{"id":"GHSA-9cx6-37pm-9jff","summary":"Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation","details":"## Summary\n\nWhen a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. `{{*n}}`), the compiled template calls `lookupProperty(decorators, \"n\")`, which returns `undefined`. The runtime then immediately invokes the result as a function, causing an unhandled `TypeError: ... is not a function` that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a `try/catch` is vulnerable to a single-request Denial of Service.\n\n## Description\n\nIn `lib/handlebars/compiler/javascript-compiler.js`, the code generated for a decorator invocation looks like:\n\n```javascript\nfn = lookupProperty(decorators, \"n\")(fn, props, container, options) || fn;\n```\n\nWhen `\"n\"` is not a registered decorator, `lookupProperty(decorators, \"n\")` returns `undefined`. The expression immediately attempts to call `undefined` as a function, producing:\n\n```\nTypeError: lookupProperty(...) is not a function\n```\n\nBecause the error is thrown inside the compiled template function and is not caught by the runtime, it propagates up as an unhandled exception and — when not caught by the application — crashes the Node.js process.\n\nThis inconsistency is notable: references to unregistered **helpers** produce a clean `\"Missing helper: ...\"` error, while references to unregistered **decorators** cause a hard crash.\n\n**Attack scenario:** An attacker submits `{{*n}}` as template content to any endpoint that calls `Handlebars.compile(userInput)()`. Each request crashes the server process; with process managers that auto-restart (PM2, systemd), repeated submissions create a persistent DoS.\n\n## Proof of Concept\n\n```javascript\nconst Handlebars = require('handlebars'); // Handlebars 4.7.8, Node.js v22.x\n\n// Any of these payloads crash the process\nHandlebars.compile('{{*n}}')({});\nHandlebars.compile('{{*decorator}}')({});\nHandlebars.compile('{{*constructor}}')({});\n```\n\nExpected crash output:\n```\nTypeError: lookupProperty(...) is not a function\n    at Function.eval [as decorator] (eval at compile (...javascript-compiler.js:134:36))\n```\n\n## Workarounds\n\n- **Wrap compilation and rendering in `try/catch`:**\n  ```javascript\n  try {\n    const result = Handlebars.compile(userInput)(context);\n    res.send(result);\n  } catch (err) {\n    res.status(400).send('Invalid template');\n  }\n  ```\n- **Validate template input** before passing it to `compile()`. Reject templates containing  decorator syntax (`{{*...}}`) if decorators are not used in your application.\n- **Use the pre-compilation workflow:** compile templates at build time and serve only pre-compiled  templates; do not call `compile()` at request time.","aliases":["CVE-2026-33939"],"modified":"2026-07-17T21:09:30.488596118Z","published":"2026-03-27T18:21:15Z","database_specific":{"cwe_ids":["CWE-754"],"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-03-27T18:21:15Z","nvd_published_at":"2026-03-27T22:16:20Z"},"references":[{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-9cx6-37pm-9jff"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33939"},{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2"},{"type":"PACKAGE","url":"https://github.com/handlebars-lang/handlebars.js"},{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9"}],"affected":[{"package":{"name":"handlebars","ecosystem":"npm","purl":"pkg:npm/handlebars"},"ranges":[{"type":"SEMVER","events":[{"introduced":"4.0.0"},{"fixed":"4.7.9"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 4.7.8","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/03/GHSA-9cx6-37pm-9jff/GHSA-9cx6-37pm-9jff.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}