Executive brief
Lettre, a popular Rust library for sending emails, contains a flaw when using the BoringSSL security backend that silently disables identity verification for mail servers. This allows a sophisticated attacker positioned on the network to impersonate a legitimate email server and intercept sensitive communications. If exploited, an attacker could steal email login credentials, read private message contents, or modify emails before they are sent.
Technical details
An inverted-boolean bug in lettre's `boring-tls` integration causes the library to skip TLS hostname verification when the default strict configuration is used. The vulnerability stems from `TlsParametersBuilder` passing its `accept_invalid_hostnames` flag directly to BoringSSL's `set_verify_hostname` API, which uses opposite logic (true means verify, false means skip). An on-path attacker with a valid certificate for any domain can successfully perform a man-in-the-middle attack against SMTP sessions, gaining access to SMTP AUTH credentials and message bodies. This issue affects both synchronous and asynchronous implementations in versions 0.10.1 through 0.11.21 when the `boring-tls` feature is enabled. The fix involves negating the flag at the BoringSSL call sites, and it is addressed in version 0.11.22.
Affected products
- lettre lettre >= 0.10.1, < 0.11.22
Timeline
- 2026-05-12: other: Downstream mitigation by Cloudflare email-fwdr
- 2026-05-14: advisory: Initial GitHub Advisory published
- 2026-07-28: advisory: Advisory updated