CVE-2026-46428CWE-295

lettre has TLS hostname verification disabled when using Boring TLS backend

Critical · published July 20, 2026

CVSS v4.0
9.1
EPSS
0%
Percentile
23.7
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v4.0
9.1 · CRITICAL
Vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:L/SI:L/SA:N
EPSS
0.00319 · 23.7th percentile
Weakness
CWE-295 · Improper Certificate Validation
Published
2026-07-20T15:37Z
EPSS history
Timeline
  • 20 JUL 15:37Z
    lettre has TLS hostname verification disabled when using Boring TLS backend
    cvelistv5