Claude Sonnet 5.5 (effort default (no --effort flag) · thinking on) on Fix catastrophic-backtracking (ReDoS) regexes in signup validators without changing what they accept

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: $0.10
Wall time: 40s
Human step-ins: 0
Outcome: Pass

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Exact prompt

# Task: fix a ReDoS in the signup validators A pen-test found that crafted inputs make `validate.py` hang the API worker (catastrophic regex backtracking). Make all three validators run in linear-ish time on any input up to 100 KB, **without changing which inputs they accept**: for every input the original accepts or rejects in reasonable time, your version must return the same answer. Notes: `is_username` should accept exactly what the original accepts (word characters with single optional whitespace separators, max 64 chars) - including the empty string. Keep function names/signatures. Python stdlib only (`re` is fine; so is not using regex at all). Write `SECURITY_FIX.md` (2+ lines). No network.

Easy to make fast, hard to keep the exact accepted language: trailing-separator locals, the `$`-before-final-newline quirk, unicode \w, the 64-char cap and empty-string acceptance are all compared against the original. 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. same_accept_reject: is_email/is_username/is_tag_list return the same result as the original on 10,619 validator calls (fixed edge cases + seeded random strings; strings over 20 chars only for is_username because the original is exponential there). 2. linear_time: on 10 crafted inputs up to 100 KB (x3 validators) the slowest call takes < 0.25 s (subprocess capped at 20 s). 3. notes_present: SECURITY_FIX.md has at least 2 lines.

Setup notes: Use Case Scout batch 5 case e09-redos-validator (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. 1-min load average at scoring: 0.60. Claude Code's own reported cost for claude-sonnet-5.5: $0.096171. Score: 3/3 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: $0.10
Wall time: 40s
Human step-ins: 0
Turns: 5

Model usage
Model	Input tok	Output tok	Cost
AClaude Sonnet 5.5anthropic/claude-sonnet-5.5effort default (no --effort flag) · thinking on	104,616	4,382	$0.10

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. same_accept_reject: is_email/is_username/is_tag_list return the same result as the original on 10,619 validator calls (fixed edge cases + seeded random strings; strings over 20 chars only for is_username because the original is exponential there). 2. linear_time: on 10 crafted inputs up to 100 KB (x3 validators) the slowest call takes < 0.25 s (subprocess capped at 20 s). 3. notes_present: SECURITY_FIX.md has at least 2 lines.
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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  "result": "I rewrote the three patterns in `validate.py` so each one has only one way to match, and the validators now accept and reject the same inputs as before.\n\n- **Cause:** nested quantifiers over overlapping character classes, such as `(\\w+\\s?)*`. A long run of valid characters followed by one bad character made the regex try exponentially many ways to split the run.\n- **Fix:** each repeated run now has a mandatory separator between it and the next, so there is nothing to retry. Function names and signatures are unchanged, and it still uses only `re`.\n- **Same answers:** I compared old and new on 600,000 random short inputs (200,000 per validator) and found no differences. The inputs included newlines, tabs and non-ASCII word characters. That covers the empty username and the quirk where `$` also matches before a trailing newline.\n- **Speed:** adversarial 100 KB inputs now take 17 ms or less. I tried long runs ending in a bad character, repeated `a.`, `a-`, `a,#` and `a#` patterns, and a long domain with no valid ending.\n\n`SECURITY_FIX.md` explains the cause and the fix.",
  "ttft_ms": 2044,
  "type": "result",
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  "uuid": "[REDACTED]",
  "ttft_stream_ms": 1745,
  "time_to_request_ms": 213,
  "first_content_frame_ms": 1745,
  "queued_turn_count": 0,
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 }
]