External risk intelligence

OpenSSH Password Authentication Bypass Vulnerability

CVE advisoryKnown Exploit

CVE-2026-123158

The vulnerability affects sshd, which is the primary service for remote system access. SSH services are designed to be internet-facing by default in many common deployment scenarios, such as Linux servers, cloud infrastructure, and remote management portals, making them a standard, public-facing gateway for administrative connectivity.

Authentication Bypass

Openbsd Openssh

before 10.610.6

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A security issue has been identified in OpenSSH, a widely used secure shell protocol, that could potentially allow unauthorized access through password authentication bypass on certain Linux configurations. This vulnerability affects systems using musl libc without PAM and could permit any password within a specific length range to grant access. The primary concern is to determine if our environment is impacted by this specific configuration and, if so, to understand the exposure.

Bypasses secure password login.
Matters for remote access security.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could target systems running OpenSSH on Linux distributions like Alpine that use musl libc without PAM. By sending a specially crafted password between 256 and 1024 characters during the authentication process, the attacker could bypass password checks and gain unauthorized access to the system.

Attacker must have network access.
Triggered by providing a long password.
Results in authentication bypass.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could allow an unauthenticated attacker to bypass password authentication on affected Linux systems using musl libc without PAM, when certain account configurations are met. This could lead to unauthorized access to systems.

System accounts and their configurations.
Password authentication could be bypassed.
Unauthorized system access may occur.

Operational Fix

Recommended remediation, mitigation, and detection steps

In real-world scenarios, teams responsible for infrastructure, platform management, and network security are typically accountable for addressing this vulnerability. The first practical step involves identifying all instances of the affected OpenSSH deployment, determining their reachability and business criticality, and then engaging the relevant system owners to plan remediation.

Identify vulnerable OpenSSH systems.
Verify external exposure and business impact.
Plan and execute vendor-coordinated updates.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is OpenSSH in the context of CVE-2026-123158?

OpenSSH is the standard suite of tools for secure remote login and file transfer over a network. It uses the SSH protocol to provide encrypted communication channels. This vulnerability specifically impacts versions before 10.6p2 running on Linux systems that utilize the musl libc library instead of the more traditional glibc, particularly when the Pluggable Authentication Modules (PAM) framework is not enabled, such as in Alpine Linux.

How does this OpenSSH password authentication bypass work?

The flaw is categorized as CWE-394, which refers to an unexpected status code or return value. In affected OpenSSH versions, the system misinterprets how it handles specific string lengths during login. Because of this logic error, the software may incorrectly signal a successful authentication for any password submitted that is between 256 and 1024 characters long, effectively skipping the verification step.

What triggers this authentication bypass?

An attacker triggers this by attempting to log in to an affected system and providing a password of a length between 256 and 1024 characters. It is important to note that this bug does not trigger for passwords outside of that specific length range. Furthermore, the vulnerability is not present if the system uses PAM for authentication or relies on public-key authentication methods instead of password-based logins.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal identifies that this vulnerability targets sshd, the service managing remote access. Because SSH is intentionally designed to be a network-accessible gateway for management, it is often exposed to the internet. If you run affected configurations, the accessibility of your SSH service makes it a potential target, as the service is a primary point of entry for remote administration.

What is the first step to address this CVE?

Begin by auditing your infrastructure to locate any Linux servers using musl libc without PAM that are running OpenSSH versions older than 10.6p2. Once identified, prioritize these systems based on their network accessibility and the sensitivity of the data they manage. Work with your system administration team to verify if the OpenSSH package can be updated to version 10.6p2 or later, which contains the necessary fix for this authentication logic error.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.