1 files changed, 76 insertions, 0 deletions
diff --git a/lib/bundler/rubygems_gem_installer.rb b/lib/bundler/rubygems_gem_installer.rb
new file mode 100644
index 0000000000..977e13d948
--- /dev/null
+++ b/lib/bundler/rubygems_gem_installer.rb
@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+require "rubygems/installer"
+
+module Bundler
+  class RubyGemsGemInstaller < Gem::Installer
+    unless respond_to?(:at)
+      def self.at(*args)
+        new(*args)
+      end
+    end
+
+    def check_executable_overwrite(filename)
+      # Bundler needs to install gems regardless of binstub overwriting
+    end
+
+    def pre_install_checks
+      super && validate_bundler_checksum(options[:bundler_expected_checksum])
+    end
+
+  private
+
+    def validate_bundler_checksum(checksum)
+      return true if Bundler.settings[:disable_checksum_validation]
+      return true unless checksum
+      return true unless source = @package.instance_variable_get(:@gem)
+      return true unless source.respond_to?(:with_read_io)
+      digest = source.with_read_io do |io|
+        digest = Digest::SHA256.new
+        digest << io.read(16_384) until io.eof?
+        io.rewind
+        send(checksum_type(checksum), digest)
+      end
+      unless digest == checksum
+        raise SecurityError, <<-MESSAGE
+          Bundler cannot continue installing #{spec.name} (#{spec.version}).
+          The checksum for the downloaded `#{spec.full_name}.gem` does not match \
+          the checksum given by the server. This means the contents of the downloaded \
+          gem is different from what was uploaded to the server, and could be a potential security issue.
+
+          To resolve this issue:
+          1. delete the downloaded gem located at: `#{spec.gem_dir}/#{spec.full_name}.gem`
+          2. run `bundle install`
+
+          If you wish to continue installing the downloaded gem, and are certain it does not pose a \
+          security issue despite the mismatching checksum, do the following:
+          1. run `bundle config disable_checksum_validation true` to turn off checksum verification
+          2. run `bundle install`
+
+          (More info: The expected SHA256 checksum was #{checksum.inspect}, but the \
+          checksum for the downloaded gem was #{digest.inspect}.)
+          MESSAGE
+      end
+      true
+    end
+
+    def checksum_type(checksum)
+      case checksum.length
+      when 64 then :hexdigest!
+      when 44 then :base64digest!
+      else raise InstallError, "The given checksum for #{spec.full_name} (#{checksum.inspect}) is not a valid SHA256 hexdigest nor base64digest"
+      end
+    end
+
+    def hexdigest!(digest)
+      digest.hexdigest!
+    end
+
+    def base64digest!(digest)
+      if digest.respond_to?(:base64digest!)
+        digest.base64digest!
+      else
+        [digest.digest!].pack("m0")
+      end
+    end
+  end
+end