{"id":"CVE-2026-33939","aliases":["GHSA-9cx6-37pm-9jff"],"url":"https://o3.security/vulnerability/CVE-2026-33939","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.","published":"2026-03-27T21:08:24.664Z","modified":"2026-10-28T02:30:51.360321154Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"npm","name":"handlebars","fixedVersion":"4.7.9"}],"fix":{"url":"https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2","label":"handlebars-lang/handlebars.js@68d8df5"},"references":[{"type":"WEB","url":"https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9"},{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33939.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:10175"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:34342"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-33939"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33939.json"},{"type":"ADVISORY","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":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2452508"},{"type":"FIX","url":"https://github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26fe9e6084c5c6aaebe67b07da2"},{"type":"PACKAGE","url":"https://github.com/handlebars-lang/handlebars.js"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:51.360321154Z"}}