CVE-2026-46428
lettre boring-tls bug disables TLS hostname verification, enabling SMTP MitM
Is CVE-2026-46428 being exploited?
Not confirmed. CVE-2026-46428 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.32% probability of exploitation in the next 30 days.
How severe is CVE-2026-46428?
CVE-2026-46428 is rated Critical with a CVSS score of 9.1. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-46428?
Yes. A fix has been recorded for CVE-2026-46428. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-46428 affect?
CVE-2026-46428 affects lettre (Rust) >=0.10.1 and <0.11.22 built with boring-tls feature (native-tls,rustls unaffected). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-46428?
Upgrade lettre to >=0.11.22 or stop using boring-tls; verify TLS hostname checks.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
On-path attacker can present any valid chain cert to intercept SMTP submission, capturing PLAIN/LOGIN credentials and email contents.
Immediate action required
- affected>= 0.10.1 and < 0.11.22
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.