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CVE-2026-46428 lettre CVE debrief

CVE-2026-46428 is a critical vulnerability in the lettre mailer library for Rust, caused by an inverted-boolean bug in lettre's `boring-tls` integration. This bug silently disables TLS hostname verification for callers using the default (strict) configuration, allowing an on-path attacker to 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.

Vendor
lettre
Product
Unknown
CVSS
CRITICAL 9.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-20
Original CVE updated
2026-07-21
Advisory published
2026-07-20
Advisory updated
2026-07-21

Who should care

Users of the lettre mailer library for Rust, particularly those using the `boring-tls` feature in version 0.10.1 to 0.11.21, should be aware of this vulnerability and take steps to mitigate it. This includes operators, platform administrators, vulnerability management teams, and security teams who manage or use lettre-based applications.

Technical summary

The vulnerability is caused by an inverted-boolean bug in lettre's `boring-tls` integration, which 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. The bug affects versions 0.10.1 to 0.11.21 of the lettre library.

Defensive priority

High

Recommended defensive actions

  • Update to version 0.11.22 or later
  • Verify TLS configuration and ensure hostname verification is enabled
  • Monitor for suspicious activity
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability was reported by an unknown source and patched by the lettre developers. The CVE record was published on 2026-07-20T16:17:01.347Z and last modified on 2026-07-21T16:17:11.147Z. Evidence is limited, and defenders should verify affected deployments and configurations. The bug affects users of the `boring-tls` feature in version 0.10.1 to 0.11.21 of the lettre library.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-46428 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-46428

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-46428 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-46428

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.