MySQL allows changing the location of the general log and the slow query log while the server is running by anybody having the SYSTEM_VARIABLES_ADMIN privilege to any location, including appending to existing files. In Percona Server for MySQL 8.0.28-19 we introduced a new system variable, secure-log-path, that can be used to restrict the location of these log files to avoid accidents or possible data corruption attacks.

When somebody with the system variables admin privilege changes these variables, the server runs a few sanity checks. Unfortunately, these checks are quite minimal, and only verify that the specified file is writable by mysqld.

Compared to this, other variables specifying write-related file and directory names are either read-only during the runtime of the server (such as datadir, tmpdir, or log_error), or have additional security checks, for example, the FILE privilege restricted by secure-file-priv.

Without a mechanism like that, these log files can be redirected to any file writable by MySQL, such as ibdata0 or any other data file in the data directory. The server only appends to these files, and no data will be deleted from them, but this still makes it possible to corrupt these files by writing unrelated log messages into them.

To fix this issue, starting with Percona Server for MySQL 8.0.28-19 we introduced a new variable, secure-log-path. This works similarly to secure-file-priv: it can be either empty (which is the default) or the name of a directory. If it is specified, the general log file and the slow query log can only be set to a file within that directory.

Similar to secure-file-priv, the server will display a warning during startup if this new variable is empty, and also if the data directory is within it.

While these two variables are similar, it is also important that they are not the same: in most setups, users with the FILE privilege shouldn’t have read access to the log files written by mysqld using SQL commands, as that could leak potentially sensitive data. To avoid this, it is important to set them to different directories.