wolfSSH server accepts server-to-client DH group exchange messages from an unauthenticated client, causing pre-authentication primality-test CPU exhaustion and key exchange role confusion
Summary
| CVE | CVE-2026-84897 |
|---|---|
| State | PUBLISHED |
| Assigner | wolfSSL |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-10-07 03:17:00 UTC |
| Updated | 2026-10-13 14:27:11 UTC |
| Description | 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. |
Risk And Classification
Primary CVSS: v4.0 6.9 MEDIUM from [email protected]
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:Y/R:X/V:X/RE:X/U:X
EPSS: 0.003010000 probability, percentile 0.209390000 (date 2026-10-07)
Problem Types: CWE-372 | CWE-400 | CWE-405 | CWE-372 CWE-372 Incomplete Internal State Distinction | CWE-405 CWE-405 Asymmetric Resource Consumption (Amplification) | CWE-400 CWE-400 Uncontrolled Resource Consumption
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 4.0 | [email protected] | Secondary | 6.9 | MEDIUM | 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/E:X/C... |
| 4.0 | CNA | CVSS | 6.9 | MEDIUM | 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 |
| 3.1 | [email protected] | Primary | 5.3 | MEDIUM | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L |
CVSS v4.0 Breakdown
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:Y/R:X/V:X/RE:X/U:X
CVSS v3.1 Breakdown
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
NVD Known Affected Configurations (CPE 2.3)
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | WolfSSL Inc. | WolfSSH | affected 1.2.0 1.5.0 semver | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| www.rfc-editor.org/rfc/rfc3526.html | [email protected] | www.rfc-editor.org | Technical Description |
| github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22 | [email protected] | github.com | Patch |
| github.com/wolfSSL/wolfssh/pull/1221 | [email protected] | github.com | Patch |
| www.rfc-editor.org/rfc/rfc4419.html | [email protected] | www.rfc-editor.org | Technical Description |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
Vendor Comments And Credit
Discovery Credit
CNA: Abdullah Al Ishtiaq, Kai Tu, Matthew Carter, Xiaotian Zhou, Ananna Rahman, Yilu Dong, Tianwei Yu, Ali Ranjbar, Syed Rafiul Hussain (en)
Additional Advisory Data
Workarounds
CNA: Build wolfSSH with WOLFSSH_NO_DH_GEX_SHA256 defined so that diffie-hellman-group-exchange-sha256 is neither offered nor accepted, which removes the message 31 dispatch path entirely. Where group exchange must stay available, lowering WOLFSSH_DEFAULT_GEXDH_MAX reduces the size of the value a peer can submit for primality testing and so the cost of a single packet, but it does not stop a server from accepting the message.