# CVE-2026-123611: rustls before 0.23.46 Key Exchange Downgrade via Unauthenticated ServerHello

Status: published. Published: 2026-11-08. Updated: 2026-11-08.

rustls before 0.23.46 contains a key exchange downgrade vulnerability that allows network attackers to poison the per-server-name key-exchange group record via an unauthenticated ServerHello. Attackers can answer one connection with a ServerHello selecting X25519, causing later ClientHellos to send only classical shares and lose post-quantum confidentiality.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- rustls / rustls (rustls): ≥ 0; < 0.23.46 (affected, semver); 0.23.46 (unaffected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 6.3 (medium); CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 3.7 (low); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-757

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/rustls/rustls/security/advisories/GHSA-w44r-93j7-qv28 (CNA: VulnCheck; vendor-advisory)

- https://www.vulncheck.com/advisories/rustls-before-0.23.46-key-exchange-downgrade-via-unauthenticated-serverhello (CNA: VulnCheck; third-party-advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-123611; retrieved 2026-11-08; source updated 2026-11-08

Revision: c77f2f014e8914604ab76e320acdd49fb05561aa91b903bdb11fa6bc0c712fb4

Canonical: https://y2.dev/cve/CVE-2026-123611

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