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Vulnerability · Published Jul 20, 2026 · Assigned by GitHub_M

CVE-2026-46428lettre has TLS hostname verification disabled when using Boring TLS backend

Critical 9.1

In plain English

A critical-severity improper certificate validation in lettre from lettre. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).

Severity (CVSS 4.0)9.1Critical
Exploit odds (30 days)0.3%top 75.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official description

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.

Affected products (as reported)

VendorProductVersions
lettre lettre >= 0.10.1, < 0.11.22

Weakness: CWE-295 · 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

References