modauthmellon through 0.14.2 has an Open Redirect via the login?ReturnTo= substring, as demonstrated by omitting the // after http: in the target URL.
{ "binaries": [ { "binary_name": "libapache2-mod-auth-mellon", "binary_version": "0.12.0-2+deb9u1build0.16.04.1" } ] }
{ "availability": "No subscription required", "binaries": [ { "binary_name": "libapache2-mod-auth-mellon", "binary_version": "0.13.1-1ubuntu0.2" } ] }
{ "availability": "No subscription required", "binaries": [ { "binary_name": "libapache2-mod-auth-mellon", "binary_version": "0.14.2-1ubuntu2" } ] }