# CVE-2026-33916: Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection > Prototype pollution in Handlebars.js allows attackers to inject malicious scripts via partial templates, leading to cross-site scripting. - **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** A user has to click or open something - **Fix available:** Yes - **Published:** 27 Mar 2026 - **Updated:** 30 Mar 2026 - **Also known as:** CLEANSTART-2026-BA07139, CLEANSTART-2026-UA73241, DEBIAN-CVE-2026-33916, GHSA-2qvq-rjwj-gvw9, MINI-44jc-3hjp-j8p9, MINI-4ffv-qvgc-9gc9, MINI-574j-mg2c-wxw9, MINI-7m99-g547-v4xv, MINI-c7fg-jx9x-gqp3, MINI-cc3c-6j7m-82wp, MINI-ff52-x699-gjm3, MINI-hf4x-xcgh-7v9x, MINI-j6x4-mww4-q7v3, MINI-pf4q-86vc-qx79, MINI-rjcf-qww2-cvqv, MINI-xm59-6gcx-rc5g, MINI-xr62-f2qr-427g, MINI-xr6g-j9cf-8phr, ROOT-APP-NPM-CVE-2026-33916, UBUNTU-CVE-2026-33916 ## What is CVE-2026-33916? CVE-2026-33916 is a medium-severity cross-site scripting vulnerability in handlebars-lang handlebars.js, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link. ### Official 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. Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes) ## Which versions are affected by CVE-2026-33916? 2 open source packages ship an affected version, including handlebars. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `handlebars` | npm | From 4.0.0 | 4.7.9 | | `handlebars` | npm | From 4.0.0 | 4.7.9 | Products: Handlebarsjs Handlebars. ## How do I fix CVE-2026-33916? Upgrade handlebars to 4.7.9 or later. ## Is CVE-2026-33916 being exploited? It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33916 exists. ## Other vulnerabilities in handlebars-lang handlebars.js - [CVE-2026-33937](https://insights.sbomify.com/CVE-2026-33937/): Handlebars.js has JavaScript Injection via AST Type Confusion (critical 9.8) - [CVE-2026-33941](https://insights.sbomify.com/CVE-2026-33941/): Handlebars.js has JavaScript Injection in CLI Precompiler via Unescaped Names and Options (high 8.3) - [CVE-2026-33938](https://insights.sbomify.com/CVE-2026-33938/): Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block (high 8.1) - [CVE-2026-33940](https://insights.sbomify.com/CVE-2026-33940/): Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial (high 8.1) - [CVE-2019-20920](https://insights.sbomify.com/CVE-2019-20920/): Arbitrary Code Execution in Handlebars (high 8.1) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-33916 and fix it if it is affected. What it is: CVE-2026-33916 is a medium-severity cross-site scripting vulnerability in handlebars-lang handlebars.js, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link. Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33916 exists. Affected packages and fixed versions: - handlebars (npm): affected from 4.0.0; fixed in 4.7.9 - handlebars (npm): affected from 4.0.0; fixed in 4.7.9 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-33916/ (Markdown: https://insights.sbomify.com/CVE-2026-33916/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33916 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33916 - OSV: https://osv.dev/vulnerability/CVE-2026-33916 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-33916/ Summaries are generated and may contain mistakes; the official description above is the record.