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Critical, CVSS 9.1amended

lettre has TLS hostname verification disabled when using Boring TLS backend

CVE-2026-46428GHSA-4pj9-g833-qx53

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (lettre 0.11.22), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • crates.io lettre

    Affected
    >= 0.10.1, < 0.11.22
    Fixed in
    0.11.22

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
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 · score source
EPSS
  • CVE-2026-46428: 0.32% estimated exploitation activity in the next 30 days; percentile 24.97 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary 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. Details boring's SslConnectorBuilder::setverifyhostname(bool) / verifyhostname(bool) API takes a verify flag — true means enforce hostname verification, false means skip it. (Boring docs: https://docs.rs/boring/latest/boring/ssl/struct.ConnectConfiguration.html#method.setverifyhostname) lettre's TlsParametersBuilder exposes the opposite-named flag acceptinvalidhostnames: bool — true means accept invalid (skip verification), false means verify. lettre passes this flag directly to boring at both TLS-upgrade sites, inverting the semantics:…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-4pj9-g833-qx53

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-4pj9-g833-qx53

Agent brief

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# Patch Next advisory brief: CVE-2026-46428

Advisory: lettre has TLS hostname verification disabled when using Boring TLS backend
IDs: CVE-2026-46428, GHSA-4pj9-g833-qx53
Record: https://patchnext.com/advisories/cve-2026-46428 (revision 3, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lettre 0.11.22. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io lettre: affected >= 0.10.1, < 0.11.22; fixed in 0.11.22

## Suggested inspection (read-only)
- crates.io lettre: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on lettre.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4pj9-g833-qx53 (OSV record GHSA-4pj9-g833-qx53 lists affected ranges: crates.io lettre >=0.10.1 <0.11.22 (fixed))
- Verified: https://github.com/advisories/GHSA-4pj9-g833-qx53 (GitHub-reviewed advisory GHSA-4pj9-g833-qx53 lists affected ranges: crates.io lettre >=0.10.1 <0.11.22)

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