The SAML2 library before 1.10.4, 2.x before 2.3.5, and 3.x before 3.1.1 in SimpleSAMLphp has a Regular Expression Denial of Service vulnerability for fraction-of-seconds data in a timestamp. When parsing a timestamp contained inside a SAML document, the SAML2 library uses a regular expression to validate its format and parse its components. The given regular expression sets no limits for the fraction of seconds part, such that an arbitrary amount of digits can be processed. This opens up the possibility to perform a denial of service attack by sending a large number of digits as the fraction of a second in a timestamp, making the processing script choke while evaluating the regular expression.

References:
https://simplesamlphp.org/security/201801-01