Filter invalid secrets on file uploads

Validates secrets provided to FileUploader in order to prevent
directory traversal attacks. We generate 32-byte hexadecimal secrets
now and 10-byte hexadecimal secrets in the past, so these are the only
two valid formats permitted.

Also adds a test that proves the exploit works without the change, and
a test that proves the change resolves the exploit.

app/controllers/concerns/uploads_actions.rb

@@ -9,6 +9,7 @@ module UploadsActions
 
   included do
     prepend_before_action :set_request_format_from_path_extension
+    rescue_from FileUploader::InvalidSecret, with: :render_404
   end
 
   def create

app/uploaders/file_uploader.rb

@@ -16,6 +16,9 @@ class FileUploader < GitlabUploader
 
   MARKDOWN_PATTERN = %r{\!?\[.*?\]\(/uploads/(?<secret>[0-9a-f]{32})/(?<file>.*?)\)}.freeze
   DYNAMIC_PATH_PATTERN = %r{.*(?<secret>\h{32})/(?<identifier>.*)}.freeze
+  VALID_SECRET_PATTERN = %r{\A\h{10,32}\z}.freeze
+
+  InvalidSecret = Class.new(StandardError)
 
   after :remove, :prune_store_dir
 
@@ -153,6 +156,10 @@ class FileUploader < GitlabUploader
 
   def secret
     @secret ||= self.class.generate_secret
+
+    raise InvalidSecret unless @secret =~ VALID_SECRET_PATTERN
+
+    @secret
   end
 
   # return a new uploader with a file copy on another project

changelogs/unreleased/security-709-secret-traversal.yml

+---
+title: Prevent directory traversal through FileUploader
+merge_request:
+author:
+type: security

spec/controllers/snippets_controller_spec.rb

@@ -260,8 +260,10 @@ describe SnippetsController do
     context 'when the snippet description contains a file' do
       include FileMoverHelpers
 
-      let(:picture_file) { "/-/system/user/#{user.id}/secret56/picture.jpg" }
-      let(:text_file) { "/-/system/user/#{user.id}/secret78/text.txt" }
+      let(:picture_secret) { SecureRandom.hex }
+      let(:text_secret) { SecureRandom.hex }
+      let(:picture_file) { "/-/system/user/#{user.id}/#{picture_secret}/picture.jpg" }
+      let(:text_file) { "/-/system/user/#{user.id}/#{text_secret}/text.txt" }
       let(:description) do
         "Description with picture: ![picture](/uploads#{picture_file}) and "\
         "text: [text.txt](/uploads#{text_file})"
@@ -284,8 +286,8 @@ describe SnippetsController do
         snippet = subject
 
         expected_description = "Description with picture: "\
-          "![picture](/uploads/-/system/personal_snippet/#{snippet.id}/secret56/picture.jpg) and "\
-          "text: [text.txt](/uploads/-/system/personal_snippet/#{snippet.id}/secret78/text.txt)"
+          "![picture](/uploads/-/system/personal_snippet/#{snippet.id}/#{picture_secret}/picture.jpg) and "\
+          "text: [text.txt](/uploads/-/system/personal_snippet/#{snippet.id}/#{text_secret}/text.txt)"
 
         expect(snippet.description).to eq(expected_description)
       end

spec/support/shared_examples/controllers/uploads_actions_shared_examples.rb

@@ -69,13 +69,39 @@ RSpec.shared_examples 'handle uploads' do
   end
 
   describe "GET #show" do
+    let(:filename) { "rails_sample.jpg" }
+
+    let(:upload_service) do
+      UploadService.new(model, jpg, uploader_class).execute
+    end
+
     let(:show_upload) do
-      get :show, params: params.merge(secret: secret, filename: "rails_sample.jpg")
+      get :show, params: params.merge(secret: secret, filename: filename)
     end
 
     before do
       allow(FileUploader).to receive(:generate_secret).and_return(secret)
-      UploadService.new(model, jpg, uploader_class).execute
+      upload_service
+    end
+
+    context 'when the secret is invalid' do
+      let(:secret) { "../../../../../../../../" }
+      let(:filename) { "Gemfile.lock" }
+      let(:upload_service) { nil }
+
+      it 'responds with status 404' do
+        show_upload
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+
+      it 'is a working exploit without the validation' do
+        allow_any_instance_of(FileUploader).to receive(:secret) { secret }
+
+        show_upload
+
+        expect(response).to have_gitlab_http_status(:ok)
+      end
     end
 
     context 'when accessing a specific upload via different model' do

spec/uploaders/file_mover_spec.rb

@@ -7,7 +7,7 @@ describe FileMover do
 
   let(:user) { create(:user) }
   let(:filename) { 'banana_sample.gif' }
-  let(:secret) { 'secret55' }
+  let(:secret) { SecureRandom.hex }
   let(:temp_file_path) { File.join("uploads/-/system/user/#{user.id}", secret, filename) }
 
   let(:temp_description) do
@@ -47,8 +47,8 @@ describe FileMover do
           subject
 
           expect(snippet.reload.description)
-            .to eq("test ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/secret55/banana_sample.gif) "\
-                   "same ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/secret55/banana_sample.gif) ")
+            .to eq("test ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/#{secret}/banana_sample.gif) "\
+                   "same ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/#{secret}/banana_sample.gif) ")
         end
 
         it 'updates existing upload record' do
@@ -75,8 +75,8 @@ describe FileMover do
           subject
 
           expect(snippet.reload.description)
-            .to eq("test ![banana_sample](/uploads/-/system/user/#{user.id}/secret55/banana_sample.gif) "\
-                   "same ![banana_sample](/uploads/-/system/user/#{user.id}/secret55/banana_sample.gif) ")
+            .to eq("test ![banana_sample](/uploads/-/system/user/#{user.id}/#{secret}/banana_sample.gif) "\
+                   "same ![banana_sample](/uploads/-/system/user/#{user.id}/#{secret}/banana_sample.gif) ")
         end
 
         it 'does not change the upload record' do
@@ -101,8 +101,8 @@ describe FileMover do
           subject
 
           expect(snippet.reload.description)
-            .to eq("test ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/secret55/banana_sample.gif) "\
-                   "same ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/secret55/banana_sample.gif) ")
+            .to eq("test ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/#{secret}/banana_sample.gif) "\
+                   "same ![banana_sample](/uploads/-/system/personal_snippet/#{snippet.id}/#{secret}/banana_sample.gif) ")
         end
 
         it 'creates new target upload record an delete the old upload' do
@@ -121,8 +121,8 @@ describe FileMover do
           subject
 
           expect(snippet.reload.description)
-            .to eq("test ![banana_sample](/uploads/-/system/user/#{user.id}/secret55/banana_sample.gif) "\
-                   "same ![banana_sample](/uploads/-/system/user/#{user.id}/secret55/banana_sample.gif) ")
+            .to eq("test ![banana_sample](/uploads/-/system/user/#{user.id}/#{secret}/banana_sample.gif) "\
+                   "same ![banana_sample](/uploads/-/system/user/#{user.id}/#{secret}/banana_sample.gif) ")
         end
 
         it 'does not change the upload record' do
@@ -138,12 +138,8 @@ describe FileMover do
       let(:temp_file_path) { File.join("uploads/-/system/user/#{user.id}", '..', 'another_subdir_of_temp') }
 
       it 'does not trigger move if path is outside designated directory' do
-        stub_file_mover("uploads/-/system/user/#{user.id}/another_subdir_of_temp")
         expect(FileUtils).not_to receive(:move)
-
-        subject
-
-        expect(snippet.reload.description).to eq(temp_description)
+        expect { subject }.to raise_error(FileUploader::InvalidSecret)
       end
     end
 

spec/uploaders/file_uploader_spec.rb

@@ -6,7 +6,8 @@ describe FileUploader do
   let(:group) { create(:group, name: 'awesome') }
   let(:project) { create(:project, :legacy_storage, namespace: group, name: 'project') }
   let(:uploader) { described_class.new(project, :avatar) }
-  let(:upload) { double(model: project, path: 'secret/foo.jpg') }
+  let(:upload) { double(model: project, path: "#{secret}/foo.jpg") }
+  let(:secret) { "55dc16aa0edd05693fd98b5051e83321" } # this would be nicer as SecureRandom.hex, but the shared_examples breaks
 
   subject { uploader }
 
@@ -14,7 +15,7 @@ describe FileUploader do
     include_examples 'builds correct paths',
                      store_dir: %r{awesome/project/\h+},
                      upload_path: %r{\h+/<filename>},
-                     absolute_path: %r{#{described_class.root}/awesome/project/secret/foo.jpg}
+                     absolute_path: %r{#{described_class.root}/awesome/project/55dc16aa0edd05693fd98b5051e83321/foo.jpg}
   end
 
   context 'legacy storage' do
@@ -51,11 +52,11 @@ describe FileUploader do
   end
 
   describe 'initialize' do
-    let(:uploader) { described_class.new(double, secret: 'secret') }
+    let(:uploader) { described_class.new(double, secret: secret) }
 
     it 'accepts a secret parameter' do
       expect(described_class).not_to receive(:generate_secret)
-      expect(uploader.secret).to eq('secret')
+      expect(uploader.secret).to eq(secret)
     end
   end
 
@@ -154,8 +155,39 @@ describe FileUploader do
 
   describe '#secret' do
     it 'generates a secret if none is provided' do
-      expect(described_class).to receive(:generate_secret).and_return('secret')
-      expect(uploader.secret).to eq('secret')
+      expect(described_class).to receive(:generate_secret).and_return(secret)
+      expect(uploader.secret).to eq(secret)
+      expect(uploader.secret.size).to eq(32)
+    end
+
+    context "validation" do
+      before do
+        uploader.instance_variable_set(:@secret, secret)
+      end
+
+      context "32-byte hexadecimal" do
+        let(:secret) { SecureRandom.hex }
+
+        it "returns the secret" do
+          expect(uploader.secret).to eq(secret)
+        end
+      end
+
+      context "10-byte hexadecimal" do
+        let(:secret) { SecureRandom.hex(10) }
+
+        it "returns the secret" do
+          expect(uploader.secret).to eq(secret)
+        end
+      end
+
+      context "invalid secret supplied" do
+        let(:secret) { "%2E%2E%2F%2E%2E%2F%2E%2E%2F%2E%2E%2F%2E%2E%2F%2E%2E%2F%2E%2E%2Fgrafana%2Fconf%2F" }
+
+        it "raises an exception" do
+          expect { uploader.secret }.to raise_error(described_class::InvalidSecret)
+        end
+      end
     end
   end
 

spec/uploaders/personal_file_uploader_spec.rb

@@ -6,12 +6,13 @@ describe PersonalFileUploader do
   let(:model) { create(:personal_snippet) }
   let(:uploader) { described_class.new(model) }
   let(:upload) { create(:upload, :personal_snippet_upload) }
+  let(:secret) { SecureRandom.hex }
 
   subject { uploader }
 
   shared_examples '#base_dir' do
     before do
-      subject.instance_variable_set(:@secret, 'secret')
+      subject.instance_variable_set(:@secret, secret)
     end
 
     it 'is prefixed with uploads/-/system' do
@@ -23,12 +24,12 @@ describe PersonalFileUploader do
 
   shared_examples '#to_h' do
     before do
-      subject.instance_variable_set(:@secret, 'secret')
+      subject.instance_variable_set(:@secret, secret)
     end
 
     it 'is correct' do
       allow(uploader).to receive(:file).and_return(double(extension: 'txt', filename: 'file_name'))
-      expected_url = "/uploads/-/system/personal_snippet/#{model.id}/secret/file_name"
+      expected_url = "/uploads/-/system/personal_snippet/#{model.id}/#{secret}/file_name"
 
       expect(uploader.to_h).to eq(
         alt: 'file_name',