---
title: "CVE-2026-46428: lettre has TLS hostname verification disabled when using Boring TLS backend"
url: https://erroneo.us/security/cve/CVE-2026-46428/
description: "CVE-2026-46428 (lettre (crates.io (Rust))): lettre is a a mailer library for Rust. Starting in version 0.10.1 and prior to version 0.11.22, an…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-07-20
date_modified: 2026-07-28
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-46428"
kev: false
cvss: 9.1
epss: 0.00319
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.3% , percentile 24: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 24% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes lettre (crates.io (Rust)) >= 0.10.1, < 0.11.22, fixed in 0.11.22 . Check your lockfile, including transitive dependencies.

## 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.

## Affected packages

- crates.io (Rust) `lettre` >= 0.10.1, < 0.11.22 → fixed: 0.11.22

## Affected products (CVE record)

- lettre lettre: >= 0.10.1, < 0.11.22

## Severity

- GitHub_M CVSS 4.0: 9.1 `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`
- GitHub CVSS 4.0: 9.1 `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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.32% probability of exploitation activity in the next 30 days; percentile 24.

## Sources

- [GitHub Advisory GHSA-4pj9-g833-qx53](https://github.com/advisories/GHSA-4pj9-g833-qx53)
- [CVE.org record CVE-2026-46428](https://www.cve.org/CVERecord?id=CVE-2026-46428)
- [NVD entry CVE-2026-46428](https://nvd.nist.gov/vuln/detail/CVE-2026-46428)
- [github.com](https://github.com/lettre/lettre/security/advisories/GHSA-4pj9-g833-qx53)
- [github.com](https://github.com/lettre/lettre/commit/f5efffc88360dbdbfcef80f465e42d5bce68ca35)
- [github.com](https://github.com/lettre/lettre/releases/tag/v0.11.22)
- [rustsec.org](https://rustsec.org/advisories/RUSTSEC-2026-0141.html)

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Source page: https://erroneo.us/security/cve/CVE-2026-46428/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt