CVE-2026-33940: CWE-94: Improper Control of Generation of Code ('Code Injection') in handlebars-lang handlebars.js
Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in `resolvePartial()` and cause `invokePartial()` to return `undefined`. The Handlebars runtime then treats the unresolved partial as a source that needs to be compiled, passing the crafted object to `env.compile()`. Because the object is a valid Handlebars AST containing injected code, the generated JavaScript executes arbitrary commands on the server. The attack requires the adversary to control a value that can be returned by a dynamic partial lookup. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). Without `compile()`, the fallback compilation path in `invokePartial` is unreachable. Second, sanitize context data before rendering: Ensure no value in the context is a non-primitive object that could be passed to a dynamic partial. Third, avoid dynamic partial lookups (`{{> (lookup ...)}}`) when context data is user-controlled.
AI Analysis
Technical Summary
Handlebars.js versions 4.0.0 to 4.7.8 contain a code injection vulnerability (CWE-94) where a crafted object in the template context bypasses guards in resolvePartial(), causing invokePartial() to return undefined. This triggers fallback compilation of the crafted object as a Handlebars AST, resulting in arbitrary JavaScript execution on the server. Exploitation requires control over values returned by dynamic partial lookups. The vulnerability is fixed in version 4.7.9. Mitigations include using the runtime-only build to disable compile(), sanitizing context data to prevent non-primitive objects in dynamic partials, and avoiding dynamic partial lookups with user-controlled input.
Potential Impact
Successful exploitation allows an unauthenticated attacker to execute arbitrary JavaScript code on the server, potentially leading to full system compromise including confidentiality, integrity, and availability impacts. The CVSS 3.1 score is 8.1 (High), reflecting network attack vector, high complexity, no privileges or user interaction required, and high impact on confidentiality, integrity, and availability.
Mitigation Recommendations
A fixed version 4.7.9 of handlebars.js is available and should be applied to remediate this vulnerability. If immediate patching is not possible, use the runtime-only build (require('handlebars/runtime')) to prevent fallback compilation paths. Additionally, sanitize all context data to ensure no non-primitive objects are passed to dynamic partial lookups, and avoid using dynamic partial lookups ({{> (lookup ...)}}) with user-controlled data. Patch status is confirmed by the vendor advisory indicating version 4.7.9 fixes the issue.
CVE-2026-33940: CWE-94: Improper Control of Generation of Code ('Code Injection') in handlebars-lang handlebars.js
Description
Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in `resolvePartial()` and cause `invokePartial()` to return `undefined`. The Handlebars runtime then treats the unresolved partial as a source that needs to be compiled, passing the crafted object to `env.compile()`. Because the object is a valid Handlebars AST containing injected code, the generated JavaScript executes arbitrary commands on the server. The attack requires the adversary to control a value that can be returned by a dynamic partial lookup. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). Without `compile()`, the fallback compilation path in `invokePartial` is unreachable. Second, sanitize context data before rendering: Ensure no value in the context is a non-primitive object that could be passed to a dynamic partial. Third, avoid dynamic partial lookups (`{{> (lookup ...)}}`) when context data is user-controlled.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Handlebars.js versions 4.0.0 to 4.7.8 contain a code injection vulnerability (CWE-94) where a crafted object in the template context bypasses guards in resolvePartial(), causing invokePartial() to return undefined. This triggers fallback compilation of the crafted object as a Handlebars AST, resulting in arbitrary JavaScript execution on the server. Exploitation requires control over values returned by dynamic partial lookups. The vulnerability is fixed in version 4.7.9. Mitigations include using the runtime-only build to disable compile(), sanitizing context data to prevent non-primitive objects in dynamic partials, and avoiding dynamic partial lookups with user-controlled input.
Potential Impact
Successful exploitation allows an unauthenticated attacker to execute arbitrary JavaScript code on the server, potentially leading to full system compromise including confidentiality, integrity, and availability impacts. The CVSS 3.1 score is 8.1 (High), reflecting network attack vector, high complexity, no privileges or user interaction required, and high impact on confidentiality, integrity, and availability.
Mitigation Recommendations
A fixed version 4.7.9 of handlebars.js is available and should be applied to remediate this vulnerability. If immediate patching is not possible, use the runtime-only build (require('handlebars/runtime')) to prevent fallback compilation paths. Additionally, sanitize all context data to ensure no non-primitive objects are passed to dynamic partial lookups, and avoid using dynamic partial lookups ({{> (lookup ...)}}) with user-controlled data. Patch status is confirmed by the vendor advisory indicating version 4.7.9 fixes the issue.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-03-24T19:50:52.103Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 69c6f6ca3c064ed76ff81ba4
Added to database: 3/27/2026, 9:29:46 PM
Last enriched: 4/4/2026, 10:49:29 AM
Last updated: 5/11/2026, 6:52:29 AM
Views: 92
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