CVE-2026-106583 in OpenSSH
Summary
by MITRE • 10/07/2026
In ssh in OpenSSH before 10.6, a $ or \ character can occur in a command-line username, leading to injection.
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Analysis
by VulDB Data Team • 10/07/2026
The vulnerability identified involves a critical input validation flaw within the SSH client implementation of OpenSSH versions prior to 10.6. This issue specifically affects how the application processes usernames provided via the command line interface. When an attacker supplies a maliciously crafted username containing special shell metacharacters, such as the dollar sign or backslash, the system fails to properly sanitize these inputs before passing them to underlying system commands for authentication processing. This lack of rigorous input validation creates a pathway for command injection attacks, allowing an adversary to execute arbitrary operating system commands with the privileges of the user running the SSH client.
From a technical perspective, this flaw stems from improper neutralization of special elements used in scripts or commands, commonly categorized under CWE-78: Improper Neutralization of Special Elements used in an OS Command. The vulnerability arises because the username field is treated as part of a command string rather than being strictly isolated as data. When characters like $ are present, they can trigger variable expansion by the shell, while backslashes can be used to escape quotes or concatenate strings, effectively altering the intended command structure. This behavior violates fundamental security principles regarding input handling and boundary enforcement between user-supplied data and executable code paths within the application logic.
The operational impact of this vulnerability is severe, particularly in environments where SSH clients are automated through scripts or configuration management tools that accept dynamic usernames. An attacker who can influence the username argument passed to OpenSSH could achieve remote code execution on the local machine running the client. This compromises not only the confidentiality and integrity of the system but also potentially allows for lateral movement if the compromised host is used as a pivot point within an internal network. The severity is further amplified in scenarios where SSH clients run with elevated privileges or when interacting with untrusted hosts, although the primary execution context remains local to the client machine rather than the remote server.
This vulnerability aligns with MITRE ATT&CK technique T1059: Command and Scripting Interpreter, specifically sub-techniques involving shell commands such as sh or bash. It represents a classic injection vector where user input is directly concatenated into system calls without adequate escaping or parameterization. The risk is mitigated by ensuring that all inputs destined for OS-level command execution are strictly validated against an allowlist of expected characters and formats, rather than attempting to block specific malicious patterns which can often be bypassed through encoding variations or context switching within the shell parser.
To address this issue, organizations must immediately upgrade OpenSSH to version 10.6 or later, where the developers have implemented stricter input sanitization routines for command-line arguments. In addition to patching, administrators should review automation scripts and deployment pipelines that invoke SSH clients to ensure they do not pass dynamic values directly into username fields without proper encoding. Implementing defense-in-depth strategies such as restricting shell access on critical systems and employing application whitelisting can further reduce the blast radius if a similar vulnerability were to be exploited in other components of the infrastructure. Regular security audits focusing on input validation practices across all network utility tools are essential to prevent recurrence of injection-based attacks.