CVE-2026-87123: hbs vulnerable to Denial of Service via unhandled exception in async helper output escaping
hbs is an Express view engine wrapper for Handlebars. Version 4.3.0 can crash the Node.js process during output escaping when an async helper, registered with registerAsyncHelper, resolves to an object whose toHTML property is truthy but not callable. Handlebars escapeExpression calls the toHTML method on any value that has a truthy toHTML, so such a value throws a TypeError, and because the async substitution runs on a later tick outside the render function's try/catch, the throw is an uncaught exception that terminates the process without sending a response. Only version 4.3.0 is affected, since the throwing escape was introduced by the fix for CVE-2026-16231 and earlier versions do not escape async helper values. It can be triggered remotely when an async helper resolves to an externally influenced object such as parsed JSON. The issue is fixed in hbs 4.3.1, and users should upgrade to hbs 4.3.1 or later.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
hbsto a version that resolves this vulnerability.Fixed in 4.3.1
Event History
Frequently Asked Questions
Which deployments are exposed to remote exploitation?
Applications using hbs 4.3.0 are exposed when they register an async helper with registerAsyncHelper and that helper can resolve to an externally influenced object, such as parsed JSON. The object must have a truthy toHTML property that is not callable.
What does an attacker need to trigger the denial of service?
An attacker must be able to cause a relevant async helper to resolve to the malformed object during rendering. No authentication or user interaction is required according to the supplied vector, but exploitation has high complexity because the required helper behavior and object shape must be reached.
Are earlier hbs releases affected?
No. Only hbs 4.3.0 is affected; earlier versions do not escape async helper values and therefore do not reach this throwing behavior.
What is the recommended remediation?
Upgrade hbs to version 4.3.1 or later. No alternative mitigation is provided in the available information.
How can teams identify potentially affected applications?
Check for hbs version 4.3.0 and uses of registerAsyncHelper. Prioritize helpers that process externally influenced values and may return objects with a truthy, non-function toHTML property, as these can cause an uncaught TypeError and terminate the Node.js process.