GHSA-2cf5-4w76-r9qv

Arbitrary Code Execution in handlebars

High npm

Severity: 7.3 / 10; High · CVSS 3.1
Exploit chance (EPSS): —; No score published
Known exploited: No; Not in CISA's catalog
Weakness: CWE-94 Improper Control of Generation of Code ('Code Injection')
Fix: 3.0.8; Upgrade handlebars to this version or later

Description

Versions of handlebars prior to 3.0.8 or 4.5.2 are vulnerable to Arbitrary Code Execution. The package's lookup helper fails to properly validate templates, allowing attackers to submit templates that execute arbitrary JavaScript in the system. It can be used to run arbitrary code in a server processing Handlebars templates or on a victim's browser (effectively serving as Cross-Site Scripting).

The following template can be used to demonstrate the vulnerability:

	{{#with split as |a|}}
		{{pop (push "alert('Vulnerable Handlebars JS');")}}
		{{#with (concat (lookup join (slice 0 1)))}}
			{{#each (slice 2 3)}}
				{{#with (apply 0 a)}}
					{{.}}
				{{/with}}
			{{/each}}
		{{/with}}
	{{/with}}
{{/with}}```


## Recommendation

Upgrade to version 3.0.8, 4.5.2 or later.

Affected versions

npm

Package	Vulnerable	Fixed in
handlebars	`< 3.0.8`	`3.0.8`
handlebars	`>= 4.0.0, < 4.5.2`	`4.5.2`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

handlebars npm → `3.0.8`

npm

npm install handlebars@3.0.8

yarn

yarn add handlebars@3.0.8

pnpm

pnpm add handlebars@3.0.8

Severity breakdown

CVSS 3.1 7.3

CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:C/C:L/I:H/A:L

Attack vector: Local
Attack complexity: Low
Privileges required: Low
User interaction: Required
Scope: Changed
Confidentiality: Low
Integrity: High
Availability: Low

Weakness

CWE-94: Improper Control of Generation of Code ('Code Injection')

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.

How to prevent it

Architecture and Design: Refactor your program so that you do not have to dynamically generate code.
Architecture and Design: Run your code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which code can be executed by your product. Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of your application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.
Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. To reduce the likelihood of code injection, use stringent allowlists that limit which constructs are allowed. If you are dynamically constructing code that invokes a function, then verifying that the input is alphanumeric might be insufficient. An attacker might still be able to reference a dangerous function that you did not intend to allow, such as system(), exec(), or exit().
Testing: Use dynamic tools and techniques that interact with the product using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The product's operation may slow down, but it should not become unstable, crash, or generate incorrect results.
Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
Operation: Run the code in an environment that performs automatic taint propagation and prevents any command execution that uses tainted variables, such as Perl's "-T" switch. This will force the program to perform validation steps that remove the taint, although you must be careful to correctly validate your inputs so that you do not accidentally mark dangerous inputs as untainted (see CWE-183 and CWE-184).
Implementation: For Python programs, it is frequently encouraged to use the ast.literal_eval() function instead of eval, since it is intentionally designed to avoid executing code. However, an adversary could still cause excessive memory or stack consumption via deeply nested structures [REF-1372], so the python documentation discourages use of ast.literal_eval() on untrusted data [REF-1373].

References

Timeline

Sep 4, 2020 Published
Sep 10, 2026 Last updated by the source

Advisory	Severity	Why it's related
GHSA-8r5x-fm3f-whwj Handlebars: JavaScript Injection via AST Type Confusion in compile (bypass of CVE-2026-33937)	Critical	Also affects handlebars
GHSA-p8wg-vrv2-v86f Handlebars: JavaScript Injection via Own Property Check Bypass	Critical	Also affects handlebars
GHSA-xw65-4hp5-5hc7 Handlebars: JavaScript Injection via Unsafe Inline Embedding of Precompiled Templates	Medium	Also affects handlebars
GHSA-7rx3-28cr-v5wh Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry	Medium	Also affects handlebars
GHSA-442j-39wm-28r2 Handlebars.js has a Property Access Validation Bypass in container.lookup	Low	Also affects handlebars
GHSA-xjpj-3mr7-gcpf Handlebars.js has JavaScript Injection in CLI Precompiler via Unescaped Names and Options	High	Also affects handlebars
GHSA-xhpv-hc6g-r9c6 Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial	High	Also affects handlebars
GHSA-9cx6-37pm-9jff Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation	High	Also affects handlebars

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