Exim 4 before 4.94.2 allows Execution with Unnecessary Privileges. Because Exim operates as root in the log directory (owned by a non-root user), a symlink or hard link attack allows overwriting critical root-owned files anywhere on the filesystem.
https://www.tenable.com/cyber-exposure/2021-threat-landscape-retrospective
https://www.exim.org/static/doc/security/CVE-2020-qualys/CVE-2020-28007-LFDIR.txt