Contao is an open source content management system. Starting in version 4.9.0 and prior to versions 4.13.40 and 5.3.4, when checking for broken links on protected pages, Contao sends the cookie header to external urls as well, the passed options for the http client are used for all requests. Contao versions 4.13.40 and 5.3.4 have a patch for this issue. As a workaround, disable crawling protected pages.

Contao: Possible cookie sharing with external domains while checking protected pages for broken links