CVE-2026-54591: AsyncSSH Path Traversal Arbitrary File Write Vulnerability

Technical summary

The vulnerability stems from improper path validation in AsyncSSH's SCP implementation. The _parse_cd_args function in scp.py accepts server-provided directory change commands and returns filenames verbatim without sanitization. When these names are subsequently joined to the destination path via _recv_files, no enforcement of directory boundaries occurs. An attacker controlling the SSH server can embed ../ sequences in filenames to traverse up the directory tree, enabling arbitrary file writes on the client filesystem. The flaw is a classic path-traversal issue (CWE-22) that bypasses intended access controls on file placement.

Business impact

Organizations using AsyncSSH for automated file transfers or secure copy operations face risk of data corruption, system compromise, or malware injection. An attacker could overwrite critical application files, configuration files, or system libraries, leading to denial of service, privilege escalation, or lateral movement. The business impact depends on the privileges of the user running the AsyncSSH client and the sensitivity of files in accessible directories. Any automation pipeline relying on AsyncSSH for untrusted or internet-facing server connections becomes an attack surface.

Affected systems

AsyncSSH versions prior to 2.23.1 are affected. The vulnerability specifically impacts SCP client functionality when connecting to SSH servers. Any Python application embedding AsyncSSH—whether for automated backups, file synchronization, or secure transfers—is potentially vulnerable if it receives connections from or connects to servers not fully under the organization's control. Systems where non-administrative users run AsyncSSH clients are somewhat less critical but still at risk.

Exploitability

Exploitation requires that a user initiate an SCP download from a malicious or compromised SSH server. The attacker must control or compromise the SSH server endpoint to inject traversal sequences into filenames. No network-level authentication bypass or complex technical steps are needed beyond hosting a hostile SSH server. However, user interaction is necessary—the client must deliberately connect and request a file transfer. The attack is straightforward to execute for any actor capable of running an SSH server, making it moderately exploitable in scenarios where users connect to untrusted servers or where server compromise occurs.

Remediation

Upgrade AsyncSSH to version 2.23.1 or later immediately. This version includes proper path-validation logic to ensure filenames cannot escape the intended destination directory. After patching, no additional configuration changes are required. Organizations should verify that all instances of AsyncSSH in use—including those embedded in custom applications or containerized services—are updated. If immediate patching is not possible, restrict SCP client usage to known, trusted SSH servers and avoid accepting file transfers from any server not fully under your control.

Patch guidance

Update the AsyncSSH package to version 2.23.1 or later via your package manager (pip install --upgrade asyncssh or equivalent). Verify the update by confirming the installed version matches or exceeds 2.23.1. If AsyncSSH is vendored or embedded in a larger application, check the application's release notes to confirm it includes the patched AsyncSSH version. Test functionality in a non-production environment before deploying widely. For organizations using multiple Python environments or virtual environments, ensure each is updated separately.

Detection guidance

Monitor for attempts to download files with ../ or similar traversal sequences in SSH/SCP logs if available. Examine file creation events for unexpected files appearing in system or application directories during SCP operations. Set up alerting on file integrity changes in critical directories. Review SSH server logs for unusual directory traversal patterns in file names during SCP sessions. Endpoint detection and response (EDR) tools should flag unexpected file writes outside expected directories by Python processes linked to AsyncSSH. Historical detection is possible by reviewing file creation timestamps and paths to identify if files were written outside intended download locations.

Why prioritize this

This vulnerability merits high priority because it enables arbitrary file writes on client systems via a commonly-used library, requires only moderate attacker capability (hosting an SSH server), and has a high CVSS score of 8.1. While user interaction is required, the attack surface is real for organizations performing automated transfers, CI/CD pipelines, or user-initiated downloads from semi-trusted sources. The integrity impact is severe—an attacker can corrupt or inject malicious files—and availability impact is possible through system file corruption. Prompt patching limits exposure window.

Risk score, explained

The CVSS 3.1 score of 8.1 (HIGH) reflects high integrity and availability impact (I:H, A:H), network-adjacent attack vector (AV:N), low attack complexity (AC:L), and no privilege requirements (PR:N). The user interaction requirement (UI:R) prevents a perfect score but remains significant. The scoring appropriately captures that while an attacker cannot silently exploit this from the network alone, a user performing a legitimate file transfer to a compromised or hostile server becomes a vector for system compromise. The unchanged scope (S:U) indicates the impact is confined to the affected system.

Frequently asked questions