A vulnerability was found in mod_auth_mellon before v0.14.2. If Apache is configured as a reverse proxy and mod_auth_mellon is configured to only let through authenticated users (with the require valid-user directive), adding special HTTP headers that are normally used to start the special SAML ECP (non-browser based) can be used to bypass authentication.
access.redhat.com/errata/RHBA-2019:0959
access.redhat.com/errata/RHSA-2019:0746
access.redhat.com/errata/RHSA-2019:0766
access.redhat.com/errata/RHSA-2019:0985
bugzilla.redhat.com/show_bug.cgi?id=CVE-2019-3878
github.com/Uninett/mod_auth_mellon/pull/196
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CNW5YMC5TLWVWNJEY6AIWNSNPRAMWPQJ/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/X7NLAU7KROWNTHAYSA2S67X347F42L2I/
usn.ubuntu.com/3924-1/