We presently get a lot of these messages in our exim logs:

2022-04-11 21:47:15 Warning: Tainted filename for search '/etc/exim4/aliases/wikimedia.org'
2022-04-11 21:47:15 Warning: Tainted filename '/etc/exim4/aliases/wikimedia.org'
2022-04-11 21:47:15 Warning: Tainted filename for search '/etc/exim4/aliases/wikimedia.org'

These are due to using $domain, which is under the control of the email sender, as the value for a filename. Exim does not allow you to use sender controlled data for filename lookups. Instead we should switch to $domain_data which is the value of a domain lookup.