CVE-2026-33940

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.

Weakness classification

• security-advisories@github.com: CWE-94
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c: CWE-94

Primary references

• security-advisories@github.com — Patch
• security-advisories@github.com — Release Notes
• security-advisories@github.com — Exploit, Vendor Advisory
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c