The validateSignature method in the SAML2\Utils class in SimpleSAMLphp before 1.14.10 and simplesamlphp/saml2 library before 1.9.1, 1.10.x before 1.10.3, and 2.x before 2.3.3 allows remote attackers to spoof SAML responses or possibly cause a denial of service by leveraging improper conversion of return values to boolean.