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Medium

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

  • Disclosed: 29.03.2026
  • Updated: 11.10.2026 10:22

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:

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.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
npm package handlebars >= 4.6.0, < 4.7.9 4.7.9

References

CVSS score 4.8/10
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Weakness (CWE)
CWE-1321
Platform
npm
Package
handlebars
Disclosed
29.03.2026
Updated
11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-7rx3-28cr-v5wh - CC BY 4.0

Vulnerability history: handlebars

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (22) in the archive
Total entries
22
First → latest
10.2018 10.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • Critical 6
  • High 10
  • Medium 5
  • Low 1

Disclosures per year

Severity:
  1. Handlebars: JavaScript Injection via AST Type Confusion in compile (bypass of CVE-2026-33937)
    Critical · 9.8 CVE-2026-106446 >= 4.0.0, < 4.7.10 4.7.10
  2. Handlebars: JavaScript Injection via Own Property Check Bypass
    Critical CVE-2026-106445 >= 3.0.0, < 4.7.10 4.7.10
  3. Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry this entry
    Medium · 4.8 ghsa-7rx3-28cr-v5wh >= 4.6.0, < 4.7.9 4.7.9
  4. Handlebars.js has JavaScript Injection in CLI Precompiler via Unescaped Names and Options
    High · 8.2 CVE-2026-33941 >= 4.0.0, < 4.7.9 4.7.9
  5. Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial
    High · 8.1 CVE-2026-33940 >= 4.0.0, < 4.7.9 4.7.9
  6. Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation
    High · 7.5 CVE-2026-33939 >= 4.0.0, < 4.7.9 4.7.9
  7. Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block
    High · 8.1 CVE-2026-33938 >= 4.0.0, < 4.7.9 4.7.9
  8. Handlebars.js has JavaScript Injection via AST Type Confusion
    Critical · 9.8 CVE-2026-33937 >= 4.0.0, < 4.7.9 4.7.9
  9. Prototype Pollution in handlebars
    Critical · 9.8 CVE-2021-23383 < 4.7.7 4.7.7
  10. Regular Expression Denial of Service in Handlebars
    High · 7.5 CVE-2019-20922 >= 4.0.0, < 4.4.5 4.4.5
  11. Arbitrary Code Execution in Handlebars
    High · 8.1 CVE-2019-20920 < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  12. Remote code execution in handlebars when compiling templates
    Critical · 9.8 CVE-2021-23369 < 4.7.7 4.7.7
  13. Arbitrary Code Execution in handlebars
    High ghsa-q2c6-c6pm-g3gh < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  14. Prototype Pollution in handlebars
    High ghsa-g9r4-xpmj-mj65 < 3.0.8, >= 4.0.0, < 4.5.3 3.0.8, 4.5.3
  15. Arbitrary Code Execution in handlebars
    High · 7.3 ghsa-2cf5-4w76-r9qv < 3.0.8, >= 4.0.0, < 4.5.2 3.0.8, 4.5.2
  16. Prototype Pollution in handlebars
    Critical · 9.8 CVE-2019-19919 >= 4.0.0, < 4.3.0, < 3.0.8 4.3.0, 3.0.8
  17. Prototype Pollution in handlebars
    High · 7.3 ghsa-q42p-pg8m-cqh6 >= 4.1.0, < 4.1.2, >= 4.0.0, < 4.0.14, < 3.0.7 4.1.2, 4.0.14, 3.0.7

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