Handlebars.js: JavaScript injection via AST type confusion in compile()

Summary

Handlebars.js versions 4.0.0 up to 4.7.9 accept pre-parsed AST objects in compile() and precompile() but validate only some AST values. An attacker who can supply an AST object instead of a template string can inject JavaScript that the compiler writes into generated code, leading to code execution in the server process. Applications that pass only template strings are not affected.

Affected

• handlebars.js from 4.0.0 up to and including 4.7.9
• Affected only if the application passes non-string AST objects to compile() or precompile()
• Check transitive dependencies in lock files; the range reported by one downstream audit starts at 3.0.0, which could not be confirmed

Fixed in

• handlebars.js 4.7.10 (released 5 October 2026)
• Precompiled templates must also be rebuilt with the fixed version

Workaround

• Allow only template strings to reach compile() and precompile() and reject all other input
• If templates are precompiled at build time, use the runtime-only build (handlebars/runtime) so that compile() is not available on the server
• These measures are interim only; patching is the complete fix

Exploitation

• CISA SSVC assessment: exploitation status 'poc', automatable: yes, technical impact: total
• No confirmed in-the-wild exploitation found; CISA KEV status unknown
• EPSS: 0.0064
• Exploitation requires that an application passes attacker-controlled objects, for example JSON from a request body, into the template compiler

Our recommendation

• Inventory all package-lock.json, yarn.lock and pnpm-lock.yaml files for handlebars below 4.7.10, including transitive and build-time dependencies
• Upgrade to 4.7.10 or later, including precompiled templates
• Review code paths that pass request data to compile() or precompile() and enforce string-only input until the upgrade is done
• Monitor the vendor advisory and CISA KEV for changes

Public exploits iAttack code is publicly available (PoC on GitHub, Metasploit or Exploit-DB).

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

• GitHub Security Advisory GHSA-8r5x-fm3f-whwj github.com
• Fix commit 703fdcc github.com
• Release v4.7.10 github.com
• Release notes v4.7.10 github.com
• Pull request #2185 github.com
• OSV entry CVE-2026-106446 osv.dev
• Red Hat CVE record access.redhat.com
• Tenable CVE record www.tenable.com

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 30 Oct 2026, 21:03.