SFTP chroot bypass via path traversal in SSH_FXP_FSETSTAT

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in Erlang OTP ssh (ssh_sftpd module) allows an authenticated SFTP user to modify file attributes outside the configured chroot directory.

The SFTP daemon (ssh_sftpd) stores the raw, user-supplied path in file handles instead of the chroot-resolved path. When SSH_FXP_FSETSTAT is issued on such a handle, file attributes (permissions, ownership, timestamps) are modified on the real filesystem path, bypassing the root directory boundary entirely.

Any authenticated SFTP user on a server configured with the root option can modify file attributes of files outside the intended chroot boundary. The prerequisite is that a target file must exist on the real filesystem at the same relative path. Note that this vulnerability only allows modification of file attributes; file contents cannot be read or altered through this attack vector.

If the SSH daemon runs as root, this enables direct privilege escalation: an attacker can set the setuid bit on any binary, change ownership of sensitive files, or make system configuration world-writable.

This vulnerability is associated with program files lib/ssh/src/ssh_sftpd.erl and program routines ssh_sftpd:do_open/4 and ssh_sftpd:handle_op/4.

This issue affects OTP from OTP 17.0 before OTP 26.2.5.20, OTP 27.3.4.11, and OTP 28.4.3, corresponding to ssh from 3.0.1 before 5.1.4.15, 5.2.11.7, and 5.5.2. Whether OTP before OTP 17.0, corresponding to ssh before 3.0.1, is affected is unknown.

• Do not use the root option in ssh_sftpd:subsystem_spec/1, and instead rely on OS-level chroot or container isolation to confine SFTP users.
• Ensure the Erlang VM is not running as a privileged OS user. Running the VM as an unprivileged user limits the impact of this vulnerability, since attribute modifications are constrained by that user's OS-level permissions.

The SFTP subsystem must be configured with the root option in ssh_sftpd:subsystem_spec/1. The root option is not set by default.