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

> An attacker can intercept SMTP traffic and steal credentials because TLS hostname verification is disabled in the Boring TLS backend.

- **Severity:** Critical 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Jul 2026
- **Updated:** 21 Jul 2026
- **Also known as:** GHSA-4pj9-g833-qx53, RUSTSEC-2026-0141

## What is CVE-2026-46428?

CVE-2026-46428 is a critical vulnerability in lettre, published on 20 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-295 (Improper Certificate Validation)

## Which versions are affected by CVE-2026-46428?

3 open source packages ship an affected version, including lettre.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `lettre` | crates.io | From 0.10.1 | 0.11.22 |
| `lettre` | crates.io | From 0.10.1 | 0.11.22 |
| `lettre` | crates.io | From 0.10.1 | 0.11.22 |

## How do I fix CVE-2026-46428?

Upgrade lettre to 0.11.22 or later.

## Is CVE-2026-46428 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-46428 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-46428 and fix it if it is affected.

What it is: CVE-2026-46428 is a critical vulnerability in lettre, published on 20 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-46428 exists.

Affected packages and fixed versions:
- lettre (crates.io): affected from 0.10.1; fixed in 0.11.22
- lettre (crates.io): affected from 0.10.1; fixed in 0.11.22
- lettre (crates.io): affected from 0.10.1; fixed in 0.11.22

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-46428/ (Markdown: https://insights.sbomify.com/CVE-2026-46428/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-46428
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-46428
- OSV: https://osv.dev/vulnerability/CVE-2026-46428

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-46428/
Summaries are generated and may contain mistakes; the official description above is the record.