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: ```ruby @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 1. Add an issue with the following description: ```markdown  ``` 1. Move the issue to the second project 1. 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