Today's spill/cache-writer hardening (tools/spill_safety.py,
write_text_exclusive/ensure_spill_dir with O_CREAT|O_EXCL|O_NOFOLLOW)
migrated tools/web_tools.py::_store_full_text() — which writes to the same
cache/web directory with the same content-hash filename scheme — but left
its near-identical sibling, tools/browser_tool.py::_store_full_snapshot(),
on the pre-fix plain open()/write_text() pattern. A pre-planted symlink at
the content-hash path redirected the write onto an arbitrary user-owned
file, same as the sites that commit fixed.
Reproduced live: with a symlink planted at the exact
browser-snapshot-<digest>.txt path (predictable from the snapshot content
hash), the pre-fix write followed the link and overwrote the link's
target with the (secret-redacted but otherwise user/page-controlled)
snapshot content.
Fix mirrors _store_full_text's exact usage: ensure_spill_dir(private=False)
+ write_text_exclusive(private=False, overwrite=True) — not private since
cache/web is bind-mounted into remote backends whose container UID must
read it; overwrite=True because re-snapshotting the same page state
legitimately reuses the same content-hash name (the overwrite path
lstat-unlinks the link itself, never following it to write through).
Added a regression test planting a symlink at the exact digest path and
asserting the link's target is untouched (only the link itself gets
safely replaced by a real file). Mutation-verified: with the fix stashed,
the pre-fix code wrote the snapshot content into the symlink's target
file, reproducing the vulnerability exactly.
pierrenode
bf8b28f27a