CVE-2026-46428 lettre CVE debrief

Who should care

Users of the lettre mailer library for Rust, particularly those using the `boring-tls` feature in version 0.10.1 to 0.11.21, should be aware of this vulnerability and take steps to mitigate it. This includes operators, platform administrators, vulnerability management teams, and security teams who manage or use lettre-based applications.

Technical summary

The vulnerability is caused by an inverted-boolean bug in lettre's `boring-tls` integration, which 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. The bug affects versions 0.10.1 to 0.11.21 of the lettre library.

Defensive priority

High

Recommended defensive actions

• Update to version 0.11.22 or later
• Verify TLS configuration and ensure hostname verification is enabled
• Monitor for suspicious activity
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability was reported by an unknown source and patched by the lettre developers. The CVE record was published on 2026-07-20T16:17:01.347Z and last modified on 2026-07-21T16:17:11.147Z. Evidence is limited, and defenders should verify affected deployments and configurations. The bug affects users of the `boring-tls` feature in version 0.10.1 to 0.11.21 of the lettre library.

Official resources