Update X-XSS-Protection recommendation

[MichaIng]

• Resolves: [Bug]: Setup check for X-XSS-Protection recommendation is deprecated #37154

Summary

While 1; mode=block was seen as the most secure value for this header, some years ago, after possible side-channel attacks have become known, this turned towards 0 being the best-practice value, disabling XSS filtering entirely for those old browsers who still support it.

MDN web docs give some explanation: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/X-XSS-Protection#security_considerations

The OWASP cheat sheet recommends 0: https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#x-xss-protection

Here the related discussion when this recommendation was updated: OWASP/CheatSheetSeries#376

A Stack Overflow question underlines the clear recommendation, adding some more details: https://stackoverflow.com/questions/9090577

Since 1; mode=block is not a large security risk, it affects only very old browsers, and a changed recommendation will (sadly) trigger a lot of issues/topics in forum and GitHub, the old value however is allowed to pass the check. So to pass the check, either page blocking needs to be enabled along with XSS filtering, or XSS filtering needs to be disabled. The warning however will always suggest to disable it.

TODO

• Update .htaccess
• Update documentation, once the change has been generally accepted

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[MichaIng]

Just found #53476 which addresses pretty much the same, and probably in a more consequent way. Keeping the check for this ancient header which is not supported anymore by any browser since 2019, doesn't really make sense. And OWASP also suggests to just not set it at all, as alternative to explicitly disabling XSS filtering.

[MichaIng]

After sleeping a night over it and another thought, I am closing this PR in favor or #53476. If for whatever reason a majority thinks it makes more sense to keep the check for now, just accepting 0 as well, it can be re-opened any time. But I vote for removing the check instead.