While BREACH requires the following three factors to be effectively exploitable we should add another mitigation:
1. Application must support HTTP compression
2. Response most reflect user-controlled input
3. Response should contain sensitive data
Especially part 2 is with ownCloud not really given since user-input is usually only echoed if a CSRF token has been passed.
To reduce the risk even further it is however sensible to encrypt the CSRF token with a shared secret. Since this will change on every request an attack such as BREACH is not feasible anymore against the CSRF token at least.
Storage status is saved in the database. Failed storages are rechecked every
10 minutes, while working storages are rechecked every request.
Using the files_external app will recheck all external storages when the
settings page is viewed, or whenever an external storage is saved.
From https://github.com/owncloud/core/issues/16287:
> This is caused by XCache at 8e59d4c64b/lib/private/util.php (L1276) where we are trying to reset the opcode cache with `XC_TYPE_PHP`.
> I suspect that while XCache is installed its opcode component is not used. Unfortunately, the XCache API is not really properly documented and thus I don't know what API we would have to call to check whether the `XC_TYPE_PHP` cache is populated. In fact, there is an [open XCache bug](http://xcache.lighttpd.net/ticket/176) since 7 years that discusses this problem and is likely to never get fixed since XCache is abandonware.
Fixes https://github.com/owncloud/core/issues/16287
This removes the hard-dependency on output buffering as requested at https://github.com/owncloud/core/issues/16013 since a lot of distributions such as Debian and Ubuntu decided to use `4096` instead of the PHP recommended and documented default value of `off`.
However, we still should encourage disabling this setting for improved performance and reliability thus the setting switches in `.user.ini` and `.htaccess` are remaining there. It is very likely that we in other cases also should disable the output buffering but aren't doing it everywhere and thus causing memory problems.
Fixes https://github.com/owncloud/core/issues/16013
