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.

The npm hbs package is an Express view engine wrapper for Handlebars. Depending on usage, users of hbs may be vulnerable to a file disclosure vulnerability. There is currently no patch for this vulnerability. hbs mixes pure template data with engine configuration options through the Express render API. By overwriting internal configuration options a file disclosure vulnerability may be triggered in downstream applications. For an example PoC see the referenced GHSL-2021-020.