Validate gem name before building compact index cache paths

lib/bundler/compact_index_client/cache.rb

@@ -50,6 +50,7 @@ def versions_etag_path = directory.join("versions.etag")
 
       def info_path(name)
         name = name.to_s
+        validate_name!(name)
         # TODO: converge this into the info_root by hashing all filenames like info_etag_path
         if /[^a-z0-9_-]/.match?(name)
           name += "-#{SharedHelpers.digest(:MD5).hexdigest(name).downcase}"
@@ -61,9 +62,19 @@ def info_path(name)
 
       def info_etag_path(name)
         name = name.to_s
+        validate_name!(name)
         @info_etag_root.join("#{name}-#{SharedHelpers.digest(:MD5).hexdigest(name).downcase}")
       end
 
+      # Gem names come from the remote index and are not otherwise
+      # validated, so refuse anything that would escape the cache
+      # directory when used as a path component.
+      def validate_name!(name)
+        return if File.basename(name) == name
+
+        raise SecurityError, "malformed gem name: #{name.inspect}"
+      end
+
       def mkdir(name)
         directory.join(name).tap do |dir|
           SharedHelpers.filesystem_access(dir) do

lib/rubygems/compact_index_client/cache.rb

@@ -60,6 +60,7 @@ def versions_etag_path = directory.join("versions.etag")
 
     def info_path(name)
       name = name.to_s
+      validate_name!(name)
       if /[^a-z0-9_-]/.match?(name)
         name += "-#{Digest::MD5.hexdigest(name).downcase}"
         @special_characters_info_root.join(name)
@@ -70,9 +71,19 @@ def info_path(name)
 
     def info_etag_path(name)
       name = name.to_s
+      validate_name!(name)
       @info_etag_root.join("#{name}-#{Digest::MD5.hexdigest(name).downcase}")
     end
 
+    # Gem names come from the remote index and are not otherwise
+    # validated, so refuse anything that would escape the cache
+    # directory when used as a path component.
+    def validate_name!(name)
+      return if File.basename(name) == name
+
+      raise Gem::Exception, "malformed gem name: #{name.inspect}"
+    end
+
     def checksum_for_file(path)
       return unless path.file?
       Digest::MD5.file(path).hexdigest

spec/bundler/compact_index_client/cache_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require "bundler/compact_index_client"
+require "bundler/compact_index_client/cache"
+require "tmpdir"
+
+RSpec.describe Bundler::CompactIndexClient::Cache do
+  let(:directory) { Pathname.new(Dir.mktmpdir("compact_index")) }
+  let(:cache) { described_class.new(directory) }
+
+  it "rejects a gem name that would escape the cache directory" do
+    expect { cache.info("../../../../pwn") }.to raise_error(Bundler::SecurityError, /malformed gem name/)
+  end
+
+  it "accepts plain gem names, including special characters" do
+    expect { cache.info("rack") }.not_to raise_error
+    expect { cache.info("Rails.js") }.not_to raise_error
+  end
+end

spec/support/shards.rb

@@ -148,6 +148,7 @@ module Shards
       ],
       shard_d: [
         "spec/bundler/compact_index_client/cache_file_spec.rb",
+        "spec/bundler/compact_index_client/cache_spec.rb",
         "spec/bundler/rubygems_ext_spec.rb",
         "spec/bundler/resolver/cooldown_spec.rb",
         "spec/install/cooldown_spec.rb",

test/rubygems/test_gem_compact_index_client_cache.rb

@@ -113,6 +113,29 @@ def test_fetch_info_fetches_without_checksum
     assert_equal "a 1.0.0\n", @dir.join("info", "a").read
   end
 
+  def test_info_rejects_name_escaping_cache_directory
+    fetcher = FakeFetcher.new("1.0.0\n")
+    cache = Gem::CompactIndexClient::Cache.new(@dir, fetcher)
+
+    e = assert_raise Gem::Exception do
+      cache.info("../../../../pwn", "no-match")
+    end
+
+    assert_includes e.message, "malformed gem name"
+    assert_empty fetcher.requests
+  end
+
+  def test_fetch_info_rejects_name_escaping_cache_directory
+    fetcher = FakeFetcher.new("1.0.0\n")
+    cache = Gem::CompactIndexClient::Cache.new(@dir, fetcher)
+
+    assert_raise Gem::Exception do
+      cache.fetch_info("../pwn")
+    end
+
+    assert_empty fetcher.requests
+  end
+
   def test_info_with_special_characters_uses_hashed_path
     fetcher = FakeFetcher.new("1.0.0\n")
     cache = Gem::CompactIndexClient::Cache.new(@dir, fetcher)