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Server-side template Injection (SSTI), the digital equivalent of a wolf in sheep’s clothing. It sneaks into your server, disguised as innocent user input, and wreaks havoc. But don’t worry, we’ve got the playbook to outwit this cunning adversary. So, let’s dive into the fascinating world of SSTI, understand its mechanics, and learn how to fortify our server-side templating.

What is Server-Side Template Injection?

Server-Side Template Injection is a sneaky vulnerability that occurs when an attacker is able to inject malicious input into a server-side template. This can lead to unauthorized data access, remote code execution, and other security nightmares. Think of it as someone finding a way to cheat in a multiplayer game, ruining the experience for everyone else.

The Basics: Understanding Template Engines

Template engines are the unsung heroes of modern web development. They dynamically generate HTML content, making websites more interactive and user-friendly. However, if an attacker gains control over the input variables, they can manipulate the template engine to execute arbitrary code.

Code Snippet: Basic Express.js Template in JavaScript

const express = require(‘express’);
const app = express();

app.get(‘/greet/:name’, (req, res) => {
  const name =;
  res.send(`Hello, ${name}!`);

app.listen(3000, () => {
  console.log(‘Server running on <http://localhost:3000/>’);

In this JavaScript snippet, we’re using Express.js, a popular Node.js framework. The /greet/:name route takes a name parameter from the URL and injects it directly into the HTML response. While this is a straightforward example, it’s also a playground for SSTI if the name parameter can be manipulated to execute arbitrary code.

Best Practices for Mitigating SSTI

Contextual Output Encoding

The first rule of thumb is to encode user input based on its context in the HTML. This ensures that it’s treated as data and not as executable code.

const escape = require(‘escape-html’);

app.get(‘/greet/:name’, (req, res) => {
  const name = escape(;
  res.send(`Hello, ${name}!`);

In this modified snippet, we’re using the escape-html npm package to HTML-encode the user input. This turns special characters into HTML entities, making them safe for rendering.

Limit Template Functionality

Some template engines allow you to disable certain risky features, reducing the attack surface.

const nunjucks = require(‘nunjucks’);

const env = nunjucks.configure(‘views’, {
  autoescape: true

Here, we’re using Nunjucks, another templating engine for JavaScript. We configure it to automatically escape all output, reducing the risk of SSTI.

Use Allowlists

Allowlists are your best friends when it comes to security. They ensure that only specific patterns or characters are allowed in user input.

app.get(‘/greet/:name’, (req, res) => {
  const name =;
  if (!/^[a-zA-Z0-9]+$/.test(name)) {
    return res.send(‘Invalid name!’);
  res.send(`Hello, ${name}!`);

In this snippet, we’re using a regular expression to validate the name parameter. If it contains anything other than alphanumeric characters, the request is rejected.

Advanced Techniques

Parameterized Templates

Some template engines support parameterized templates, which can offer an additional layer of security.

const safeName = escape(;
res.render(‘greeting’, { name:, safeName });

Here, we’re using both the original name and a safeName that has been HTML-encoded. This allows you to use the safe version where needed without affecting other parts of the template.

Content Security Policy (CSP)

Implementing a strong Content Security Policy can add another layer of protection by restricting the sources and types of content that can be executed by the browser.

Logging and Monitoring

Keep an eye on your server logs for any unusual template rendering behavior.

const winston = require(‘winston’);`Rendering template with name: ${name}`);

In this JavaScript snippet, we’re using the Winston logging library to log the name being used in the template. This can help you spot any suspicious activity.


Server-side template injection, often lurking in the shadows of web applications, presents a significant threat to the security of your digital infrastructure. This insidious adversary can wreak havoc if left unchecked, but armed with the right strategies and cutting-edge tools, you can fortify your template engine against potential breaches. Book in a demo to see how can be meticulously tailored to meet your unique requirements.


About ShiftLeft

ShiftLeft empowers developers and AppSec teams to dramatically reduce risk by quickly finding and fixing the vulnerabilities most likely to reach their applications and ignoring reported vulnerabilities that pose little risk. Industry-leading accuracy allows developers to focus on security fixes that matter and improve code velocity while enabling AppSec engineers to shift security left.

A unified code security platform, ShiftLeft CORE scans for attack context across custom code, APIs, OSS, containers, internal microservices, and first-party business logic by combining results of the company’s and Intelligent Software Composition Analysis (SCA). Using its unique graph database that combines code attributes and analyzes actual attack paths based on real application architecture, ShiftLeft then provides detailed guidance on risk remediation within existing development workflows and tooling. Teams that use ShiftLeft ship more secure code, faster. Backed by SYN Ventures, Bain Capital Ventures, Blackstone, Mayfield, Thomvest Ventures, and SineWave Ventures, ShiftLeft is based in Santa Clara, California. For information, visit:


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