CVE-2026-33916: handlebars-lang handlebars.js Cross-site Scripting

Vulnerability summary

CVE-2026-33916 is a medium (CVSS 4.7) Cross-site Scripting vulnerability in handlebars-lang handlebars.js. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 30.6% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 27-03-2026
   PublishedVulnerability first disclosed.
2. 30-03-2026
   Last modifiedVulnerability information updated.

Description

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, `resolvePartial()` in the Handlebars runtime resolves partial names via a plain property lookup on `options.partials` without guarding against prototype-chain traversal. When `Object.prototype` has been polluted with a string value whose key matches a partial reference in a template, the polluted string is used as the partial body and rendered without HTML escaping, resulting in reflected or stored XSS. Version 4.7.9 fixes the issue. Some workarounds are available. Apply `Object.freeze(Object.prototype)` early in application startup to prevent prototype pollution. Note: this may break other libraries, and/or use the Handlebars runtime-only build (`handlebars/runtime`), which does not compile templates and reduces the attack surface.

• CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
• CWE-1321Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.