What’s Vulnerable › Vulnerabilities

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

• Severity: critical (CVSS 9.1 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-20 · updated 2026-07-23
• Weakness: CWE-295

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

• lettre: from 0.10.1, < 0.11.22

References

• https://github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35
• https://github.com/lettre/lettre/releases/tag/v0.11.22
• https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53
• https://rustsec.org/advisories/RUSTSEC-2026-0141.html
• https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53

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