Vulnerability-Lookup

CVE-2026-102824 (GCVE-0-2026-102824)

Vulnerability from cvelistv5 – Published: 2026-09-29 18:27 – Updated: 2026-09-30 20:24

Title

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

Summary

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.

Severity

4.3 (Medium)


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

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-09-30 20:24 UTC

CWE

CWE-327 - Use of a Broken or Risky Cryptographic Algorithm

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7838 records in the last 180 days details

References

3 references

URL	Tags
https://github.com/Eugeny/russh/security/advisori…	x_refsource_CONFIRM
https://github.com/Eugeny/russh/commit/8da8967f19…	x_refsource_MISC
https://github.com/Eugeny/russh/releases/tag/v0.63.0	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
Eugeny	russh	Affected: < 0.63.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-102824

Vulnerability from fkie_nvd - Published: 2026-09-29 19:17 - Updated: 2026-09-30 21:17

Severity

4.3 (Medium) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/Eugeny/russh/commit/8da8967f196472576b1565d518a0ed60fce60f0c
	security-advisories@github.com	https://github.com/Eugeny/russh/releases/tag/v0.63.0
	security-advisories@github.com	https://github.com/Eugeny/russh/security/advisories/GHSA-w3jg-pjxf-73p4

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "russh",
          "vendor": "Eugeny",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.63.0"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
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    {
      "lang": "en",
      "value": "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\u0027s intended fallback protection if ML-KEM is later weakened. This issue is fixed in version 0.63.0."
    }
  ],
  "id": "CVE-2026-102824",
  "lastModified": "2026-09-30T21:17:05.920",
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GHSA-W3JG-PJXF-73P4

Vulnerability from github – Published: 2026-09-30 23:26 – Updated: 2026-09-30 23:26

Summary

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

Details

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_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:

// 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.

Severity

4.3 (Medium)


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

Show details on source website

JSON

To clipboard

{
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      "package": {
        "ecosystem": "crates.io",
        "name": "russh"
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      "ranges": [
        {
          "events": [
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          "type": "ECOSYSTEM"
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    }
  ],
  "aliases": [
    "CVE-2026-102824"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-327"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-30T23:26:48Z",
    "nvd_published_at": "2026-09-29T19:17:24Z",
    "severity": "MODERATE"
  },
  "details": "## Vulnerability\n\nThe hybrid ML-KEM 768 + X25519 key exchange implementation in `russh/src/kex/hybrid_mlkem.rs` does not validate that the remote peer\u0027s 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.\n\nAffected code at HEAD (v0.62.4, commit 0089c89):\n\n**Server-side** (`server_dh`, lines 92-93):\n```rust\nlet mut c_pk1 = MontgomeryPoint([0; 32]);\nc_pk1.0.copy_from_slice(c_pk1_bytes);\n// No zero-point check - proceeds to: let k_cl = s_secret * c_pk1;\n```\n\n**Client-side** (`compute_shared_secret`, lines 154-155):\n```rust\nlet mut s_pk1 = MontgomeryPoint([0; 32]);\ns_pk1.0.copy_from_slice(s_pk1_bytes);\n// No zero-point check - proceeds to: let k_cl = x25519_secret * s_pk1;\n```\n\n## Root Cause\n\nCommit 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:\n- `server_dh` line 77: `if client_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }`\n- `compute_shared_secret` line 122: `if remote_pubkey.0 == [0u8; 32] { return Err(crate::Error::Kex); }`\n\nThe same X25519 scalar multiplication pattern appears in `hybrid_mlkem.rs`, but the fix was not applied there.\n\n## Proof of Concept\n\nA 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.\n\nWhen the server computes `k_cl = s_secret * c_pk1`, the result is the zero point regardless of the server\u0027s 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.\n\nThe 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.\n\n## Impact\n\nThe 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.\n\nThis matters in two scenarios:\n1. 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.\n2. An active network attacker who can intercept KEX could downgrade the hybrid exchange to effectively single-algorithm security without either peer detecting it.\n\nThe 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.\n\n## Suggested Fix\n\nAdd zero-point checks in `hybrid_mlkem.rs` matching the ones in `curve25519.rs`:\n\n```rust\n// In server_dh, after line 93:\nlet mut c_pk1 = MontgomeryPoint([0; 32]);\nc_pk1.0.copy_from_slice(c_pk1_bytes);\nif c_pk1.0 == [0u8; 32] {\n    return Err(Error::Kex);\n}\n\n// In compute_shared_secret, after line 155:\nlet mut s_pk1 = MontgomeryPoint([0; 32]);\ns_pk1.0.copy_from_slice(s_pk1_bytes);\nif s_pk1.0 == [0u8; 32] {\n    return Err(Error::Kex);\n}\n```\n\nIdeally, 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.\n\n### AI tooling\n\nAI assistancewas used for the code audit and for drafting this report. The finding was manually verified against the project\u0027s source at the location cited above before reporting it, and the severity and impact assessment are my own.",
  "id": "GHSA-w3jg-pjxf-73p4",
  "modified": "2026-09-30T23:26:48Z",
  "published": "2026-09-30T23:26:48Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-w3jg-pjxf-73p4"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-102824"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/commit/8da8967f196472576b1565d518a0ed60fce60f0c"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Eugeny/russh"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Eugeny/russh/releases/tag/v0.63.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Russh: Missing X25519 zero-point validation in hybrid ML-KEM key exchange"
}

RHSA-2026:74586

Vulnerability from csaf_redhat - Published: 2026-10-01 17:20 - Updated: 2026-10-02 10:21

Summary

Red Hat Security Advisory: Red Hat Hardened Images RPMs Security Update

Severity

Important

Scope

4 vulnerabilities in this advisory, including CVE-2026-102824.

CVE-2026-102824

A flaw was found in russh. The library fails to validate peer public keys during hybrid cryptographic key exchange, accepting an all-zero key. A remote attacker acting as a malicious Secure Shell (SSH) peer can exploit this flaw during connection negotiation to force the classical encryption component of the shared secret to zero. This causes the connection to rely entirely on post-quantum algorithms, undermining the intended fallback protection of the hybrid exchange and weakening session security.

4.3 (Medium)


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

Fixed: 7 products

https://access.redhat.com/errata/RHSA-2026:74586

Open the full advisory

UBUNTU-CVE-2026-102824 (CVE-2026-102824)

Vulnerability from osv_ubuntu – Published: 2026-09-29 19:17 – Updated: 2026-09-29 19:17 – Source website

Details

Severity

4.3 (Medium)


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

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-102824	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-102824	REPORT
	https://github.com/Eugeny/russh/security/advisori…	REPORT
	https://github.com/Eugeny/russh/commit/8da8967f19…	REPORT
	https://github.com/Eugeny/russh/releases/tag/v0.63.0	REPORT

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "librust-russh-dev",
            "binary_version": "0.57.1-2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:26.10",
        "name": "rust-russh",
        "purl": "pkg:deb/ubuntu/rust-russh@0.57.1-2?arch=source\u0026distro=stonking"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.57.1-2"
      ]
    }
  ],
  "aliases": [],
  "details": "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\u0027s intended fallback protection if ML-KEM is later weakened. This issue is fixed in version 0.63.0.",
  "id": "UBUNTU-CVE-2026-102824",
  "modified": "2026-09-29T19:17:00Z",
  "published": "2026-09-29T19:17:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-102824"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-102824"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/Eugeny/russh/security/advisories/GHSA-w3jg-pjxf-73p4"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/Eugeny/russh/commit/8da8967f196472576b1565d518a0ed60fce60f0c"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/Eugeny/russh/releases/tag/v0.63.0"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-102824"
  ]
}

Sightings

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Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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Action not permitted

CVE-2026-102824 (GCVE-0-2026-102824)

FKIE_CVE-2026-102824

GHSA-W3JG-PJXF-73P4

Vulnerability

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AI tooling

RHSA-2026:74586

UBUNTU-CVE-2026-102824 (CVE-2026-102824)

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