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EUVD-2026-339211

fkie_cve-2026-46428

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.

GHSA-4pj9-g833-qx53 — lettre has TLS hostname verification disabled when using Boring TLS backend

Affected: crates.io: lettre

### 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_verify_hostname(bool)` / `verify_hostname(bool)` API takes a *verify* flag — `true` means enforce hostname verification, `false` means skip it. (Boring docs: https://docs.rs/boring/latest/boring/ssl/struct.ConnectConfiguration.html#method.set_verify_hostname) lettre's `TlsParametersBuilder` exposes the opposite-named flag `accept_invalid_hostnames: bool` — `true` means *accept invalid* (skip verification), `false` means verify. lettre passes this flag directly to boring at both TLS-upgrade sites, inverting the semantics: src/transport/smtp/client/net.rs:202 (sync) .verify_hostname(*accept_invalid_hostnames) src/transport/smtp/client/async_net.rs:377 (async) config.set_verify_hostname(accept_invalid_hostnames); Concrete behaviour under `boring-tls`: * accept_invalid_hostnames = false (the strict default) → set_verify_hostname(false) → hostname verification SKIPPED (wrong; the caller asked for strict verification) * accept_invalid_hostnames = true (explici…

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

Affected: crates.io: lettre

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).