Exim Mail Server 4.100.1 Fixes 4 Security Flaws Including SMTP Smuggling and Heap Corruption
Exim 4.100.1 patches four flaws, including high-severity Proxy Protocol heap corruption, uninitialized data leak, GnuTLS use-after-free, and SMTP smuggling.
Exim released version 4.100.1 on September 18, 2026, fixing four vulnerabilities affecting versions from 4.83 to 4.100 depending on configuration. GCVE-25-2026-09-50-1 is an out-of-bounds write in Proxy Protocol v1 handling allowing a ~230-byte over-read and NUL-byte write; GCVE-25-2026-09-55-1 leaks stack data via uninitialized memory in Proxy Protocol v2 handling. A low-severity GnuTLS use-after-free (GCVE-25-2026-09-51-1) requires the non-default tls_early_banner_hosts setting, and GCVE-25-2026-09-56-1 permits SMTP smuggling that can alter submitted email content. Administrator upgrades to 4.100.1 are strongly advised; issues were reported by McCaulay Hudson of watchTowr.
- Proxy Protocol v1 flaw allows heap over-read of ~230 bytes and NUL-byte write
- Proxy Protocol v2 bug leaks server stack data via uninitialized memory
- GnuTLS use-after-free can crash mail reception, needs tls_early_banner_hosts enabled
- SMTP smuggling lets attackers submit content differing from logged messages
- All admins running Exim 4.83–4.100 with affected configs urged to upgrade
- Products
- Exim 4.100.1
- Organizations
- watchTowr
From the original article
2 min read at cybersecuritynews.com
Exim 4. 100. 1 fixes four security vulnerabilities, including high-severity Proxy Protocol and GnuTLS use-after-free flaws and a medium-severity SMTP smuggling issue that could let attackers alter submitted email content.
By Abinaya · cybersecuritynews.com
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