Claude Sonnet 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 Sonnet 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: $0.07
Wall time: 22s
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-sonnet-5.5: $0.067955. 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: $0.07
Wall time: 22s
Human step-ins: 0
Turns: 4

Model usage
Model	Input tok	Output tok	Cost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on	80,085	2,471	$0.07

Reproduce this run

Use the use case prompt on the use case page.
Same setup: Claude Sonnet 5.5 (anthropic/claude-sonnet-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

[
 {
  "duration_api_ms": 20062,
  "stop_reason": "end_turn",
  "session_id": "[REDACTED]",
  "total_cost_usd": 0.0679546,
  "usage": {
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   "cache_creation_input_tokens": 9031,
   "cache_read_input_tokens": 71046,
   "output_tokens": 2471,
   "output_tokens_details": {
    "thinking_tokens": 354
   },
   "server_tool_use": {
    "web_search_requests": 0,
    "web_fetch_requests": 0
   },
   "service_tier": "standard",
   "cache_creation": {
    "ephemeral_1h_input_tokens": 9031,
    "ephemeral_5m_input_tokens": 0
   },
   "inference_geo": "global",
   "iterations": [
    {
     "input_tokens": 2,
     "output_tokens": 606,
     "cache_read_input_tokens": 21008,
     "cache_creation_input_tokens": 562,
     "cache_creation": {
      "ephemeral_5m_input_tokens": 0,
      "ephemeral_1h_input_tokens": 562
     },
     "type": "message"
    }
   ],
   "speed": "standard",
   "fallback_credit": null
  },
  "modelUsage": {
   "claude-sonnet-5-5": {
    "inputTokens": 8,
    "outputTokens": 2471,
    "cacheReadInputTokens": 71046,
    "cacheCreationInputTokens": 9031,
    "webSearchRequests": 0,
    "costUSD": 0.0679546,
    "contextWindow": 1000000,
    "maxOutputTokens": 128000,
    "thinkingTokens": 354,
    "canonicalModel": "claude-sonnet-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,
   "spawned_by_subagents": 0,
   "completed": 0,
   "failed": 0,
   "killed": {
    "parent": 0,
    "user": 0,
    "system": 0
   },
   "refused": {
    "depth_limit": 0,
    "concurrency_limit": 0,
    "budget": 0
   },
   "by_type": {}
  },
  "safety_stops": 0,
  "is_error": false,
  "num_turns": 4,
  "subtype": "success",
  "api_error_status": null,
  "result": "I fixed the path traversal in `server.py`. Every attack request I tried now gets a 404, and the legitimate ones still return 200 with the right Content-Type. I tested against a running copy on localhost with `curl`.\n\n**What I changed:** requests now go through a new `resolve()` method that decodes the path once, then rejects NUL bytes, backslashes and `..` segments. It follows symlinks to the real file and requires the result to be inside the real `public/` directory. Any file or decoding error returns 404 instead of a 5xx. The command line, `E05_PORT`, default port 18501 and the 127.0.0.1 bind are unchanged.\n\n**Attacks tested, all 404:**\n- `/../config.env`\n- `/%2e%2e/config.env`\n- `/%252e%252e/config.env`\n- `/..%2fconfig.env`\n- `/docs/..%5c..%5cconfig.env`\n- `/docs/../../config.env`\n- `//etc/passwd` and `/etc/passwd`\n- `/a%00b` and `/%00`\n- A temporary symlink in `public/` pointing at `config.env`.\n\n**Legitimate requests tested:**\n- `/` \u2192 200 `text/html`\n- `/docs/readme.txt` \u2192 200 `text/plain`\n- `/docs/my%20file.txt` \u2192 200 `text/plain`\n- `/docs/readme.txt?x=1` \u2192 200 `text/plain`\n- A temporary symlink to a file inside `public/` \u2192 200 `text/plain`\n- `/docs/`, `/docs` and an empty directory \u2192 404\n- Invalid UTF-8 (`/docs/%ff`) \u2192 404, no crash\n\nI removed the temporary symlinks and the empty directory after testing. `SECURITY_FIX.md` is written with what was wrong and what changed.",
  "ttft_ms": 2102,
  "type": "result",
  "duration_ms": 21515,
  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1693,
  "time_to_request_ms": 198,
  "first_content_frame_ms": 1693,
  "queued_turn_count": 0,
  "result_index": 0
 }
]