Overview
Recently, NSFOCUS CERT monitored that OpenSSH released a security update fixing the OpenSSH/musl password authentication bypass vulnerability (CVE-2026-123158). Because crypt() in musl libc returns “*” upon failure, when the target account’s password field is “*” and other accounts using password hashes exist on the system while SSH password authentication is enabled, an unauthenticated attacker can submit an excessively long password. This causes the error return value of musl crypt() to falsely match the target account’s password field, thereby bypassing password authentication and logging into the target account. If the account has sudo/doas privileges, the attacker can further gain root privileges on the system. The CVSS score is 8.1. Vulnerability details and PoCs are currently public, and affected users are advised to take defensive measures as soon as possible.
OpenSSH is a free and open-source implementation of the SSH (Secure Shell) protocol, providing components such as sshd, ssh, and sftp, and is widely used for remote login, file transfer, and secure tunneling. musl libc is a lightweight C standard library designed for Linux systems, commonly found in environments such as Alpine Linux.
NSFOCUS has successfully reproduced this vulnerability:
Reference Link: https://www.openssh.org/txt/release-10.6p2
Scope of Impact
Affected version
- 7.3p1 <= OpenSSH < 10.6p2
Affects Linux systems using musl libc without PAM enabled for password authentication, with a primary focus on Alpine Linux as well as qualifying OpenWrt, Buildroot, Yocto systems, Docker images, and containers.
Unaffected version
- OpenSSH >= 10.6p2
Vulnerability Detection
Manual Detection
Users can determine whether their current system is within the affected scope by checking the OpenSSH version, C runtime library type, and relevant configurations:
Check the OpenSSH version command:
| ssh -V |
Check if the system uses musl libc:
| ldd /usr/sbin/sshd |
Check SSH configuration:
*Check the main configuration file:
grep -iE ‘PasswordAuthentication|UsePAM’ /etc/ssh/sshd_config
*Check the sub-configuration directory (if any):
grep -iE ‘PasswordAuthentication|UsePAM’ /etc/ssh/sshd_config.d/* 2>/dev/null
If the current version is within the affected range, PasswordAuthentication is set to yes, and PAM is not enabled on the system, security risks may exist.
Mitigation
Official Upgrade
The vendor has released a new version to fix this vulnerability. Affected users should upgrade as soon as possible for protection.
- Download Link: https://www.openssh.com/portable.html
Other Mitigation Measures
If business operations permit, affected users can apply the following temporary mitigations:
- Disable SSH password authentication by setting PasswordAuthentication no in /etc/ssh/sshd_config and restarting sshd.
- Set PermitRootLogin to prohibit-password or no.
Statement
This advisory is only used to describe a potential risk. NSFOCUS does not provide any commitment or promise on this advisory. NSFOCUS and the author will not bear any liability for any direct and/or indirect consequences and losses caused by transmitting and/or using this advisory. NSFOCUS reserves all the rights to modify and interpret this advisory. Please include this statement paragraph when reproducing or transferring this advisory. Do not modify this advisory, add/delete any information to/from it, or use this advisory for commercial purposes without permission from NSFOCUS.
About NSFOCUS
NSFOCUS, a pioneering leader in cybersecurity, is dedicated to safeguarding telecommunications, Internet service providers, hosting providers, and enterprises from sophisticated cyberattacks.
Founded in 2000, NSFOCUS operates globally with over 3000 employees at two headquarters in Beijing, China, and Santa Clara, CA, USA, and over 50 offices worldwide. It has a proven track record of protecting over 25% of the Fortune Global 500 companies, including four of the five largest banks and six of the world’s top ten telecommunications companies.
Leveraging technical prowess and innovation, NSFOCUS delivers a comprehensive suite of security solutions, including the Intelligent Security Operations Platform (ISOP) for modern SOC, DDoS Protection, Continuous Threat Exposure Management (CTEM) Service and Web Application and API Protection (WAAP). All the solutions and services are augmented by the Security Large Language Model (SecLLM), ML, patented algorithms and other cutting-edge research achievements developed by NSFOCUS.
