Prevent directory traversal via root prefix bypass.

Masamuneee · ioquatix · commit f9bde3bc2dde · 2026-02-16T16:24:15.000+13:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -7,6 +7,7 @@ All notable changes to this project will be documented in this file. For info on
 ### Security
 
 - [CVE-2026-25500](https://github.com/advisories/GHSA-whrj-4476-wvmp) XSS injection via malicious filename in `Rack::Directory`.
+- [CVE-2026-22860](https://github.com/advisories/GHSA-mxw3-3hh2-x2mh) Directory traversal via root prefix bypass in `Rack::Directory`.
 
 ### Fixed
 
diff --git a/lib/rack/directory.rb b/lib/rack/directory.rb
@@ -82,6 +82,7 @@ def DIR_FILE_escape(htmls)
     # Set the root directory and application for serving files.
     def initialize(root, app = nil)
       @root = ::File.expand_path(root)
+      @root_with_separator = @root.end_with?(::File::SEPARATOR) ? @root : "#{@root}#{::File::SEPARATOR}"
       @app = app || Files.new(@root)
       @head = Head.new(method(:get))
     end
@@ -118,7 +119,9 @@ def check_bad_request(path_info)
     # Rack response to use for requests with paths outside the root, or nil if path is inside the root.
     def check_forbidden(path_info)
       return unless path_info.include? ".."
-      return if ::File.expand_path(::File.join(@root, path_info)).start_with?(@root)
+
+      expanded_path = ::File.expand_path(::File.join(@root, path_info))
+      return if expanded_path == @root || expanded_path.start_with?(@root_with_separator)
 
       body = "Forbidden\n"
       [403, { CONTENT_TYPE => "text/plain",
diff --git a/test/spec_directory.rb b/test/spec_directory.rb
@@ -149,6 +149,21 @@ def setup
     res.must_be :forbidden?
   end
 
+  it "not allow directory traversal via root prefix bypass" do
+    Dir.mktmpdir do |dir|
+      root = File.join(dir, "root")
+      outside = "#{root}_test"
+      FileUtils.mkdir_p(root)
+      FileUtils.mkdir_p(outside)
+      FileUtils.touch(File.join(outside, "test.txt"))
+
+      app = Rack::Directory.new(root)
+      res = Rack::MockRequest.new(app).get("/../#{File.basename(outside)}/")
+
+      res.must_be :forbidden?
+    end
+  end
+
   it "not allow dir globs" do
     Dir.mktmpdir do |dir|
       weirds = "uploads/.?/.?"
