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April 13, 2021

GHSL-2021-024: Reflected Cross Site Scripting in eta

Agustin Gianni

Coordinated Disclosure Timeline

Executive Summary

The Express render API was designed to only pass in template data. By allowing template engine configuration options to be passed through the Express render API directly, downstream users of an Express template engine may inadvertently introduce insecure behavior into their applications with impacts ranging from Cross Site Scripting (XSS) to Remote Code Execution (RCE).

Technical Summary

Express JS allows developers to use a variety of template rendering engines. These engines substitute things inside the template by inspecting an object that the application has supplied. For example, the following snippet renders a template named “index” and passes an object with two elements, a title, and a message.

app.get('/', function (req, res) {
 res.render('index', { title: 'Hey', message: 'Hello there!' })

Template engines often need a way to set their configuration parameters, such as the path to the template directory, the name of the template, and other engine-specific parameters. To accomplish this many template engines have opted to receive their configuration options directly through the Express render API.

Passing template engine configuration parameters through the Express render API can lead to vulnerabilities if the object is user controlled. Downstream applications often opt to pass their template data in directly through the remote user-controlled req.query object. This results in a scenario where a remote attacker may be able to subvert the vulnerable application through malicious template engine configuration options.

The security impact is specific to the engine used by the application but ranges from XSS to RCE.

IMPORTANT: this is a library/engine level API misuse resulting in a potential vulnerability in downstream application code. Express did not intend for render engines to mix template data with configuration options in the same object. We have confirmed this in discussion with the ExpressJS team.

Real world downstream vulnerabilities manifest when applications pass a user controlled object (e.g. req.query) directly into a render engine that accepts config options through the Express render interface.

Our research has shown that this vulnerability pattern occurs in the wild and that many template engines are following this unintended Express render API pattern of use, resulting in an unknown number of affected downstream applications.

By reporting this API misuse at the engine level, we hope to capture this issue more broadly than trying to pursue every single affected application as well as prevent future API misuse.



Tested Version



Issue: template engine configuration options are passed through Express render API leading to reflected XSS

eta mixes pure template data with engine configuration options through the Express render API. A vulnerable application that passes user controlled request objects to the eta template engine may introduce reflected XSS vulnerabilities by allowing an attacker to disable any intended HTML escape functionality.

Example vulnerable application code:

const express = require('express')
const app = express()
const port = 3000
const eta = require("eta")

app.engine("eta", eta.renderFile)

app.set("view engine", "eta")

app.set("views", __dirname)

app.get('/', (req, res) => {
   res.render('index', req.query)

app.listen(port, () => { console.log(`Listening at http://localhost:${port}`) })

module.exports = app;

Using the following template file (index.eta):

My favorite template engine is <%= it.message %>

The following POC would trigger a reflected XSS vulnerability by setting settings[view options][autoEscape] to false.

curl "http://localhost:3000/?message=%3Cscript%3Ealert(1)%3C/script%3E&settings%5Bview%20options%5D%5BautoEscape%5D)="


Reflected Cross Site Scripting



This issue was discovered and reported by GHSL team member @agustingianni (Agustin Gianni).


You can contact the GHSL team at, please include a reference to GHSL-2021-024 in any communication regarding this issue.