CRITICAL GitHub
CVE-2026-46428
lettre has TLS hostname verification disabled when using Boring TLS backend
### Summary
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.
### Details
boring's `SslConnectorBuilder::set
Affected Products
- rust/lettre >= 0.10.1, < 0.11.22
References
- https://github.com/advisories/GHSA-4pj9-g833-qx53
- https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53
- https://nvd.nist.gov/vuln/detail/CVE-2026-46428
- https://github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35
This critical severity vulnerability was published on 2026-07-28 via GitHub. Affected: rust/lettre >= 0.10.1, < 0.11.22.
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