CVE-2026-24061: Analysis of GNU Inetutils telnetd Authentication Bypass Vulnerability | CIP Blog

CVE-2026-24061 is a high-risk vulnerability rated CVSS 9.8 (Critical), and it is a remote authentication bypass issue occurring in the telnetd service of GNU Inetutils. The vulnerability arises from telnetd blindly trusting the USER environment variable supplied during Telnet connections, without performing proper validation. This flaw allows an attacker to completely bypass authentication and obtain root privileges.

In this article, we will look at the operating principle and actual exploitation method of CVE-2026-24061, and also examine the internet exposure status based on Criminal IP data.

Vulnerability Overview and Root Cause Analysis

A structure where an authentication bypass option is directly delivered through the USER environment variable

When a remote client connects via Telnet, telnetd internally calls /bin/login to perform user authentication. During this process, the login username is derived from the USER environment variable.

The USER value is passed directly to /bin/login as an execution argument, without any validation.

The process in which the USER value is delivered as an option to execute `/bin/login` without validation

The core issue is the absence of validation logic to prevent option injection via the USER value. If an attacker supplies a value such as -f root in the USER environment variable, it is interpreted not as a username, but as a command-line option passed to /bin/login.

The -f option used in /bin/login is designed to skip password verification by treating the user as already authenticated. In other words, if -f root is passed, /bin/login treats the root user as already authenticated and completes the login process immediately without checking a password.

As a result, telnetd treats the request as a legitimate login and spawns a root-privileged shell without requiring credentials.

Exploitation Scenario and Impact

An attacker can exploit this flaw by injecting a malicious option into the USER environment variable during a Telnet connection, as shown below.

The appearance of directly injecting an option value into the USER environment variable

USER=”-f root” telnet -a 127.0.0.1 XXXX

As a result, the password authentication process is completely skipped, and the attacker can immediately log in as the root user and obtain a root-privileged shell.

The appearance of being able to access core system files such as /etc/passwd

This enables unrestricted access to critical system files such as /etc/passwd, potentially leading to account takeover, persistent backdoor installation, and long-term system compromise.

Affected Versions

GNU Inetutils telnetd
- 1.9.3 ≤ version ≤ 2.7

Any system running a vulnerable version with externally accessible Telnet services is effectively exposed to full system compromise.

Actual Exposure Observed Through Criminal IP

The real-world risk of a vulnerability is determined not by the existence of a PoC, but by how widely the affected service is exposed to the internet.

To confirm this, the following search query was applied in Criminal IP Asset Search..

Criminal IP Search Query: product:telnetd

Results of searching `product:telnetd` in Criminal IP Asset Search

As of February 3, 2026, Criminal IP identified 87,440 internet-exposed assets. This shows that Telnet-based management interfaces are still being operated in a state of broad exposure to external networks across many regions worldwide.

One of the internet-exposed Telnet services identified through Criminal IP Asset Search

After reviewing one of the identified assets, it was confirmed that the Telnet service was directly exposed to the internet, and the product and vendor information was clearly revealed through the login banner. As a result, attackers can easily determine whether the service is vulnerable during the reconnaissance stage.

If such an asset is running a vulnerable version of telnetd, CVE-2026-24061 becomes immediately exploitable.

Response and Recommendations

Immediately disable the Telnet service wherever possible.

If unavoidable:

Apply the latest patch for telnetd
Block external access (Firewall / ACL)
Switch to secure protocols such as SSH
Use Criminal IP Asset Search to proactively identify externally exposed assets.
Identify legacy management interfaces

FAQ

Q1. Is it still dangerous even if the Telnet service is only used internally?

Even for internal services, external access may become possible due to configuration errors, firewall policy changes, or exposure of test environments, so a pre-check is necessary.

Q2. Is applying the patch alone sufficient?

If the external exposure check is not performed together with patching, the same risk can be repeated.

Conclusion

CVE-2026-24061 is not a simple implementation flaw. It is a case that demonstrates how an authentication bypass vulnerability can immediately lead to full system compromise when legacy management services such as Telnet are exposed to the internet.

Effective mitigation therefore depends not only on patching, but on proactively identifying externally exposed assets and rigorously controlling legacy management interfaces.

In relation to this you can refer to Clawdbot / Moltbot: Security Analysis of an Autonomous AI Agent Exposure Risk.

You can subscribe to Criminal IP (criminalip.io/register) and start detecting vulnerable assets right away. Also you can request a demo through the button below, and try Criminal IP’s threat intelligence (TI) analysis of externally exposed assets on the Enterprise level.

This report is based on data from Criminal IP, a Cyber Threat Intelligence search engine. Sign up for a free Criminal IP account today to explore the search results mentioned in the report and delve into comprehensive threat intelligence.

Source: Criminal IP (https://www.criminalip.io/ko), GitHub(https://github.com/h3athen/CVE-2026-24061)