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export interface UaPolicy {
allow: string[];
log: string[];
block: string[];
}
export type UaVerdict = "allow" | "block" | "log";
/**
* The LONGEST entry in `list` occurring in the (already-lowercased) UA, lowercased, or null.
* Longest rather than first-hit so the containment test in uaVerdict() compares against the
* most specific entry a multi-token UA matched, not an arbitrary earlier one.
*/
function bestMatch(lowerUa: string, list: string[]): string | null {
return list.reduce<string | null>((best, entry) => {
const needle = entry.toLowerCase();
if (!lowerUa.includes(needle)) return best;
return best === null || needle.length > best.length ? needle : best;
}, null);
}
export function uaVerdict(ua: string, policy: UaPolicy): UaVerdict {
const lowerUa = ua.toLowerCase();
// Explicit blocks take precedence — a request-controlled UA string that spoofs an
// allow-listed substring (e.g. "Googlebot EvilScraper") must not be able to bypass a
// block entry just by also matching the allow list.
if (bestMatch(lowerUa, policy.block) !== null) return "block";
// A log match WINS over any competing allow match, unconditionally. `log` is a telemetry
// verdict, not a denial (the request is served either way), so resolving a contest the
// wrong way is asymmetric: choosing `allow` loses the ua-log event permanently, while
// choosing `log` costs one log line. A UA presenting BOTH an allow token and a log token
// (e.g. "GPTBot/1.0 DuckDuckBot") is exactly the shape worth recording.
//
// There used to be a carve-out here: a matched allow entry that strictly CONTAINED the
// matched log entry won, so a policy could express a narrow allow exception inside a
// broader log entry (log "Foo", allow "Foo-Search"). It is gone, for two reasons. It was
// spoofable — containment was tested between the two matched ENTRIES, never against the
// UA's own token structure, so a caller writing "Bytespider/2.0 Bytespider-Search/1.0"
// matched both entries as independent tokens and bought itself `allow`, and the UA
// string is entirely request-controlled. And it bought nothing: no entry pair in
// ua-policy.json takes that branch. The pair the shipped policy does depend on runs the
// OTHER way — Apple ships "Applebot" (search, allow) and "Applebot-Extended" (AI
// training, log), where the log entry is the longer one, so there is no containment and
// log wins regardless. Losing the carve-out costs a future narrow-allow vendor variant
// nothing worse than being logged as well as served.
return bestMatch(lowerUa, policy.log) === null ? "allow" : "log"; // unknown UAs fail open
}
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