CVE-2026-46428: Lettre

Critical severity, CVSS 9.1. EPSS: 0.3% chance of exploitation in the next 30 days.

lettre is a a mailer library for Rust. Starting in version 0.10.1 and prior to version 0.11.22, an inverted-boolean bug in lettre's `boring-tls` integration silently disables TLS hostname verification for callers using the default (strict) configuration. An on-path attacker presenting any chain-valid certificate for any domain can intercept SMTP submission, including PLAIN/LOGIN credentials and message contents, against any lettre user built with the `boring-tls` feature. Other TLS backends (`native-tls`, `rustls`) are unaffected. Version 0.11.22 patches the issue.

Affected products

• Lettre Lettre: from 0.10.1, before 0.11.22 (fixed in 0.11.22)

Published 2026-07-20. Last modified 2026-07-23.