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fix: stop CSP allow-lists triggering challenge detection (#264) #266

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fix: stop CSP allow-lists triggering challenge detection (#264) #266

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10 changes: 10 additions & 0 deletions src/fetch/classify.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,16 @@ describe('fetch classification', () => {
it('recognizes explicit challenges but not bare forbidden responses', () => {
expect(isChallengeResponse(403, { server: 'cloudflare' }, 'Just a moment...')).toBe(true);
expect(isChallengeResponse(403, {}, 'forbidden')).toBe(false);
expect(isChallengeResponse(403, { server: 'cloudflare' }, 'forbidden')).toBe(true);
});
it('ignores third-party allow-lists in CSP and friends', () => {
const csp = "default-src 'self'; script-src https://www.google.com/recaptcha/ https://cdnjs.cloudflare.com/";
expect(isChallengeResponse(200, { 'content-security-policy': csp }, '<html>Hacker News</html>')).toBe(false);
expect(isChallengeResponse(403, { 'content-security-policy': csp }, 'forbidden')).toBe(false);
});
it('does not treat a served 200 as a challenge on headers alone', () => {
expect(isChallengeResponse(200, { server: 'cloudflare' }, '<html>real page</html>')).toBe(false);
expect(isChallengeResponse(200, { server: 'cloudflare' }, 'Just a moment...')).toBe(true);
});
it('recognizes script-heavy app shells', () => expect(isJavaScriptShell('<div id="root"></div><script src="/app.js"></script><script>boot()</script>')).toBe(true));
});
10 changes: 8 additions & 2 deletions src/fetch/classify.ts
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Original file line number Diff line number Diff line change
@@ -1,8 +1,14 @@
const challengeMarkers = /cloudflare|cf-chl|datadome|perimeterx|px-captcha|akamai|captcha|just a moment|verify you are human/i;
// Headers that say something about *this* response. CSP/report-to/link are allow-lists of third
// parties (cdnjs.cloudflare.com, google.com/recaptcha) and are evidence of nothing.
const signalHeaders = /^(?:server|cf-mitigated|cf-chl-[\w-]+|x-datadome[\w-]*|set-cookie)$/i;

export function isChallengeResponse(status: number, headers: Record<string, string>, body: string): boolean {
const evidence = `${Object.entries(headers).map(([key, value]) => `${key}:${value}`).join('\n')}\n${body.slice(0, 20_000)}`;
return challengeMarkers.test(evidence) && (status === 403 || status === 429 || status === 503 || status === 200);
if (status !== 403 && status !== 429 && status !== 503 && status !== 200) return false;
if (challengeMarkers.test(body.slice(0, 20_000))) return true;
// A 200 with a real body is a served page; headers alone (server: cloudflare) never prove otherwise.
if (status === 200) return false;
return Object.entries(headers).some(([key, value]) => signalHeaders.test(key) && challengeMarkers.test(value));
}

export function isJavaScriptShell(body: string): boolean {
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