diff --git a/lib/rack/utils.rb b/lib/rack/utils.rb
index c05a94042..1808660cd 100644
--- a/lib/rack/utils.rb
+++ b/lib/rack/utils.rb
@@ -346,6 +346,18 @@ def byte_ranges(env, size)
     end
     module_function :byte_ranges
 
+    # Constant time string comparison.
+    def secure_compare(a, b)
+      return false unless bytesize(a) == bytesize(b)
+
+      l = a.unpack("C*")
+
+      r, i = 0, -1
+      b.each_byte { |v| r |= v ^ l[i+=1] }
+      r == 0
+    end
+    module_function :secure_compare
+
     # Context allows the use of a compatible middleware at different points
     # in a request handling stack. A compatible middleware must define
     # #context which should take the arguments env and app. The first of which
diff --git a/test/spec_utils.rb b/test/spec_utils.rb
index 3a3ee8f4f..f98515470 100644
--- a/test/spec_utils.rb
+++ b/test/spec_utils.rb
@@ -301,6 +301,11 @@ def kcodeu
     Rack::Utils.bytesize("FOO\xE2\x82\xAC").should.equal 6
   end
 
+  should "should perform constant time string comparison" do
+    Rack::Utils.secure_compare('a', 'a').should.equal true
+    Rack::Utils.secure_compare('a', 'b').should.equal false
+  end
+
   should "return status code for integer" do
     Rack::Utils.status_code(200).should.equal 200
   end