Fix parsing of defined? blocks for complex expressions

YARD evaluates these blocks which can lead to arbitrary execution
of code. Thanks to Nelson Elhage <nelhage@nelhage.com> for
reporting this issue.

Fixes #1177

CHANGELOG.md

@@ -1,3 +1,13 @@
+# master
+
+# [0.9.15] - July 17th, 2018
+
+[0.9.15]: https://github.com/lsegal/yard/compare/v0.9.14...v0.9.15
+
+- Fixed security issue in parsing of Ruby code that could allow for arbitrary
+  execution. Credit to Nelson Elhage <nelhage@nelhage.com> for discovering this
+  issue.
+
 # [0.9.14] - June 2nd, 2018
 
 [0.9.14]: https://github.com/lsegal/yard/compare/v0.9.13...v0.9.14

lib/yard/handlers/ruby/class_condition_handler.rb

@@ -52,12 +52,17 @@ def parse_condition
       # defined? keyword used, let's see if we can look up the name
       # in the registry, then we'll try using Ruby's powers. eval() is not
       # *too* dangerous here since code is not actually executed.
-      name = statement.condition[0].source
-      obj = YARD::Registry.resolve(namespace, name, true)
-      begin
-        condition = true if obj || Object.instance_eval("defined? #{name}")
-      rescue SyntaxError, NameError
-        condition = false
+      arg = statement.condition.first
+
+      if arg.type == :var_ref
+        name = arg.source
+        obj = YARD::Registry.resolve(namespace, name, true)
+
+        begin
+          condition = true if obj || (name && Object.instance_eval("defined? #{name}"))
+        rescue SyntaxError, NameError
+          condition = false
+        end
       end
     when :var_ref
       var = statement.condition[0]

lib/yard/handlers/ruby/legacy/class_condition_handler.rb

@@ -41,7 +41,7 @@ def parse_condition
     case statement.tokens[1..-1].to_s.strip
     when /^(\d+)$/
       condition = $1 != "0"
-    when /^defined\?\s*\(?(.+?)\)?$/
+    when /^defined\?\s*\(?\s*([A-Za-z0-9:_]+?)\s*\)?$/
       # defined? keyword used, let's see if we can look up the name
       # in the registry, then we'll try using Ruby's powers. eval() is not
       # *too* dangerous here since code is not actually executed.

spec/handlers/class_condition_handler_spec.rb

@@ -65,4 +65,23 @@ def no_undoc_error(code)
     eof
     no_undoc_error "if caller.none? { |l| l =~ %r{lib/rails/generators\\.rb:(\\d+):in `lookup!'$} }; end"
   end
+
+  it "only parses identifiers or namespaces from defined? expressions" do
+    module A
+      @@value = nil
+      def self.b(value) @@value = value end
+      def self.value; @@value end
+    end
+
+    YARD.parse_string <<-eof
+      if defined? A.b(42).to_i
+        class Foo; end
+      else
+        class Bar; end
+      end
+    eof
+    expect(A.value).to be_nil
+    expect(YARD::Registry.at('Foo')).not_to be_nil
+    expect(YARD::Registry.at('Bar')).not_to be_nil
+  end
 end