Reject unsigned LogoutRequest when signature validation is enabled

Previously, ValidateEncodedLogoutRequestPOST silently accepted unsigned
SAML LogoutRequest messages when SkipSignatureValidation was false.
When validateElementSignature returned ErrMissingSignature, the code
fell through and processed the unverified element. Unlike Response
handling, LogoutRequest has no secondary assertion signature check,
so this allowed completely unsigned requests to be accepted.

Return an error instead of falling through when the signature is missing.

Author: russellhaering

decode_logout_request.go

@@ -49,7 +49,7 @@ func (sp *SAMLServiceProvider) ValidateEncodedLogoutRequestPOST(encodedRequest s
 	}
 
 	// Parse the raw request - parseResponse is generic
-	doc, el, err := parseResponse(raw, sp.MaximumDecompressedBodySize)
+	_, el, err := parseResponse(raw, sp.MaximumDecompressedBodySize)
 	if err != nil {
 		return nil, err
 	}
@@ -58,8 +58,7 @@ func (sp *SAMLServiceProvider) ValidateEncodedLogoutRequestPOST(encodedRequest s
 	if !sp.SkipSignatureValidation {
 		el, err = sp.validateElementSignature(el)
 		if err == dsig.ErrMissingSignature {
-			// Unfortunately we just blew away our Response
-			el = doc.Root()
+			return nil, fmt.Errorf("logout request is not signed")
 		} else if err != nil {
 			return nil, err
 		} else if el == nil {