Impact

The admin url can be discovered without prior knowledge of it's location by exploiting the X-Original-Url header on some configurations.

Patches

The bug comes from the Zend library and is patche by unsetting the header in the bootstrap process.

Workarounds

Unset the X-Original-Url header in the web server configuration.

References

The activation of these headers is coming from the ZendController module. It appears this has been known to some degree since 2016 - https://peterocallaghan.co.uk/2016/12/magento-poisoning-cache/ (dead link now..)

Credit

Anees Hyder ( @anees0xdev ) via HackerOne https://hackerone.com/anees0xdev/hacktivity