ILIAS 5.1.x, 5.2.x, and 5.3.x before 5.3.5 redirects a logged-in user to a third-party site via the return_to_url parameter.

References

• advisory https://www.ilias.de/docu/goto.php?target=st_229
• patch https://github.com/ILIAS-eLearning/ILIAS/commit/01a24cf04fe8dddf1da59ca497580637973482b6