Summary
Russh: Missing X25519 zero-point validation in hybrid ML-KEM key exchange
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):
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):
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_dhline 77:if client_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }compute_shared_secretline 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.
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.
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:
- 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.
- 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.
The application uses a cryptographic algorithm known to have weaknesses, such as MD5, SHA-1, or DES. Typical impact: compromised confidentiality or integrity of protected data.
CVE-2026-102824 has a CVSS score of 4.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.63.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Add zero-point checks in hybrid_mlkem.rs matching the ones in curve25519.rs:
// 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.
Frequently Asked Questions
- What is CVE-2026-102824? CVE-2026-102824 is a medium-severity use of a broken or risky cryptographic algorithm vulnerability in russh (rust), affecting versions < 0.63.0. It is fixed in 0.63.0. The application uses a cryptographic algorithm known to have weaknesses, such as MD5, SHA-1, or DES.
- How severe is CVE-2026-102824? CVE-2026-102824 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of russh are affected by CVE-2026-102824? russh (rust) versions < 0.63.0 is affected.
- Is there a fix for CVE-2026-102824? Yes. CVE-2026-102824 is fixed in 0.63.0. Upgrade to this version or later.
- Is CVE-2026-102824 exploitable, and should I be worried? Whether CVE-2026-102824 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-102824 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-102824? Upgrade
russhto 0.63.0 or later.