Russh: client wrong-length X25519 clone_from_slice panic (pre-auth DoS)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-73429: 0.51% estimated exploitation activity in the next 30 days; percentile 43.30 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A malicious SSH server can crash a russh client session with a single malformed key-exchange reply, causing a pre-authentication Denial-of-Service before the server host key is verified. The embedding process itself stays up, but the connection is killed deterministically. Details Every other kex path in russh validates the peer ephemeral length before cloning: - Curve25519Kex::serverdh (russh/src/kex/curve25519.rs:61-65) checks if pubkeylen != 32 { return Err(crate::Error::Kex); } before clonefromslice. - The hybrid ML-KEM, ECDH-NIST, and DH/GEX paths all validate lengths. Only the client-side curve25519 computesharedsecret is missing the check. This asymmetric validation gap makes the bug easy to miss in code review: a malicious client cannot panic a russh server this way (the server path checks the length), but a malicious server can panic a russh client. Incriminated source code (repo-relative paths): - Vulnerable computesharedsecret: russh/src/kex/curve25519.rs:110-117…

Published by the source

24 Jul 2026, 16:47 UTC

Last modified by the source

12 Aug 2026, 20:53 UTC

Source last checked

26 Oct 2026, 15:43 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g9hv-x236-4qp3

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# Patch Next advisory brief: CVE-2026-73429

Advisory: Russh: client wrong-length X25519 `clone_from_slice` panic (pre-auth DoS)
IDs: CVE-2026-73429, GHSA-g9hv-x236-4qp3
Record: https://patchnext.com/advisories/cve-2026-73429 (revision 1, source checked 26 Oct 2026, 15:43 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: russh 0.62.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io russh: affected <= 0.62.3; fixed in 0.62.4

## Suggested inspection (read-only)
- crates.io russh: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on russh.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g9hv-x236-4qp3 (GitHub-reviewed advisory GHSA-g9hv-x236-4qp3 lists affected ranges: crates.io russh >=0 <=0.62.3)

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