App::Ack versions through 3.10.0 for Perl read arbitrary files via --files-from in a project .ackrc.
ack searches up the directory hierarchy from the current directory for a project .ackrc and loads its options. The project-source option blocklist in App::Ack::ConfigLoader does not include --files-from, so a project .ackrc can set it to a path whose listed files ack then reads and searches. Version 3.10.0 added --follow to the blocklist; --files-from remains accepted.
A project .ackrc committed to an untrusted repository can make ack read files outside the project and print their matching lines.
Run ack with --noenv, or avoid running ack in a directory tree that contains an untrusted .ackrc.
Upgrade to a future ack release.