When working with Handlebars, developers often don’t realize how using templates the wrong way can expose serious injection flaws. Although Handlebars JS automatically escapes output, unsafe patterns such as triple braces or poorly written Handlebars helpers can bypass protections. As a result, attackers may inject XSS payloads, steal data, or run malicious code.
This guide shows you how to apply safe Handlebars usage, highlights dangerous versus secure examples, and explains how to automate template validation in pipelines with tools like Xygeni.
Why Handlebar Templates Can Be Risky
By default, Handlebars JS escapes values to block direct injection. But many developers turn this off without knowing the risks. For example, using triple braces shows raw HTML:
<!-- Unsafe usage -->
<div>{{{userInput}}}</div>
If userInput is <script>alert('XSS')</script>, the script executes in the browser. As a result, even one unsafe template can compromise the entire app.
These basic mistakes highlight the risk. Consequently, history has shown that misuse of Handlebars led to major exploits in the wild. Let’s review some of the most notable cases.
Real-World Examples of Handlebars Vulnerabilities
Although Handlebars JS escapes values by default, history shows that unsafe patterns or weak Handlebars helpers have caused serious problems:
1. Prototype Pollution in Handlebars (npm advisory GHSA-2cf5-4w76-r9qv)
In 2021, a bug was found in the handlebars package itself. The flaw allowed prototype pollution, where attackers could inject properties into global objects through crafted templates.
- Impact: Prototype pollution can let attackers run code, gain higher access, or steal data.
- Exploit: Attackers sent malicious input like
__proto__properties. When templates were rendered, these polluted objects changed how the app behaved. - Lesson: Even the template engine can be risky if not updated. Running dependency scans and keeping packages patched is critical.
2. XSS in Asana’s Custom Handlebars Helpers (2019)
In 2019, security researchers discovered that Asana, a task management tool, exposed users to XSS because of unsafe custom helpers. Developers wrote helpers that returned raw strings using SafeString, bypassing Handlebars’ built-in escaping.
- Impact: Attackers injected malicious JavaScript into shared tasks or comments.
- Exploit: Payloads like
<script>alert('XSS')</script>ran in the victim’s browser when they opened those fields. - Lesson: Never disable escaping unless you have no other choice. Always review custom Handlebars helpers before deploying them.
Types of Vulnerabilities in Handlebars
Unsafe usage of Handlebars JS templates can expose different classes of vulnerabilities. Understanding them helps developers know exactly what to prevent:
Cross-Site Scripting (XSS)
This is the most common flaw. It occurs when developers use triple braces {{{}}} or raw helpers that disable escaping.
- Impact: Attackers can inject
<script>tags, HTML, or iframes that run in the user’s browser. - Example:
<!-- Unsafe -->
<div>{{{userInput}}}</div>
If
userInputcontains<script>alert('XSS')</script>, it executes immediately.
Prototype Pollution
As seen in the npm advisory GHSA-2cf5-4w76-r9qv, malicious input can modify global objects through crafted templates.
- Impact: This can lead to arbitrary behavior, privilege escalation, or data exfiltration.
- Lesson: Always keep
handlebarsupdated and run dependency checks in CI/CD.
Server-Side Template Injection (SSTI)
SSTI happens when untrusted input is passed directly into a template engine running on the server. With Handlebars JS, if developers compile templates using raw user input, an attacker can execute payloads at the server level.
// Insecure Handlebars usage (server-side)
const template = Handlebars.compile(req.query.view);
res.send(template({}));
{{#with "require('fs').readdirSync('.')"}}
the engine may try to evaluate it, exposing server-side files.
- Impact: Unlike XSS, which only affects the browser, SSTI can provide direct access to the server environment.
- Prevention: Never compile templates from untrusted sources. Instead, only use predefined templates and sanitize inputs before rendering.
Similar risks appear in other ecosystems. For example, see our guide on Python Dependency Injection to learn safe practices in a different context.
Logic Abuse in Helpers
Custom Handlebars helpers can be dangerous if they allow raw strings or work with unvalidated input.
- Impact: Attackers can bypass escaping or trick helpers into exposing sensitive data.
- Example:
Handlebars.registerHelper('raw', input => new Handlebars.SafeString(input));
This helper disables escaping entirely and should be avoided.
Data Leakage
Sometimes helpers or misconfigured templates reveal sensitive information.
- Impact: Tokens, config values, or database credentials could be rendered in HTML output.
- Lesson: Never expose secrets in templates; validate and scan for accidental leaks in your repositories.
Together, these categories show the range of risks developers face. As a result, even small mistakes in Handlebars JS can turn into supply chain problems if left unfixed.
Why This Matters for Developers
These vulnerabilities prove that unsafe usage is not theoretical, it has been exploited in real platforms like Asana and npm packages. Moreover, with Handlebars JS widely used in Node.js and frontend projects, insecure helpers or outdated dependencies quickly become a software supply chain risk.
Want to block these risks automatically? Start a free trial of Xygeni and add guardrails in your pipeline today.
Best Practices for Safe Handlebars Usage
To avoid injection flaws, follow these secure coding practices:
1. Always Use Double Braces
<!-- Safe -->
<div>{{userInput}}</div>
This ensures the template engine escapes HTML before rendering.
2. Validate and Sanitize Input
In addition, validate inputs before passing them to templates. Libraries like validator.js
3. Restrict Dangerous Helpers
Poorly designed Handlebars helpers often cause vulnerabilities:
// Dangerous helper
Handlebars.registerHelper('raw', function (input) {
return new Handlebars.SafeString(input);
});
This bypasses escaping entirely. However, safe helpers should be restricted to simple operations like formatting dates or trimming text.
4. Use Content Security Policy (CSP)
Moreover, enforce strong CSP headers to reduce the blast radius of any injection.
These best practices are important during development. In addition, you can add automated checks so unsafe code never makes it into production.
Automating Validation in CI/CD Pipelines
Manual reviews are not enough. To clarify, unsafe Handlebars patterns can slip into production unless automated checks are in place:
- SAST rules: Flag
{{{in.hbsfiles. - Secrets scanners: Detect credentials embedded in templates.
- CI/CD guardrails: Break builds when unsafe Handlebars helpers or triple braces appear.
For example, you can enforce a guardrail in pipelines:
# Example Guardrail Rule
if: contains('{{{')
then: fail_build("Unsafe Handlebars triple braces detected")
These checks reduce the chance of insecure templates, but automation at scale needs a platform. This is where Xygeni adds extra protection.
You can also see how securing pipelines in Bitbucket works in our Bitbucket Security FAQs.
How Xygeni Helps Prevent Unsafe Handlebars Usage
Writing safe code is important, but real protection comes from automation. Xygeni makes Handlebars JS safer by adding checks into your workflow:
- SAST for Templates: Scans
.hbsfiles to catch unsafe{{{or risky custom helpers. - Guardrails: Set simple rules in GitHub, GitLab, or Bitbucket. If insecure Handlebars JS code is found, the build stops.
- AutoFix: Suggests secure fixes in pull requests, swapping triple braces for safe double braces or trusted helper calls.
- Secrets & Config Checks: Finds tokens or credentials hidden in templates before they leak.
- CI/CD Security: Blocks insecure snippets during merge, so only safe code reaches production.
With Xygeni, safe Handlebars JS usage is no longer just a guideline. It becomes part of your automated pipeline.
Putting It All Together
Handlebars is mostly safe out of the box, but misuse can still open the door to injection attacks. Following best practices, validating input, and using safe helpers helps reduce risks. However, the real advantage comes when these checks are automated in your pipelines.
In conclusion, combining good coding habits with tools like Xygeni helps prevent injection flaws and keeps templates secure, without slowing down delivery.
