SFTP root escape via component-agnostic prefix check in ssh_sftpd

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability in Erlang OTP (ssh_sftpd module) allows Path Traversal.

This vulnerability is associated with program files lib/ssh/src/ssh_sftpd.erl and program routines ssh_sftpd:is_within_root/2.

The SFTP server uses string prefix matching via lists:prefix/2 rather than proper path component validation when checking if a path is within the configured root directory. This allows authenticated users to access sibling directories that share a common name prefix with the configured root directory. For example, if root is set to /home/user1, paths like /home/user10 or /home/user1_backup would incorrectly be considered within the root.

This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.

• Use OS-level chroot to run the Erlang VM/SFTP server process in an isolated filesystem environment.
• Ensure that no sensitive or precious data is readable or writable by the OS user running the Erlang VM.
• Ensure that the SFTP server port is not reachable from untrusted machines.
• Use directory naming conventions that avoid common prefixes (e.g., /home/users/alice/ instead of /home/user1/).

The SFTP subsystem must be enabled on the SSH server, the SSH port must be reachable by the attacker, and a root directory must be configured. This is the case when ssh_sftpd is included in the subsystems option with a root parameter and there exist sibling directories sharing the same name prefix as the root.