Merge commit from fork

* Use correct XPaths and resolve to correct elements

* Update xml_security.rb

* Block references that resolve to multiple nodes to prevent signature wrapping attacks

lib/xml_security.rb

@@ -310,17 +310,29 @@ def validate_signature(base64_cert, soft = true)
       canon_string = noko_signed_info_element.canonicalize(canon_algorithm)
       noko_sig_element.remove
 
+      # get signed info
+      signed_info_element = REXML::XPath.first(
+        sig_element,
+        "./ds:SignedInfo",
+        { "ds" => DSIG }
+      )
       # get inclusive namespaces
       inclusive_namespaces = extract_inclusive_namespaces
 
       # check digests
-      ref = REXML::XPath.first(sig_element, "//ds:Reference", {"ds"=>DSIG})
+      ref = REXML::XPath.first(signed_info_element, "./ds:Reference", {"ds"=>DSIG})
 
-      hashed_element = document.at_xpath("//*[@ID=$id]", nil, { 'id' => extract_signed_element_id })
+      reference_nodes = document.xpath("//*[@ID=$id]", nil, { 'id' => extract_signed_element_id })
+
+      if reference_nodes.length > 1 # ensures no elements with same ID to prevent signature wrapping attack.
+        return append_error("Digest Mismatch", soft)
+      end
+
+      hashed_element = reference_nodes[0]
 
       canon_algorithm = canon_algorithm REXML::XPath.first(
-        ref,
-        '//ds:CanonicalizationMethod',
+        signed_info_element,
+        './ds:CanonicalizationMethod',
         { "ds" => DSIG }
       )
 
@@ -330,13 +342,13 @@ def validate_signature(base64_cert, soft = true)
 
       digest_algorithm = algorithm(REXML::XPath.first(
         ref,
-        "//ds:DigestMethod",
+        "./ds:DigestMethod",
         { "ds" => DSIG }
       ))
       hash = digest_algorithm.digest(canon_hashed_element)
       encoded_digest_value = REXML::XPath.first(
         ref,
-        "//ds:DigestValue",
+        "./ds:DigestValue",
         { "ds" => DSIG }
       )
       digest_value = Base64.decode64(OneLogin::RubySaml::Utils.element_text(encoded_digest_value))
@@ -362,7 +374,7 @@ def validate_signature(base64_cert, soft = true)
     def process_transforms(ref, canon_algorithm)
       transforms = REXML::XPath.match(
         ref,
-        "//ds:Transforms/ds:Transform",
+        "./ds:Transforms/ds:Transform",
         { "ds" => DSIG }
       )