GitLab: Arbitrary file read via the UploadsRewriter when moving and issue

Summary

The UploadsRewriter does not validate the file name, allowing arbitrary files to be copied via directory traversal when moving an issue to a new project.

The pattern used to look for references is :

  MARKDOWN_PATTERN = %r{\!?\[.*?\]\(/uploads/(?<secret>[0-9a-f]{32})/(?<file>.*?)\)}.freeze

This is used by the UploadsRewriter when copying an issue to also copy across the files:

   @text.gsub(@pattern) do |markdown|
          file = find_file(@source_project, $~[:secret], $~[:file])
          break markdown unless file.try(:exists?)

          klass = target_parent.is_a?(Namespace) ? NamespaceFileUploader : FileUploader
          moved = klass.copy_to(file, target_parent)
...
   def find_file(project, secret, file)
        uploader = FileUploader.new(project, secret: secret)
        uploader.retrieve_from_store!(file)
        uploader
      end

As there is no restriction on what file can be, path traversal can be used to copy any file.

Demo
{F757226}

Steps to reproduce

1. Create two projects
2. Add an issue with the following description:

![a](/uploads/11111111111111111111111111111111/../../../../../../../../../../../../../../etc/passwd)

1. Move the issue to the second project
2. The file will have been copied to the project

Impact

Allows an attacker to read arbitrary files on the server, including tokens, private data, configs, etc

What is the current bug behavior?

The file name and path are not checked when copying an issue between projects

What is the expected correct behavior?

The file or path should be validated before copying files.

Output of checks

Results of GitLab environment info

System information
System:		Ubuntu 18.04
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.6.5p114
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.7
Git Version:	2.24.1
Sidekiq Version:5.2.7
Go Version:	unknown

GitLab information
Version:	12.8.7-ee
Revision:	2643fd87200
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.12
URL:		http://gitlab-vm.local
HTTP Clone URL:	http://gitlab-vm.local/some-group/some-project.git
SSH Clone URL:	[email protected]:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers:

GitLab Shell
Version:	11.0.0
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Impact

Allows an attacker to read arbitrary files on the server, including tokens, private data, configs, etc