HTTPRedirect.php in the saml2 library in SimpleSAMLphp before 1.15.4 has an incorrect check of return values in the signature validation utilities, allowing an attacker to get invalid signatures accepted as valid by forcing an error during validation. This occurs because of a dependency on PHP functionality that interprets a -1 error code as a true boolean value.
github.com/FriendsOfPHP/security-advisories/blob/master/simplesamlphp/saml2/CVE-2018-7711.yaml
github.com/simplesamlphp/saml2/commit/4f6af7f69f29df8555a18b9bb7b646906b45924d
lists.debian.org/debian-lts-announce/2018/03/msg00017.html
nvd.nist.gov/vuln/detail/CVE-2018-7711
simplesamlphp.org/security/201803-01