Is CVE-2026-46428 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-46428 being exploited in the wild.

How severe is CVE-2026-46428?

Critical severity, with a CVSS score of 4, published 20 July 2026.

Which products and versions are affected?

lettre (crates.io) SEMVER:introduced=0.10.1,fixed=0.11.22; Lettre >= 0.10.1, < 0.11.22.

Is there a fix for CVE-2026-46428?

Yes. lettre (crates.io): upgrade to 0.11.22 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-46428 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-46428?

Check whether you run lettre (crates.io) or Lettre at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-46428: lettre TLS hostname verification bypass in boring-tls backend

Q: Is there a fix for CVE-2026-46428?

Yes. lettre (crates.io): upgrade to 0.11.22 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

Q: How is CVE-2026-46428 attacked?

It is attacked over the network, in a software library.

Q: Am I affected by CVE-2026-46428?

Check whether you run lettre (crates.io) or Lettre at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-46428 · Severity: critical · CVSS 4 · Published 2026-07-20

Lettre, a popular Rust library for sending emails, contains a flaw when using the BoringSSL security backend that silently disables identity verification for mail servers. This allows a sophisticated attacker positioned on the network to impersonate a legitimate email server and intercept sensitive communications. If exploited, an attacker could steal email login credentials, read private message contents, or modify emails before they are sent.

Technologies: lettre (crates.io). Vendors: crates.io.

Quick answers

Is CVE-2026-46428 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-46428 being exploited in the wild.
How severe is CVE-2026-46428?: Critical severity, with a CVSS score of 4, published 20 July 2026.
Which products and versions are affected?: lettre (crates.io) SEMVER:introduced=0.10.1,fixed=0.11.22; Lettre >= 0.10.1, < 0.11.22.
Is there a fix for CVE-2026-46428?: Yes. lettre (crates.io): upgrade to 0.11.22 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-46428 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-46428?: Check whether you run lettre (crates.io) or Lettre at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

lettre (crates.io): upgrade to 0.11.22 or later
```
cargo update -p lettre
```

Technical details

An inverted-boolean bug in lettre's `boring-tls` integration causes the library to skip TLS hostname verification when the default strict configuration is used. The vulnerability stems from `TlsParametersBuilder` passing its `accept_invalid_hostnames` flag directly to BoringSSL's `set_verify_hostname` API, which uses opposite logic (true means verify, false means skip). An on-path attacker with a valid certificate for any domain can successfully perform a man-in-the-middle attack against SMTP sessions, gaining access to SMTP AUTH credentials and message bodies. This issue affects both synchronous and asynchronous implementations in versions 0.10.1 through 0.11.21 when the `boring-tls` feature is enabled. The fix involves negating the flag at the BoringSSL call sites, and it is addressed in version 0.11.22.

Affected products

lettre lettre >= 0.10.1, < 0.11.22

Identifiers

CVE-2026-46428
GHSA-4pj9-g833-qx53
RUSTSEC-2026-0141

Timeline

2026-05-12: other: Downstream mitigation by Cloudflare email-fwdr
2026-05-14: advisory: Initial GitHub Advisory published
2026-07-28: advisory: Advisory updated

References

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