Claude Haiku 5.5 (effort default (no --effort flag) · thinking on) on Close a path-traversal hole (encodings, backslashes, NUL, symlinks) in a stdlib static file server

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  "result": "The path-traversal hole is closed. `server.py` now resolves every request to a real file under `public/` and refuses anything else with a 404.\n\n**What was wrong:** the original joined the request path onto `public/` and checked only that the result was a file. Nothing confirmed it stayed inside `public/`. Against the original, `/../config.env`, `/docs/%2e%2e/%2e%2e/config.env`, `/docs/..%2f..%2fconfig.env`, a symlink to `../config.env`, and a symlink to `/etc/passwd` all returned file contents.\n\n**What I changed** (all in `resolve()` in `server.py`):\n- Strip `?` and `#`, then percent-decode once. Double-encoded input becomes a literal name, so it can't climb out.\n- Refuse NUL bytes and backslashes.\n- Drop empty segments and refuse any `.` or `..` segment.\n- Resolve the candidate with `realpath()` and require it to sit under `realpath(public/)`. This catches symlinks that point outside, and it still allows symlinks that point inside.\n- Require a regular file. A directory gets `index.html` only when the request ends in `/`, so a directory without one returns 404.\n- Any refusal or `OSError`/`ValueError` returns 404, so the handler never raises.\n\nContent-Type, query-string handling, the command line, `E05_PORT`, the default port and the 127.0.0.1 bind are unchanged. It is still stdlib only.\n\n**Verification:** I ran the new server and the original against a copy of the tree with extra symlinks, so the real `public/` stayed untouched. About 40 request variants went to each.\n- The new server returned 404 for every traversal variant, and it stayed up.\n- `/`, `/docs/readme.txt`, `/docs/my%20file.txt`, `/docs/readme.txt?x=1`, and the inside-pointing symlink returned 200 with the expected Content-Type.\n- The original leaked `config.env` and `/etc/passwd` through the requests above, so the probe does detect the bug.\n\n**Caveats:**\n- A file whose name contains a literal backslash can no longer be served. These names are rejected outright.\n- The check runs before the file is opened. Exploiting that needs write access to `public/` to retarget a symlink, which I judged outside the threat model. This is noted in `SECURITY_FIX.md`.\n\nI wrote `SECURITY_FIX.md` next to `server.py`. The work directory is not a git repo, so I made no commit.",
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