Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry

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## Summary The prototype method blocklist in `lib/handlebars/internal/proto-access.js` blocks `constructor`, `__defineGetter__`, `__defineSetter__`, and `__lookupGetter__`, but omits the symmetric `__lookupSetter__`. This omission is only exploitable when the non-default runtime option `allowProtoMethodsByDefault: true` is explicitly set — in that configuration `__lookupSetter__` becomes accessible while its counterparts remain blocked, creating an inconsistent security boundary. `4.6.0` is the version that introduced `protoAccessControl` and the `allowProtoMethodsByDefault` runtime option. ## Description In `lib/handlebars/internal/proto-access.js`: ```javascript const methodWhiteList = Object.create(null); methodWhiteList['constructor'] = false; methodWhiteList['__defineGetter__'] = false; methodWhiteList['__defineSetter__'] = false; methodWhiteList['__lookupGetter__'] = false; // __lookupSetter__ intentionally blocked in CVE-2021-23383, // but omitted here — creating an asymmetric blocklist ``` All four legacy accessor helpers (`__defineGetter__`, `__defineSetter__`, `__lookupGetter__`, `__lookupSetter__`) were involved in the exploit chain addressed by CVE-2021-23383. Three of the four were explicitly blocked; `__lookupSetter__` was left out. When `allowProtoMethodsByDefault: true` is set, any prototype method **not present** in `methodWhiteList` is permitted by default. Because `__lookupSetter__` is absent from the list, it passes the `checkWhiteList` check and is accessible in templates, while `__lookupGetter__` (its sibling) is correctly denied. ## Workarounds - Do **not** set `allowProtoMethodsByDefault: true`. The default configuration is not affected. - If `allowProtoMethodsByDefault` must be enabled, ensure templates do not reference `__lookupSetter__` through untrusted input.

Source published

29 Mar 2026 15:17:15 UTC

Source modified

10 Sep 2026 03:50:38 UTC

Saved here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

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