A request path beginning with "../" escaped the path the server was
started on, letting a client list, read, create, overwrite and delete
objects outside it.
The check added for CVE-2026-59733 rejected non-canonical paths by
comparing them with path.Clean, but path.Clean cannot resolve leading
".." elements in a relative path so it leaves them in place and the
comparison comes out equal. Only interior traversal such as "a/../../x"
was rejected. Whether a path then escaped depended on the backend:
those which join the root with the remote before encoding it - webdav,
ftp, sftp, http and memory - resolved the ".." away, while local and s3
encode the dot elements first and were unaffected.
A bare "." was accepted for the same reason, which on bucket backends
addresses the served directory's own key.
Validate with io/fs.ValidPath instead, which rejects ".", ".." and empty
elements wherever they appear. The empty path stays valid as the root of
the API, and "." is excluded explicitly because ValidPath accepts it as
the root of an FS.
Nick Craig-Wood
cc5a189f00