russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler

Exploitation

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

CVSS

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

EPSS

• CVE-2026-42189: 0.48% estimated exploitation activity in the next 30 days; percentile 41.53 (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 pre-authentication denial-of-service vulnerability exists in the server's keyboard-interactive authentication handler. A malicious client can crash any russh-based server that implements keyboard-interactive auth (e.g., for 2FA/TOTP) with a single malformed packet, requiring no credentials. Vulnerability Details In russh/src/server/encrypted.rs, the function readuserauthinforesponse decodes a u32 count from the client's SSHMSGUSERAUTHINFORESPONSE and passes it directly to Vec::withcapacity(): An attacker can send n = 0x10000000 (268M) or larger in a minimal packet (~50 bytes after encryption). The server attempts to allocate n ~24 bytes (size of Option<Bytes) = ~6.4GB, causing an OOM crash. Attack Flow 1. Attacker connects via TCP, completes key exchange (no credentials needed -- this is the anonymous DH handshake, not authentication) 2. Sends USERAUTHREQUEST with method keyboard-interactive 3. Server handler returns Auth::Partial with prompts (standard for 2FA/TOTP) 4.…

Published by the source

24 Apr 2026, 15:39 UTC

Last modified by the source

13 May 2026, 13:34 UTC

Source last checked

26 Oct 2026, 15:42 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-f5v4-2wr6-hqmg

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# Patch Next advisory brief: CVE-2026-42189

Advisory: russh has pre-auth DoS via unbounded allocation in its keyboard-interactive auth handler
IDs: CVE-2026-42189, GHSA-f5v4-2wr6-hqmg
Record: https://patchnext.com/advisories/cve-2026-42189 (revision 1, source checked 26 Oct 2026, 15:42 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: russh 0.60.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io russh: affected < 0.60.1; fixed in 0.60.1

## 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-f5v4-2wr6-hqmg (GitHub-reviewed advisory GHSA-f5v4-2wr6-hqmg lists affected ranges: crates.io russh >=0 <0.60.1)

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