TL;DR Exim shipped version 4.100.1 on September 18, 2026. It fixes four flaws, including two high-severity Proxy Protocol bugs, a GnuTLS use-after-free, and SMTP smuggling. Three of the four have no workaround, so upgrading is the only fix. Why This Exim Vulnerability Matters Exim runs on a large share of the Internet's mail servers. As a result, any flaw in how it handles inbound connections reaches a wide install base.