CVE-2026-16231: hbs vulnerable to XSS via registerAsyncHelper output-escaping bypass
hbs is an Express view engine that wraps Handlebars. Its registerAsyncHelper API bypasses Handlebars' automatic HTML escaping: an async helper returns an opaque placeholder during the first render pass, so the double-brace expression escapes only the placeholder, and after rendering hbs substitutes the placeholder with the raw callback return value without escaping it, across the cached, uncached, and layout render paths. An application that passes attacker-influenced data, for example user-supplied content from a database, into an async helper callback can therefore have arbitrary HTML and JavaScript injected into the server-rendered page, resulting in stored or reflected cross-site scripting. Versions 2.1.0 through 4.2.1 are affected, and the issue is fixed in 4.3.0, which HTML-escapes async helper output. Applications that intentionally emit raw HTML from an async helper can opt in explicitly with hbs.SafeString. Users should upgrade to 4.3.0.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
hbsto a version that resolves this vulnerability.Fixed in 4.3.0
Event History
Frequently Asked Questions
Which applications are exposed to exploitation?
Applications using hbs versions 2.1.0 through 4.2.1 are exposed if they pass attacker-influenced data, such as user-supplied database content, into a registerAsyncHelper callback whose output is rendered into a page.
Does exploitation require authentication or a special rendering path?
No authentication is required by the vulnerability itself, although exploitation requires a victim to view a rendered page containing attacker-controlled async-helper output. The escaping bypass occurs in cached, uncached, and layout render paths.
What should be done if an async helper intentionally returns HTML?
Upgrade to hbs 4.3.0, which HTML-escapes async helper output. Helpers that intentionally need to emit raw HTML must explicitly opt in by using hbs.SafeString.