CVE-2026-54590 in AsyncSSH
Summary
by MITRE • 07/08/2026
AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Version 2.23.0 contains an incomplete fix for CVE-2026-45309 in SSHServerConfig._set_tokens that blocks /, , and .. before %u substitution in AuthorizedKeysFile but does not block a leading ~ or ${ENV}, allowing later expansion in _expand_val and Path(filename).expanduser() to escape the intended authorized-keys directory. This issue is fixed in version 2.23.1.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability in AsyncSSH version 2.23.0 represents a critical path traversal flaw that undermines the security of SSH key management mechanisms. This issue specifically affects the SSHServerConfig._set_tokens method where the developers implemented an incomplete mitigation for CVE-2026-45309. The original fix attempted to prevent directory traversal by blocking forward slashes, backslashes, and double dots before %u substitution in AuthorizedKeysFile paths. However, this remediation failed to account for alternative escape sequences that could bypass the restrictions through leading tilde characters or environment variable expansions within the ${ENV} syntax. The vulnerability stems from a fundamental misunderstanding of how path expansion works in Unix-like systems and the Python pathlib library's expanduser() method.
The technical exploitation of this vulnerability occurs when an attacker crafts malicious entries in authorized keys files that contain leading ~ characters or environment variable references such as ${HOME} or ${USER}. These patterns bypass the initial filtering mechanism and subsequently get processed through the _expand_val function and Path(filename).expanduser() which performs actual user expansion. This creates a scenario where attackers can manipulate the path resolution process to access files outside of the intended authorized-keys directory, potentially leading to privilege escalation or unauthorized file access. The flaw operates at the intersection of path manipulation and environment variable expansion, making it particularly insidious as it leverages legitimate system functionality against its intended purpose.
The operational impact of this vulnerability extends beyond simple path traversal to encompass potential security breaches within SSH server configurations. Attackers could exploit this weakness to gain access to sensitive files that should remain isolated within the authorized-keys directory structure. This includes potentially accessing other users' keys, configuration files, or even system-level information that might be accessible through the expanded paths. The vulnerability affects any system running AsyncSSH version 2.23.0 where SSH servers are configured with AuthorizedKeysFile directives that accept user-supplied input, creating a persistent threat vector for systems that rely on this library for secure remote access management.
Security practitioners should note that this vulnerability aligns with CWE-22 Path Traversal and CWE-78 Improper Neutralization of Special Elements used in OS Command. The attack pattern follows techniques described in the MITRE ATT&CK framework under T1059 Command and Scripting Interpreter and T1566 Phishing, as attackers may craft malicious authorized keys that exploit this path traversal to achieve unauthorized access. The issue demonstrates the complexity of securing input validation mechanisms when dealing with multiple expansion points within a single system. The fix implemented in version 2.23.1 addresses this by ensuring comprehensive filtering that accounts for all potential escape sequences including tilde expansion and environment variable substitution, thereby preventing the bypass that occurred in the previous implementation. Organizations should urgently upgrade to version 2.23.1 or later and conduct thorough audits of their SSH configurations to ensure no malicious entries have been introduced into authorized-keys files.