brain_tunnel passes caller-supplied source and target paths to
WriteTunnel, which checked only the first segments ("wiki/<wing>/...")
and joined them onto the brain dir. "wiki/b/x/../../../../tmp/f.md"
resolved outside the brain, and appendSeeAlso read and rewrote that file
(verified by execution): any MCP token holder could append to any
existing file the pod user can write, inside the brain (raw/, sessions/,
classification.yaml) or outside it.
- checkNotePath: only a clean, local wiki/.../<slug>.md path, with no
".", ".." or empty segments.
- Both notes are read and rewritten through brainfs. New
brainfs.WriteRegular: never creates; Lstat through the root, open with
O_NOFOLLOW|O_NONBLOCK, and require the opened handle to be the same
regular file (os.SameFile). os.Root.OpenFile follows an in-root final
symlink even with O_NOFOLLOW (verified on go1.27), so the flag alone
does not refuse it.
- The source is no longer rewritten when the target is refused.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
mathiasandClaude Opus 5.5
e548b97ba4