RUSTSEC-2026-0141: TLS hostname verification disabled when using Boring TLS backend

What to do

1. See which version you have: cargo tree -i lettre (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p lettre. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example lettre = "0.11.22") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

The bug was introduced in v0.10.1 and persists through v0.11.21 (latest).

References

• Package: crates.io/crates/lettre
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0141.html
• Web: github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for lettre

• Vulnerability MediumRUSTSEC-2020-0069 · 2020-11-11Argument injection in sendmail transport
• VulnerabilityRUSTSEC-2021-0069 · 2021-05-22SMTP command injection in body

Full security history of lettre