Russh: Missing X25519 zero-point validation in hybrid ML-KEM key exchange

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Russh is a Rust SSH client and server library. Prior to 0.63.0, the hybrid ML-KEM 768 and X25519 implementation in russh/src/kex/hybrid_mlkem.rs accepts an all-zero 32-byte peer X25519 public key in both server_dh and compute_shared_secret, forcing the X25519 contribution to the combined shared secret to zero. A malicious SSH peer can therefore make the combined secret depend only on ML-KEM, defeating the hybrid exchange's intended fallback protection if ML-KEM is later weakened. This issue is fixed in version 0.63.0.

Source published

29 Sep 2026 18:27:41 UTC

Source modified

30 Sep 2026 20:24:58 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Russh is a Rust SSH client and server library. Prior to 0.63.0, the hybrid ML-KEM 768 and X25519 implementation in russh/src/kex/hybrid_mlkem.rs accepts an all-zero 32-byte peer X25519 public key in both server_dh and compute_shared_secret, forcing the X25519 contribution to the combined shared secret to zero. A malicious SSH peer can therefore make the combined secret depend only on ML-KEM, defeating the hybrid exchange's intended fallback protection if ML-KEM is later weakened. This issue is fixed in version 0.63.0.

Source published

Sep 29, 2026, 6:27:41 PM (timezone not stated)

Source modified

Sep 30, 2026, 8:24:58 PM (timezone not stated)

Observed here

11 Nov 2026 05:29:30 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Russh is a Rust SSH client and server library. Prior to 0.63.0, the hybrid ML-KEM 768 and X25519 implementation in russh/src/kex/hybrid_mlkem.rs accepts an all-zero 32-byte peer X25519 public key in both server_dh and compute_shared_secret, forcing the X25519 contribution to the combined shared secret to zero. A malicious SSH peer can therefore make the combined secret depend only on ML-KEM, defeating the hybrid exchange's intended fallback protection if ML-KEM is later weakened. This issue is fixed in version 0.63.0.

Source published

29 Sep 2026 19:17:24 UTC

Source modified

30 Sep 2026 21:17:05 UTC

Observed here

25 Oct 2026 04:17:26 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 4.3 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

## Vulnerability The hybrid ML-KEM 768 + X25519 key exchange implementation in `russh/src/kex/hybrid_mlkem.rs` does not validate that the remote peer's X25519 public key is not the zero point (all-zero 32-byte value). This allows a remote peer to force the X25519 contribution to the combined shared secret to zero, reducing the hybrid KEX to a single-algorithm exchange. Affected code at HEAD (v0.62.4, commit 0089c89): **Server-side** (`server_dh`, lines 92-93): ```rust let mut c_pk1 = MontgomeryPoint([0; 32]); c_pk1.0.copy_from_slice(c_pk1_bytes); // No zero-point check - proceeds to: let k_cl = s_secret * c_pk1; ``` **Client-side** (`compute_shared_secret`, lines 154-155): ```rust let mut s_pk1 = MontgomeryPoint([0; 32]); s_pk1.0.copy_from_slice(s_pk1_bytes); // No zero-point check - proceeds to: let k_cl = x25519_secret * s_pk1; ``` ## Root Cause Commit a7fc1eb (2026-07-22, "fix mpint encoding and validate curve25519 keys") added zero-point validation to the standalone Curve25519 KEX in `russh/src/kex/curve25519.rs` at two locations: - `server_dh` line 77: `if client_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }` - `compute_shared_secret` line 122: `if remote_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }` The same X25519 scalar multiplication pattern appears in `hybrid_mlkem.rs`, but the fix was not applied there. ## Proof of Concept A malicious SSH client negotiating `mlkem768x25519-sha256` can send a KEX_HYBRID_INIT message containing a valid ML-KEM 768 encapsulation key followed by 32 zero bytes as the X25519 component. When the server computes `k_cl = s_secret * c_pk1`, the result is the zero point regardless of the server's secret scalar. The combined shared secret `K = SHA-256(k_pq || k_cl)` then depends only on the ML-KEM component. The same attack works in reverse against a client connecting to a malicious server. The zero X25519 public key passes all existing validation (the length check on line 78 succeeds since 32 bytes is correct). No panic or crash occurs - the exchange completes successfully with a weakened shared secret. ## Impact The purpose of hybrid key exchange is defense-in-depth: if either the classical algorithm (X25519) or the post-quantum algorithm (ML-KEM 768) is broken, the combined secret remains secure. By injecting a zero X25519 public key, an attacker eliminates the classical contribution entirely, reducing security to depend solely on ML-KEM. This matters in two scenarios: 1. If ML-KEM 768 is later found to have a weakness (the explicit threat model hybrid KEX is designed to mitigate), sessions where the X25519 component was zeroed out lose their fallback protection. 2. An active network attacker who can intercept KEX could downgrade the hybrid exchange to effectively single-algorithm security without either peer detecting it. The severity is MEDIUM rather than HIGH because the ML-KEM component still provides strong security today, and an active attacker who can modify KEX messages can already perform other attacks unless strict KEX is negotiated. ## Suggested Fix Add zero-point checks in `hybrid_mlkem.rs` matching the ones in `curve25519.rs`: ```rust // In server_dh, after line 93: let mut c_pk1 = MontgomeryPoint([0; 32]); c_pk1.0.copy_from_slice(c_pk1_bytes); if c_pk1.0 == [0u8; 32] { return Err(Error::Kex); } // In compute_shared_secret, after line 155: let mut s_pk1 = MontgomeryPoint([0; 32]); s_pk1.0.copy_from_slice(s_pk1_bytes); if s_pk1.0 == [0u8; 32] { return Err(Error::Kex); } ``` Ideally, also validate against the other small-subgroup points on Curve25519 (there are a small number of low-order points that also yield a zero shared secret), matching the comprehensive validation OpenSSH performs. ### AI tooling AI assistancewas used for the code audit and for drafting this report. The finding was manually verified against the project's source at the location cited above before reporting it, and the severity and impact assessment are my own.

Source published

30 Sep 2026 23:26:48 UTC

Source modified

30 Sep 2026 23:30:05 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Publisher rating · MODERATE