Arbitrary file read via the UploadsRewriter when moving an issue

HackerOne report #827052 by vakzz on 2020-03-23, assigned to @dcouture:

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
file-read.mp4

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:	git@gitlab-vm.local: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

Attachments

Warning: Attachments received through HackerOne, please exercise caution!

• file-read.mp4