The SOH/STX control frame carries a client-supplied filename that was passed
straight to FSCom open/remove/exists, so a ".." component could write, read, or
delete outside the filesystem root. On embedded LittleFS this is largely inert
(no parent of the partition root); on the Portduino daemon FSCom is the host
filesystem under a mountpoint, so it is a real arbitrary-path write/read/delete.
Validate the filename before any FS access: reject empty and any ".." path
component, and NAK the transfer. Absolute and subdirectory paths are still
accepted - the file manager transfers them from the manifest and PortduinoFS
confines them to its mountpoint - so only traversal out of the root is blocked.
Reachable only from a local client connection (PhoneAPI: BLE/USB/serial/TCP),
not over the RF mesh; on the daemon the TCP API makes it network-reachable.
native-suite-count goes to 34: +1 for the new test_xmodem suite and +1 correcting
a pre-existing miscount (it read 32 for 33 suite directories).
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
