Claude Opus 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

AClaude Opus 5.5default · thinking on · Claude Code 2.1.299 (headless -p) · cloud · 2026-10-17 · by @bobsiopener

Trust 80%Fully autonomoussuccess · first tryCompare

Estimated cost: $0.20
Wall time: 47s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Task: close the path-traversal hole in the docs server `server.py` serves files from `public/`. A security report says it can be made to return files outside `public/` (for example `config.env` next to it). Fix it. Requirements: - No request may ever return the contents of a file outside `public/` - whatever the encoding (`..`, percent-encoding incl. double encoding, backslashes, absolute paths, NUL bytes) and including via symlinks inside `public/` that point outside it. Such requests get 404 (or 400/403) and never a 5xx or a crash. - Legitimate files keep working exactly as before: `/` and `/docs/readme.txt`, percent-encoded names like `/docs/my%20file.txt`, query strings ignored, correct Content-Type. - Directories without index.html return 404. Symlinks inside `public/` that point to files *inside* `public/` still work. - Keep it stdlib only, same command line (`python3 server.py [port]`, env `E05_PORT`, default 18501, bind 127.0.0.1). Write a short `SECURITY_FIX.md` (what was wrong, what you changed). No network beyond localhost.

Naive '..' string filters fail on double encoding, backslashes, absolute paths and especially symlinks pointing out of the web root; over-strict fixes break encoded names or in-root symlinks. Original benchmark task written for usecaserank.com (not based on a specific public post). Headless, offline run on a reproducible fixture. Each run's notes say how its cost was measured (estimates are marked).

Success criteria: 1. blocks_traversal: none of 16 attack requests returns the random canary planted in config.env and in a file outside the workdir. 2. attack_status_4xx: every attack request gets a 4xx status (no 5xx, no dropped connection). 3. serves_index: / returns index.html as text/html. 4. serves_file_query: /docs/readme.txt?x=1 returns the file as text/plain. 5. serves_encoded_name: /docs/my%20file.txt works. 6. inside_symlink_ok: a symlink inside public/ pointing to a file inside public/ is served. 7. dir_404: a directory without index.html and a missing file return 404. 8. server_survived: the server process is still running after all requests. 9. notes_present: SECURITY_FIX.md exists and is not empty.

Setup notes: Use Case Scout batch 5 case e05-static-server-traversal (offline fixture). Prepared with batch5/prepare.sh; hidden key/tests kept outside the agent's work dir. Prompt = TASK.md verbatim. Scored with the scout's scorer: `python score.py WORKDIR KEYDIR`. Tools: Read/Write/Edit/Glob/Grep/Bash, web disabled. Single run, no retries. Claude Code's own reported cost for claude-opus-5.5: $0.203259. Score: 9/9 checks. Cost is an ESTIMATE: measured tokens x pricing.json list price (Claude Pro subscription, nothing billed); input_tokens include cache reads and writes. Single run.

Estimated cost List-price estimate; not actually billed.

Metrics

Estimated cost: $0.20
Wall time: 47s
Human step-ins: 0
Turns: 6

Model usage
Model	Input tok	Output tok	Cost
AClaude Opus 5.5anthropic/claude-opus-5.5effort default (no --effort flag) · thinking on	124,400	4,372	$0.20

Reproduce this run

Use the use case prompt on the use case page.
Same setup: Claude Opus 5.5 (anthropic/claude-opus-5.5, effort default (no --effort flag) · thinking on) via Claude Code 2.1.299 (headless -p), cloud.
It counts as a success only if it meets every success criterion: 1. blocks_traversal: none of 16 attack requests returns the random canary planted in config.env and in a file outside the workdir. 2. attack_status_4xx: every attack request gets a 4xx status (no 5xx, no dropped connection). 3. serves_index: / returns index.html as text/html. 4. serves_file_query: /docs/readme.txt?x=1 returns the file as text/plain. 5. serves_encoded_name: /docs/my%20file.txt works. 6. inside_symlink_ok: a symlink inside public/ pointing to a file inside public/ is served. 7. dir_404: a directory without index.html and a missing file return 404. 8. server_survived: the server process is still running after all requests. 9. notes_present: SECURITY_FIX.md exists and is not empty.
Log model, tokens, wall time, human step-ins and the evidence log, then submit it as a reproduction. A different model? Submit a head-to-head rerun.

How runs are scored: Methodology.

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Automated checks

Trust 80% comes from these checks (how Trust % works).

Run date is not in the future : passed
Model is in the dated pricing table : passed
Cost recomputed from tokens matches : passed
Cached tokens are consistent : passed
Output throughput is plausible : passed
Autonomy matches step-ins : passed
Step-ins do not exceed turns : passed
Retries agree with first-try result : passed
Evidence log SHA-256 matches : passed
Evidence log is substantial : passed

Evidence log

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   "server_tool_use": {
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     "cache_creation": {
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     },
     "type": "message"
    }
   ],
   "speed": "standard",
   "fallback_credit": null
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  "modelUsage": {
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    "outputTokens": 4372,
    "cacheReadInputTokens": 112735,
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    "webSearchRequests": 0,
    "costUSD": 0.203259,
    "contextWindow": 1000000,
    "maxOutputTokens": 128000,
    "thinkingTokens": 973,
    "canonicalModel": "claude-opus-5-5",
    "provider": "firstParty",
    "costBasis": "list"
   }
  },
  "permission_denials": [],
  "terminal_reason": "completed",
  "fast_mode_state": "off",
  "fast_mode_disabled_reason": "sdk_opt_in_required",
  "subagent_stats": {
   "spawned": 0,
   "requested": {
    "background": 0,
    "foreground": 0,
    "unset": 0
   },
   "started_in_background": 0,
   "max_depth": 0,
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   "killed": {
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   "refused": {
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   "by_type": {}
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  "safety_stops": 0,
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  "num_turns": 6,
  "subtype": "success",
  "api_error_status": null,
  "result": "I fixed the path-traversal hole in `server.py`: no request can return a file outside `public/` any more, and the normal pages still work. I tested it on a temporary copy of the project and every attack request got a 404.\n\n**What was wrong:** the server decoded the URL and joined it straight onto `public/` without checking where the result pointed. So `/../config.env`, `/%2e%2e/config.env` and similar requests returned files outside `public/`, and symlinks inside `public/` were followed wherever they led.\n\n**What I changed:** all lookups now go through a new `resolve()` function. It:\n- removes the query string and fragment, then decodes the path **exactly once**;\n- rejects NUL bytes and backslashes;\n- resolves the full real path, including every symlink, and only serves it if it is still inside `public/` and is a regular file.\n\nAny rejection or file error returns 404, so nothing can cause a 5xx. It's still stdlib only, with the same command line and port settings.\n\n**What I tested:** 28 paths plus two raw requests sent without curl, against a copy of the project with extra symlinks and a symlink loop added. The real `public/` was not modified.\n- **Still work:** `/`, `/index.html`, `/docs/readme.txt`, `/docs/my%20file.txt`, `?query` ignored, a symlink pointing to a file inside `public/` and a symlinked directory inside it. Content-Type is correct.\n- **Return 404 with no leak:** directories without `index.html`, the symlink loop, `..`, single- and double-encoded dots, encoded `/` and `\\`, a literal backslash, `//etc/passwd`, NUL bytes, symlinks pointing outside `public/` (including one to `/`), invalid encodings and a 5000-character path. The raw `GET http://x/../config.env` and `GET ../config.env` requests also got 404.\n\nOne gap remains, which `SECURITY_FIX.md` notes: there is a tiny window between checking a path and opening it. Someone who can already write files inside `public/` could swap in a symlink during it, but that needs local write access to the served folder.\n\nFiles are in the project folder:\n- `server.py`\n- `SECURITY_FIX.md`",
  "ttft_ms": 2060,
  "type": "result",
  "duration_ms": 46343,
  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1575,
  "time_to_request_ms": 217,
  "first_content_frame_ms": 1575,
  "queued_turn_count": 0,
  "result_index": 0
 }
]