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Vulnerabilities

Vulnerability · RUSTSEC-2026-0349

Unauthenticated ServerHello can downgrade later connections to classical key exchange

Published Oct 14, 2026 · catalog updated Oct 14, 2026 · RustSec Rust

Open the advisory

Exploitation likelihood is not on this record for RUSTSEC-2026-0349. A missing EPSS score is not a low risk. Compare RustSec Rust with the linked advisory before you change production.

Gaps on this record

  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
  • EPSS is missing. A missing score is not a low risk.
Severity
Low3.7
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
Awaiting synchronization with FIRST
Remediation
Advisory link
Affected products
crates.io rustls

The client recorded the server-selected key exchange group for each server name when processing the ServerHello, before the server was authenticated. This record is used to predict which key share to send in future ClientHellos to that server name. A network attacker can answer one connection attempt with a ServerHello selecting X25519, and then close the connection. Later ClientHellos for that name carry only an X25519 key share, while still lists X25519MLKEM768 first. If the genuine server accepts that share rather than sending a HelloRetryRequest for its preferred hybrid group, the handshake completes with classical-only key exchange and no error. This persists for as long as the entry stays in the client's session store. The impact is loss of post-quantum confidentiality for affected connections ("harvest now, decrypt later"). There is no key compromise and no authentication bypass. The following are not affected: - connections to servers which send a HelloRetryRequest for their preferred group, - clients with resumption disabled (as nothing is stored), and - other server names. This behaviour is discussed in [draft-ietf-tls-key-share-prediction](https://www.ietf.org/archive/id/draft-ietf-tls-key-share-prediction-01.html#name-security-considerations), so the practical impact depends largely on server policy.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • crates.io rustls0.0.0-0 → 0.23.46

How to fix RUSTSEC-2026-0349

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

Exploitation likelihood is not on this record for RUSTSEC-2026-0349. A missing EPSS score is not a low risk. Compare RustSec Rust with the linked advisory before you change production.

Which versions are affected?

Compare what you have installed with 0.0.0-0 → 0.23.46. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS is not on this record yet. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Oct 14, 2026 · GitHub Advisory Database

    Detected

    The client recorded the server-selected key exchange group for each server name when processing the ServerHello, before the server was authenticated. This record is used to predict which key share to send in future ClientHellos to that server name. A network attacker can answer one connection attempt with a ServerHello selecting X25519, and then close the connection. Later ClientHellos for that na

  2. Oct 14, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46

  3. Oct 14, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46

  4. Oct 14, 2026 · GitHub Advisory Database

    Updated

    The client recorded the server-selected key exchange group for each server name when processing the ServerHello, before the server was authenticated. This record is used to predict which key share to send in future ClientHellos to that server name. A network attacker can answer one connection attempt with a ServerHello selecting X25519, and then close the connection. Later ClientHellos for that na

Sources

  • GitHub Advisory DatabasePackage advisoryGHSA-w44r-93j7-qv28

    GitHub Advisory Database record mirrored by OSV.

    github.com
  • OpenSSF OSV / OSV ScannerPackage advisoryRUSTSEC-2026-0349

    Unauthenticated ServerHello can downgrade later connections to classical key exchange

    osv.dev
  • RustSec Advisory DBVendor advisoryRUSTSEC-2026-0349

    The client recorded the server-selected key exchange group for each server name when processing the ServerHello, before the server was authenticated. This record is used to predict which key share to send in future ClientHellos to that server name. A network attacker can answer o

    crates.io

References

Freshness

Last verified in this catalog Oct 14, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.