From 4e206183021b2463e25b2495d3986e9ccc3fb08e Mon Sep 17 00:00:00 2001
From: Aaron Patterson <aaron.patterson@gmail.com>
Date: Fri, 6 Oct 2017 11:11:40 -0700
Subject: [PATCH] Whitelist classes and symbols that are in Gem spec YAML

This patch adds a method for loading YAML specs from a gem and
whitelists classes and symbols that are allowed in the spec.  Then it
changes calls to YAML.load to call the whitelisted "safe" loader
instead.

[CVE-2017-0903]
---
 lib/rubygems.rb               |  3 ++-
 lib/rubygems/config_file.rb   |  2 +-
 lib/rubygems/package.rb       |  2 +-
 lib/rubygems/package/old.rb   |  2 +-
 lib/rubygems/safe_yaml.rb     | 48 +++++++++++++++++++++++++++++++++++++++++++
 lib/rubygems/specification.rb |  2 +-
 6 files changed, 54 insertions(+), 5 deletions(-)
 create mode 100644 lib/rubygems/safe_yaml.rb

diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index d819bdee..ab004e8e 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -690,7 +690,7 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
 
     unless test_syck
       begin
-        gem 'psych', '>= 1.2.1'
+        gem 'psych', '>= 2.0.0'
       rescue Gem::LoadError
         # It's OK if the user does not have the psych gem installed.  We will
         # attempt to require the stdlib version
@@ -714,6 +714,7 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
     end
 
     require 'yaml'
+    require 'rubygems/safe_yaml'
 
     # If we're supposed to be using syck, then we may have to force
     # activate it via the YAML::ENGINE API.
diff --git a/lib/rubygems/config_file.rb b/lib/rubygems/config_file.rb
index b98d30cc..a4efed0f 100644
--- a/lib/rubygems/config_file.rb
+++ b/lib/rubygems/config_file.rb
@@ -354,7 +354,7 @@ if you believe they were disclosed to a third party.
     return {} unless filename and File.exist? filename
 
     begin
-      content = YAML.load(File.read(filename))
+      content = Gem::SafeYAML.load(File.read(filename))
       unless content.kind_of? Hash
         warn "Failed to load #{filename} because it doesn't contain valid YAML hash"
         return {}
diff --git a/lib/rubygems/package.rb b/lib/rubygems/package.rb
index c36e71d8..77811ed5 100644
--- a/lib/rubygems/package.rb
+++ b/lib/rubygems/package.rb
@@ -468,7 +468,7 @@ EOM
 
     @checksums = gem.seek 'checksums.yaml.gz' do |entry|
       Zlib::GzipReader.wrap entry do |gz_io|
-        YAML.load gz_io.read
+        Gem::SafeYAML.safe_load gz_io.read
       end
     end
   end
diff --git a/lib/rubygems/package/old.rb b/lib/rubygems/package/old.rb
index 88193b98..f6e6e67c 100644
--- a/lib/rubygems/package/old.rb
+++ b/lib/rubygems/package/old.rb
@@ -101,7 +101,7 @@ class Gem::Package::Old < Gem::Package
       header << line
     end
 
-    YAML.load header
+    Gem::SafeYAML.safe_load header
   end
 
   ##
diff --git a/lib/rubygems/safe_yaml.rb b/lib/rubygems/safe_yaml.rb
new file mode 100644
index 00000000..b98cfaa5
--- /dev/null
+++ b/lib/rubygems/safe_yaml.rb
@@ -0,0 +1,48 @@
+module Gem
+
+  ###
+  # This module is used for safely loading YAML specs from a gem.  The
+  # `safe_load` method defined on this module is specifically designed for
+  # loading Gem specifications.  For loading other YAML safely, please see
+  # Psych.safe_load
+
+  module SafeYAML
+    WHITELISTED_CLASSES = %w(
+      Symbol
+      Time
+      Date
+      Gem::Dependency
+      Gem::Platform
+      Gem::Requirement
+      Gem::Specification
+      Gem::Version
+      Gem::Version::Requirement
+      YAML::Syck::DefaultKey
+      Syck::DefaultKey
+    )
+
+    WHITELISTED_SYMBOLS = %w(
+      development
+      runtime
+    )
+
+    if ::YAML.respond_to? :safe_load
+      def self.safe_load input
+        ::YAML.safe_load(input, WHITELISTED_CLASSES, WHITELISTED_SYMBOLS, true)
+      end
+
+      def self.load input
+        ::YAML.safe_load(input, [::Symbol])
+      end
+    else
+      warn "YAML safe loading is not available. Please upgrade psych to a version that supports safe loading (>= 2.0)."
+      def self.safe_load input, *args
+        ::YAML.load input
+      end
+
+      def self.load input
+        ::YAML.load input
+      end
+    end
+  end
+end
diff --git a/lib/rubygems/specification.rb b/lib/rubygems/specification.rb
index a23ffa22..2d71d184 100644
--- a/lib/rubygems/specification.rb
+++ b/lib/rubygems/specification.rb
@@ -1124,7 +1124,7 @@ class Gem::Specification < Gem::BasicSpecification
     Gem.load_yaml
 
     input = normalize_yaml_input input
-    spec = YAML.load input
+    spec = Gem::SafeYAML.safe_load input
 
     if spec && spec.class == FalseClass then
       raise Gem::EndOfYAMLException
-- 
2.11.0