CVE-2026-33938

Summary

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, 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. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (require('handlebars/runtime')). The compile() method is absent, eliminating the vulnerable fallback path. Second, audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only. Third, avoid registering helpers from third-party packages (such as handlebars-helpers) in contexts where templates or context data can be influenced by untrusted input.

Affected Software

VendorProductVersion RangeStatus
handlebars-langhandlebars.js>= 4.0.0, < 4.7.9affected

Weaknesses

  • CWE-94: CWE-94: Improper Control of Generation of Code ('Code Injection')
  • CWE-843: CWE-843: Access of Resource Using Incompatible Type ('Type Confusion')

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: poc
    • Automatable: no
    • Technical Impact: total

Additional References

handlebars: Handlebars: Arbitrary code execution via @partial-block overwrite

Additional References

References