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
In Handlebars.js versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in the resolvePartial() function, causing invokePartial() to return undefined. This triggers the Handlebars runtime to treat the unresolved partial as a source needing compilation, passing the crafted object to env.compile(). Because the object is a valid Handlebars AST containing injected code, this results in arbitrary JavaScript code execution on the server. Exploitation requires control over a value returned by a dynamic partial lookup. Version 4.7.9 addresses this vulnerability. Workarounds include using the runtime-only build (which lacks compile()), sanitizing context data to prevent non-primitive objects from being passed as dynamic partials, and avoiding dynamic partial lookups with user-controlled data.
Potential Impact
Successful exploitation allows an unauthenticated attacker to execute arbitrary code on the server hosting Handlebars.js, potentially leading to full system compromise. The vulnerability affects confidentiality, integrity, and availability of the affected system.
Mitigation Recommendations
An official fix is available in Handlebars.js version 4.7.9. Users should upgrade to this version to remediate the vulnerability. If upgrading immediately is not possible, apply workarounds: use the runtime-only build to avoid fallback compilation paths, sanitize all context data to ensure no non-primitive objects can be passed as dynamic partials, and avoid dynamic partial lookups when context data is user-controlled.
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.
CVSS v3.1
Score 8.1high
Affected software
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AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
In Handlebars.js versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in the resolvePartial() function, causing invokePartial() to return undefined. This triggers the Handlebars runtime to treat the unresolved partial as a source needing compilation, passing the crafted object to env.compile(). Because the object is a valid Handlebars AST containing injected code, this results in arbitrary JavaScript code execution on the server. Exploitation requires control over a value returned by a dynamic partial lookup. Version 4.7.9 addresses this vulnerability. Workarounds include using the runtime-only build (which lacks compile()), sanitizing context data to prevent non-primitive objects from being passed as dynamic partials, and avoiding dynamic partial lookups with user-controlled data.
Potential Impact
Successful exploitation allows an unauthenticated attacker to execute arbitrary code on the server hosting Handlebars.js, potentially leading to full system compromise. The vulnerability affects confidentiality, integrity, and availability of the affected system.
Mitigation Recommendations
An official fix is available in Handlebars.js version 4.7.9. Users should upgrade to this version to remediate the vulnerability. If upgrading immediately is not possible, apply workarounds: use the runtime-only build to avoid fallback compilation paths, sanitize all context data to ensure no non-primitive objects can be passed as dynamic partials, and avoid dynamic partial lookups when context data is user-controlled.
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
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-33940","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:10175","vendor":"Red Hat"}]
Threat ID: 69c6f6ca3c064ed76ff81ba4
Added to database: 03/27/2026, 21:29:46 UTC
Last enriched: 07/15/2026, 13:21:33 UTC
Last updated: 07/31/2026, 19:22:58 UTC
Views: 205
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