Summary
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.
Description
The root cause is in lib/handlebars/runtime.js inside resolvePartial() and invokePartial():
// Vulnerable: plain bracket access traverses Object.prototype
partial = options.partials[options.name];
hasOwnProperty is never checked, so if Object.prototype has been seeded with a key whose name matches a partial reference in the template (e.g. widget), the lookup succeeds and the polluted string is returned. The runtime emits a prototype-access warning, but the partial is still resolved and its content is inserted into the rendered output unescaped. This contradicts the documented security model and is distinct from CVE-2021-23369 and CVE-2021-23383, which addressed data property access rather than partial template resolution.
Prerequisites for exploitation:
- The target application must be vulnerable to prototype pollution (e.g. via
qs, minimist, or
any querystring/JSON merge sink).
- The attacker must know or guess the name of a partial reference used in a template.
Proof of Concept
const Handlebars = require('handlebars');
// Step 1: Prototype pollution (via qs, minimist, or another vector)
Object.prototype.widget = '<img src=x onerror="alert(document.domain)">';
// Step 2: Normal template that references a partial
const template = Handlebars.compile('<div>Welcome! {{> widget}}</div>');
// Step 3: Render — XSS payload injected unescaped
const output = template({});
// Output: <div>Welcome! <img src=x onerror="alert(document.domain)"></div>
The runtime prints a prototype access warning claiming "access has been denied," but the partial still resolves and returns the polluted value.
Workarounds
- Apply
Object.freeze(Object.prototype) early in application startup to prevent prototype pollution. Note: this may break other libraries.
- Use the Handlebars runtime-only build (
handlebars/runtime), which does not compile templates and reduces the attack surface.
References
ByamB4 Reporter
Summary
resolvePartial()in the Handlebars runtime resolves partial names via a plain property lookup onoptions.partialswithout guarding against prototype-chain traversal. WhenObject.prototypehas 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.Description
The root cause is in
lib/handlebars/runtime.jsinsideresolvePartial()andinvokePartial():hasOwnPropertyis never checked, so ifObject.prototypehas been seeded with a key whose name matches a partial reference in the template (e.g.widget), the lookup succeeds and the polluted string is returned. The runtime emits a prototype-access warning, but the partial is still resolved and its content is inserted into the rendered output unescaped. This contradicts the documented security model and is distinct from CVE-2021-23369 and CVE-2021-23383, which addressed data property access rather than partial template resolution.Prerequisites for exploitation:
qs,minimist, orany querystring/JSON merge sink).
Proof of Concept
Workarounds
Object.freeze(Object.prototype)early in application startup to prevent prototype pollution. Note: this may break other libraries.handlebars/runtime), which does not compile templates and reduces the attack surface.References