AsyncSSH, a Python SSH library, has a flaw where the SCP client does not enforce directory boundaries. As CWE-22, it allows a malicious SSH server to send filenames containing traversal sequences (../) that the client accepts verbatim and joins to a destination path. This can lead to arbitrary files being written anywhere on the client’s filesystem, potentially overwriting critical files or inserting malicious payloads, thereby compromising confidentiality, integrity, or availability.

The product affected is AsyncSSH by ronf. All releases prior to 2.23.1 are vulnerable. This issue is fixed in version 2.23.1 and later. Any application or service that uses an older AsyncSSH release is impacted.

The CVSS score is 8.1, indicating high severity. The EPSS score is less than 1%, and the vulnerability is not listed in the CISA KEV catalog. The likely attack vector is remote: an attacker controlling an SSH server can send a malicious SCP request to a vulnerable client, enabling arbitrary file writes without needing local access. Once exploited, the attacker can compromise or disrupt the client system.