GHSA-7RX3-28CR-V5WH

Vulnerability from github – Published: 2026-03-29 15:17 – Updated: 2026-03-29 15:17
VLAI
Summary
Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry
Details

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:

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.
Severity
4.8 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.7.8"
      },
      "package": {
        "ecosystem": "npm",
        "name": "handlebars"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.6.0"
            },
            {
              "fixed": "4.7.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-29T15:17:15Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe 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 \u2014 in that configuration `__lookupSetter__` becomes accessible while its counterparts remain blocked, creating an inconsistent security boundary.\n\n`4.6.0` is the version that introduced `protoAccessControl` and the `allowProtoMethodsByDefault` runtime option.\n\n## Description\n\nIn `lib/handlebars/internal/proto-access.js`:\n\n```javascript\nconst methodWhiteList = Object.create(null);\nmethodWhiteList[\u0027constructor\u0027]      = false;\nmethodWhiteList[\u0027__defineGetter__\u0027] = false;\nmethodWhiteList[\u0027__defineSetter__\u0027] = false;\nmethodWhiteList[\u0027__lookupGetter__\u0027] = false;\n// __lookupSetter__ intentionally blocked in CVE-2021-23383,\n// but omitted here \u2014 creating an asymmetric blocklist\n```\n\nAll 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.\n\nWhen `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.\n\n## Workarounds\n\n- Do **not** set `allowProtoMethodsByDefault: true`. The default configuration is not affected.\n- If `allowProtoMethodsByDefault` must be enabled, ensure templates do not reference  `__lookupSetter__` through untrusted input.",
  "id": "GHSA-7rx3-28cr-v5wh",
  "modified": "2026-03-29T15:17:15Z",
  "published": "2026-03-29T15:17:15Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-7rx3-28cr-v5wh"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-765h-qjxv-5f44"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/handlebars-lang/handlebars.js"
    },
    {
      "type": "WEB",
      "url": "https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry"
}



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