{ "actor": { "kind": "model", "lab": "anthropic", "name": "claude-opus-5-5" }, "at": "2026-11-05T22:55:48Z", "attempt_log": [ { "attempt": 1, "guard": { "added_required_params": [], "line_delta": 6, "new_imports": [], "ok": true, "path": "webhook.py", "public_api_preserved": true, "removed_lines": 0, "removed_public": [], "violations": [], "warnings": [] }, "reasons": [], "verdict": "VERIFIED_FIX" } ], "claude": { "attempts": 1, "first_attempt": { "input_tokens": 808, "model": "claude-opus-5-5", "output_tokens": 1400, "request_id": null, "seconds": 14.4, "stop_reason": "end_turn" }, "input_tokens": 808, "model": "claude-opus-5-5", "output_tokens": 1400, "request_id": null, "seconds": 14.4, "stop_reason": "end_turn" }, "claude_test_code": "import hashlib\nimport hmac\n\nimport webhook\n\n\ndef _sig(secret, body):\n return hmac.new(secret, body, hashlib.sha256).hexdigest()\n\n\ndef test_valid_signature_accepted():\n assert webhook.check_webhook(b\"k\", b\"payload\", _sig(b\"k\", b\"payload\")) is True\n\n\ndef test_wrong_signature_rejected():\n assert webhook.check_webhook(b\"k\", b\"payload\", _sig(b\"other\", b\"payload\")) is False\n assert webhook.check_webhook(b\"k\", b\"payload\", \"\") is False\n\n\ndef test_non_ascii_signature_rejected_without_error():\n assert webhook.check_webhook(b\"k\", b\"payload\", \"\\u00e9\" * 64) is False\n\n\ndef test_uses_constant_time_compare():\n calls = []\n original = hmac.compare_digest\n\n def spy(a, b):\n calls.append((a, b))\n return original(a, b)\n\n hmac.compare_digest = spy\n try:\n result = webhook.check_webhook(b\"k\", b\"body\", _sig(b\"k\", b\"body\"))\n finally:\n hmac.compare_digest = original\n assert result is True\n assert len(calls) == 1\n", "diff": "--- a/webhook.py\n+++ b/webhook.py\n@@ -4,5 +4,11 @@\n \n def check_webhook(secret: bytes, body: bytes, signature: str) -> bool:\n \"\"\"True if `signature` is the hex HMAC-SHA256 of body under secret.\"\"\"\n+ if not isinstance(signature, str):\n+ return False\n expected = hmac.new(secret, body, hashlib.sha256).hexdigest()\n- return signature == expected\n+ # Compare as bytes so non-ASCII input cannot raise TypeError, and use a\n+ # constant-time comparison so timing does not leak the expected digest.\n+ return hmac.compare_digest(\n+ signature.encode(\"utf-8\"), expected.encode(\"ascii\")\n+ )\n", "explanation": "The signature check compared the attacker-supplied signature against the expected HMAC with `==`, which short-circuits on the first differing character and can leak the expected digest through timing, so the function did not provide the safe verification a webhook check implies. The fix computes the same HMAC-SHA256 hex digest and compares it with hmac.compare_digest. Both sides are encoded to bytes first so a non-ASCII signature is rejected instead of raising TypeError, and a non-str signature returns False. The tests check that valid signatures are accepted, invalid and non-ASCII ones are rejected, and that hmac.compare_digest is actually called. The original file fails that last test.", "falsework_after": { "findings": [], "grade": "REAL" }, "falsework_before": { "findings": [ { "check": "secret_compare_not_constant_time", "detail": "`expected` is compared with ==/!=, which is not constant-time: use hmac.compare_digest.", "line": 8, "severity": "MEDIUM", "snippet": "return signature == expected" } ], "grade": "PARTIAL" }, "finding": { "check": "secret_compare_not_constant_time", "detail": "`expected` is compared with ==/!=, which is not constant-time: use hmac.compare_digest.", "line": 8, "severity": "MEDIUM", "snippet": "return signature == expected" }, "finding_fingerprint": "4a648d97ef4b57c445404a2fe4f1ab1f21e7f3f7c367634d3f580db5c995a007", "fixed_code": "import hashlib\nimport hmac\n\n\ndef check_webhook(secret: bytes, body: bytes, signature: str) -> bool:\n \"\"\"True if `signature` is the hex HMAC-SHA256 of body under secret.\"\"\"\n if not isinstance(signature, str):\n return False\n expected = hmac.new(secret, body, hashlib.sha256).hexdigest()\n # Compare as bytes so non-ASCII input cannot raise TypeError, and use a\n # constant-time comparison so timing does not leak the expected digest.\n return hmac.compare_digest(\n signature.encode(\"utf-8\"), expected.encode(\"ascii\")\n )\n", "fixed_sha256": "017e0a778995097c4448cb6b0f7dd8f069374ec4b32e7563718109757958ab3b", "guard": { "added_required_params": [], "line_delta": 6, "new_imports": [], "ok": true, "path": "webhook.py", "public_api_preserved": true, "removed_lines": 0, "removed_public": [], "violations": [], "warnings": [] }, "lines_changed": 8, "original_code": "import hashlib\nimport hmac\n\n\ndef check_webhook(secret: bytes, body: bytes, signature: str) -> bool:\n \"\"\"True if `signature` is the hex HMAC-SHA256 of body under secret.\"\"\"\n expected = hmac.new(secret, body, hashlib.sha256).hexdigest()\n return signature == expected\n", "original_sha256": "28c72a441c26f9cb26f279d9b58662595eef8079e4f0604a8b74f19be93e9d5d", "path": "webhook.py", "prev_record_hash": "92801b41518909581778f57e225c9e2e3a40186cce5d20d78041dd8191fa04ee", "public_key_b64": "b6hfA2rpAdV29FOdhNXGkk4t91mCV0AnfFYwq0HO0R8=", "reasons": [], "record_hash": "171e88f4ee9cf58ca0d42a3eb28423d689a3858bde07f0254e584fa3f1fcd3ff", "reference_test_author": "customer:live-probe", "reference_test_author_lab": "customer", "reference_test_independence": "independent", "reference_test_origin": "caller", "reference_test_sha256": "efd2ff535a84c27e0211ca6e7cf17411432a5db9848d3dbe39eb9f20567b07cd", "run_id": "ks-20261105T225548Z-4f35c76d", "sample_id": null, "schema": "cain42.keystone.fix-proof.v1", "sequence": 10, "signature_b64": "gj7KK9XF7VVKkQkU7K73F9BS1sAI9HYjIYkOzEhWd5BJb9SWr9g7RWwX6QfEnqpD5QX9nQXObkLXUiGnRj3TDQ==", "standard": "cain42.keystone.proof-standard.v2", "tests": { "claude": { "after": { "all_passed": true, "ran": true, "results": [ { "file": "test_claude.py", "outcome": "passed", "test": "test_non_ascii_signature_rejected_without_error" }, { "file": "test_claude.py", "outcome": "passed", "test": "test_uses_constant_time_compare" }, { "file": "test_claude.py", "outcome": "passed", "test": "test_valid_signature_accepted" }, { "file": "test_claude.py", "outcome": "passed", "test": "test_wrong_signature_rejected" } ], "some_not_passed": false }, "before": { "all_passed": false, "ran": true, "results": [ { "file": "test_claude.py", "outcome": "passed", "test": "test_non_ascii_signature_rejected_without_error" }, { "detail": "assertion", "file": "test_claude.py", "outcome": "not_passed", "test": "test_uses_constant_time_compare" }, { "file": "test_claude.py", "outcome": "passed", "test": "test_valid_signature_accepted" }, { "file": "test_claude.py", "outcome": "passed", "test": "test_wrong_signature_rejected" } ], "some_not_passed": true } }, "reference": { "after": { "all_passed": true, "ran": true, "results": [ { "file": "test_reference.py", "outcome": "passed", "test": "test_comparison_is_constant_time" }, { "file": "test_reference.py", "outcome": "passed", "test": "test_genuine_signature_is_accepted" }, { "file": "test_reference.py", "outcome": "passed", "test": "test_wrong_signature_is_rejected" } ], "some_not_passed": false }, "before": { "all_passed": false, "ran": true, "results": [ { "detail": "assertion", "file": "test_reference.py", "outcome": "not_passed", "test": "test_comparison_is_constant_time" }, { "file": "test_reference.py", "outcome": "passed", "test": "test_genuine_signature_is_accepted" }, { "file": "test_reference.py", "outcome": "passed", "test": "test_wrong_signature_is_rejected" } ], "some_not_passed": true } } }, "verdict": "VERIFIED_FIX" }