---
id: CVE-2026-84897
title: >-
  src/internal.c in wolfSSL wolfSSH through 1.5.0 admits the server-to-client
  Diffie-Hellman group exchange messages SSH_MSG_KEX_DH_GEX_GROUP (31) and
  SSH_MSG_KEX_DH_GEX_REPLY (33) when a server receives them from an
  unauthenticated client…
summary: >-
  src/internal.c in wolfSSL wolfSSH through 1.5.0 admits the server-to-client
  Diffie-Hellman group exchange messages SSH_MSG_KEX_DH_GEX_GROUP (31) and
  SSH_MSG_KEX_DH_GEX_REPLY (33) when a server receives them from an
  unauthenticated client…
severity: medium
cvss: 6.9
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/AU:Y'
cwe:
  - CWE-372
  - CWE-400
  - CWE-405
vendor: wolfSSL Inc.
product: wolfSSH
affected:
  - wolfSSH >= 1.2.0 <= 1.5.0
published: '2026-10-07'
updated: '2026-10-07'
sourceUpdated: '2026-10-07T19:17:44.647'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-84897'
references:
  - url: >-
      https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22
    label: facts@wolfssl.com
  - url: 'https://github.com/wolfSSL/wolfssh/pull/1221'
    label: facts@wolfssl.com
  - url: 'https://www.rfc-editor.org/rfc/rfc3526.html'
    label: facts@wolfssl.com
  - url: 'https://www.rfc-editor.org/rfc/rfc4419.html'
    label: facts@wolfssl.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-07T18:36:45.750446Z'
cvssSource: cna
ingestedAt: '2026-10-07T03:28:55.183Z'
---

## Overview

src/internal.c in wolfSSL wolfSSH through 1.5.0 admits the server-to-client Diffie-Hellman group exchange messages SSH_MSG_KEX_DH_GEX_GROUP (31) and SSH_MSG_KEX_DH_GEX_REPLY (33) when a server receives them from an unauthenticated client. IsMessageAllowedServer() applies no direction check to the key exchange message range: when the peer is keying and no particular message is expected, which is the state a server is in for the whole window after it processes the client's KEXINIT because nothing sets handshake->expectMsgId there, the function falls out of its expectation branch without a verdict and reaches a numeric bound that admits every message id from 30 through 34. A client that negotiates diffie-hellman-group-exchange-sha256 and then sends message 31 makes the server run the client-side handler DoKexDhGexGroup(), which validates the attacker-supplied group with two 8-round Miller-Rabin primality tests, one on p and one on (p-1)/2, on a value of up to 8192 bits. The handler then returns success: the server stores the attacker's prime and generator, generates a Diffie-Hellman key pair in the attacker's group, and sends the client-role message SSH_MSG_KEX_DH_GEX_INIT (32) back to the attacker. Published RFC 3526 safe primes are the worst-case input and cost the attacker nothing to obtain. The primality validation was added in 1.5.0; versions from 1.2.0 through 1.4.22 admit the same message and enter the same client-role path without the primality cost. Message 33 is admitted as well, but on a server it is rejected before any cryptography because no public key check callback is registered, so it carries no comparable cost. Builds that define WOLFSSH_NO_DH_GEX_SHA256, which is implied by WOLFSSH_NO_DH or NO_SHA256, are unaffected.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.