CVE-2026-59846

Undergoing Analysis Undergoing Analysis - In Progress

libssh ProxyCommand Username Injection Vulnerability

Publication date: 2026-07-21

Last updated on: 2026-07-30

Assigner: Red Hat, Inc.

Description

A flaw was found in libssh. A malicious username expanded through %r in ProxyCommand handling can inject shell metacharacters, exposing environment variables and causing unintended shell behavior.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-07-21

Last Modified

2026-07-30

Generated

2026-08-10

AI Q&A

2026-07-21

EPSS Evaluated

2026-08-09

NVD

CVE-2026-59846

EUVD

EUVD-2026-46254

Affected Vendors & Products

Vendor	Product	Version / Range
redhat	enterprise_linux	8.0
redhat	enterprise_linux	9.0
libssh	libssh	*
redhat	enterprise_linux	10.0
redhat	hardened_images	*

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-77	The product constructs all or part of a command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended command when it is sent to a downstream component.

Attack-Flow Graph

Executive Summary

This vulnerability in libssh involves improper handling of username expansion in ProxyCommand. When a username contains %r and is expanded, malicious shell metacharacters can be injected. This bypasses validation in ssh_check_username_syntax() and allows attackers to manipulate shell behavior, potentially exposing environment variables or causing unintended actions.

Detection Guidance

Check libssh versions for affected releases and inspect logs for ProxyCommand handling with username expansion containing %r. Look for unusual shell metacharacters in username fields during SSH connections.

Impact Analysis

If you use libssh with ProxyCommand and untrusted username input, an attacker could craft a username to execute shell commands or leak sensitive environment variables. This requires the attacker to control part of the username and the system to use ProxyCommand with shell evaluation.

Compliance Impact

This vulnerability could lead to information disclosure, which may violate GDPR (data protection) or HIPAA (privacy) if sensitive data like environment variables are exposed. Compliance depends on whether the affected system processes regulated data and the impact of the disclosure.

Mitigation Strategies

Update libssh to the latest patched version. Disable ProxyCommand usage if not required. Validate all username inputs to prevent shell metacharacter injection before expansion.

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