fixnx
criticalCVE-2026-46428

CVE-2026-46428 lettre vulnerability

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.

Browse Cryptography security risks
Productlettre
CVSS9.1
EPSSNot scored yet
UpdatedJuly 21, 2026

Quick answer

lettre should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.

Who is affected

Affected versions

  • >= 0.10.1, < 0.11.22

Fixed versions

  • Apply the latest vendor-supported patched version.

How to fix it

CVE-2026-46428 affects lettre and involves cryptographic validation failure. A validation failure may make the application trust an invalid peer, signature, token, or cryptographic result. Treat this as a high-priority fix, especially when the affected component or endpoint is reachable by untrusted users. Upgrade lettre to 0.11.22 or a later vendor-supported fixed release.

  1. Inventory every deployment, device, package, plugin, container, build, and environment that uses lettre.
  2. Compare the installed version with the recorded affected range: >= 0.10.1, < 0.11.22.
  3. Upgrade lettre to 0.11.22 or a later vendor-supported fixed release.
  4. Until patched, disable the affected integration or require an independently verified trusted endpoint and certificate or signature policy.
  5. Review configuration, roles, tokens, network paths, and exposed endpoints connected to lettre; remove access that is not required.
  6. Review TLS, authentication, signature, and connection logs for untrusted peers, certificate errors, downgrade behavior, or unexpected trust decisions.
  7. If exploitation is suspected, isolate affected assets, preserve evidence, rotate exposed credentials or keys, restore trusted data, and rebuild compromised systems before returning them to service.

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Verify the fix

  • Confirm every affected asset now runs the fixed vendor-supported release or has the documented mitigation for CVE-2026-46428 in place.
  • Use a safe regression test to confirm the affected cryptographic validation failure path no longer produces the vulnerable behavior.
  • Confirm temporary access controls, endpoint restrictions, network blocks, and least-privilege settings remain effective after the update.
  • Review post-remediation logs and rerun Fixnx plus the relevant package, dependency, firmware, browser, or product-specific security check.
  • Document affected assets, versions, changes, validation evidence, reviewed logs, and any credential rotation, cleanup, or replacement completed.

Related categories

Cryptography

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Trusted references

Source advisoryNVDSource advisoryNVDSource advisoryNVDSource advisoryNVDSource advisoryNVD

FAQ

What is affected by CVE-2026-46428?

lettre versions listed as affected should be reviewed: >= 0.10.1, < 0.11.22.

What should I fix first?

Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.

How do I confirm the fix worked?

Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.

How are Fixnx security risk categories chosen?

Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.