into remote systems and execute commands as if they were sitting directly at the console. However, in the modern era, Telnet has become a relic—largely obsolete due to its fundamental security flaws. Unlike secure alternatives like SSH, Telnet transmits all data, including usernames, passwords, and commands, in plaintext over the network. This makes it incredibly vulnerable to eavesdropping (e.g., via packet sniffers like Wireshark or tcpdump), man-in-the-middle attacks, and session hijacking.

In this room, we'll simulate CVE-2026-24061. Disclosed in early 2026, this critical flaw affects certain misconfigured or outdated implementations of telnetd (the Telnet daemon). It allows attackers to bypass authentication entirely by manipulating environment variables passed during the connection negotiation phase. Specifically, by crafting a malicious USER variable (e.g., "-f root"), an attacker can force the login process to grant unauthenticated root access, leading to a full system compromise. This CVE highlights why legacy protocols like Telnet should never be used in production environments—it's rated CVSS 9.8 (Critical) for its ease of exploitation and high impact.

Getting Started:

1. Deploy the Machine: Click the "Start Machine" button in the top-right corner of this room. It will spin up a vulnerable Ubuntu-based server in the TryHackMe cloud. Wait about 1-2 minutes for it to fully boot.

2. Connect via VPN: Ensure you're connected to the TryHackMe VPN (download the OpenVPN config from your Access page if needed). This puts you on the same network as the target.

3. Note the Target IP: Once deployed, the machine's IP address will appear in the task pane (e.g., something like 10.10.x.x). We'll refer to it as <target_ip> throughout.

4. Verify Connectivity: Open a terminal on your attack box (Kali/Parrot via TryHackMe's browser-based access or your local setup) and run:

Confirm the machine is up by pinging it. No answer needed—just proceed.

What is the CVE ID for the Telnet authentication bypass vulnerability disclosed in 2026?

CVE-2026-24061

Task 2 Understanding Telnet and Its Protocol

In this task, we'll dive deeper into how the Telnet protocol functions at a technical level. Telnet operates as a client-server protocol over TCP, typically on port 23, and is defined by RFC 854 (with extensions in later RFCs like RFC 1408 for environment variables). It emulates a terminal session, allowing bidirectional communication between a client (e.g., your terminal) and a remote server. However, unlike modern protocols, Telnet doesn't encrypt anything—every byte of data, including sensitive information like passwords, is sent in cleartext. This makes it trivial for attackers to intercept traffic using tools like Wireshark or tcpdump.

Key aspects of the protocol:

• Negotiation Phase: When a connection starts, the client and server exchange "options" using IAC (Interpret As Command) bytes (value 255 in decimal). These negotiate features like echo mode, line mode, or environment variables (via RFC 1408 for NEW-ENVIRON). This phase is where vulnerabilities like our simulated CVE can arise, as untrusted input (e.g., env vars) might be passed to the login process.

• Data Transmission: After negotiation, data flows in plaintext. Commands are sent as ASCII characters, with special sequences for control (e.g., Ctrl+C as ^C).

• Environment Variables: Telnet clients can pass vars like USER, DISPLAY, or TERM to the server. In vulnerable setups, these can be abused—e.g., setting USER to a malicious value that tricks /bin/login into bypassing auth.

• Common Implementations: Servers like inetutils-telnetd or BSD telnetd; clients are built-in on most OSes (e.g., telnet command on Linux/Windows).

To get hands-on, we'll start with basic enumeration. Ensure your machine is deployed from Task 1 and use your attack box terminal for the following:

• Scan for the service: Run nmap -sV -p 23 <target_ip> to confirm Telnet is running and grab the version (this might hint at vuln status).

• Basic connection: Try telnet <target_ip> 23 to see the banner and login prompt. Note how it asks for username/password in cleartext—don't enter real creds yet!

This foundational knowledge sets up our exploitation chain. Remember, in real scenarios, always use tools like ssh for secure remote access. For more on historical Telnet vulnerabilities that inspired our simulation, check CVE-1999-0024 (environment variable overflows) and CVE-2011-4862 (buffer overflows in telnetd).

What is the default port for Telnet?

23

Task 3 The Core Vulnerability: Plaintext Transmission and Injection Risks

Now that you understand Telnet's basic mechanics, let's explore its primary security weaknesses in detail. The core issue with Telnet is its complete lack of encryption—all communication occurs in plaintext over the wire. This means anyone with access to the network (e.g., via a shared Wi-Fi, compromised router, or man-in-the-middle position) can passively eavesdrop on sessions using packet capture tools. Attackers can read usernames, passwords, commands, and even output from executed programs without any effort.

Beyond eavesdropping, Telnet is susceptible to active attacks like injection: During the negotiation phase or data transmission, malicious actors can insert forged packets to alter commands or hijack sessions. For instance, if an attacker spoofs TCP sequences, they could inject shell commands mid-session. Our simulated CVE-2026-24061 builds on these flaws by exploiting how Telnet handles environment variables during login. In vulnerable telnetd implementations, the server passes client-supplied env vars (like USER) directly to /bin/login without proper sanitization. By setting USER to something like "-f root" (where -f forces login without password), the auth process can be bypassed entirely, granting immediate root access.

This vulnerability chain starts with:

1. Establishing a connection and negotiating options (where env vars are sent).

2. The server trusting unsanitized input.

3. Exploitation leading to unauthenticated shell.

Task 4 Service Enumeration

Before exploiting, identify open services. Use tools like Nmap to scan the target and confirm Telnet is running. Telnet typically listens on port 23, but always verify—it could be on a non-standard port for obfuscation. This task builds enumeration skills, crucial for real pentesting (e.g., as outlined in the OWASP Testing Guide or MITRE ATT&CK's Reconnaissance techniques).

Steps to Follow:

1. Run a full port scan: nmap -p- <target_ip>. This scans all 65,535 ports to find any open ones (expect 1-3 for our lab).

2. Perform a service version scan on open ports: nmap -sV -p <open_ports> <target_ip>. Replace <open_ports> with results from step 1 (e.g., -p 22,23,80). This grabs banners and versions.

3. Look for Telnet (port 23) and note the version (e.g., GNU Inetutils telnetd). If present, it confirms our target service.

For deeper insights, consider aggressive scans like nmap -A <target_ip> (enables OS detection, version, scripts, traceroute) but use sparingly in real ops to avoid alerts. In our scenario, the scan will reveal the vulnerable telnetd, setting up the bypass. Remember, ethical hacking requires permission—tools like Nmap are legal in labs like this but not on unauthorized systems.

What port is Telnet running on?

23

What is the full service banner for Telnet?

Openwall GNU/*/Linux telnetd

Is the service version vulnerable? Research quickly using the CVE details.

yay

Now that you've identified Telnet, research known vulnerabilities. CVE-2026-24061 exploits how telnetd handles the USER environment variable, passing it unsanitized to the login program. By setting USER to "-f root", attackers can force pre-authenticated root access.

Focus on understanding the root cause: argument injection (CWE-88) in the telnetd invocation of /bin/login.

Steps to Follow:

1. Search for "CVE-2026-24061" using a search engine or vulnerability databases like NVD.

2. Read about the exploit mechanism (e.g., environment variable injection).

3. Note affected versions and exploitation requirements (no auth needed, network access sufficient).

Answer the questions below

What environment variable is manipulated in the exploit?

USER

What specific value for USER bypasses auth as root?

-f root

What is the CVSS score for this vulnerability?

9.8

Task 6 Exploitation

Time to exploit! Use a Telnet client to connect while injecting the malicious USER variable. This should grant an immediate root shell. Remember, in real scenarios, disable Telnet entirely—use SSH instead.

Warning: This is a simulated environment; never attempt this on unauthorized systems.

Steps to Follow:

1. Install a Telnet client if needed (e.g., sudo apt install telnet on Kali).

2. Craft the exploit command: Set the USER environment variable and connect with auto-login.

3. Once in, verify root access with whoami or id.

4. Navigate to /root or /home for flags.

Exploit Command (Polished for Clarity): USER="-f root" telnet -a <target_ip>

Answer the questions below

What is the output of whoami after successful exploitation?

root

Find the flag in /root/flag.txt. What is it?

THM{Telnet is dead}