{"id":"CVE-2026-84897","revision":"1e19ebe81831435f9b5e09c7a03e6a10","records":[{"revision":"1e19ebe81831435f9b5e09c7a03e6a10","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-84897","aliases":[],"title":"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","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.","state":0,"publishedAt":"2026-10-07T02:42:28.388+00:00","modifiedAt":"2026-10-07T18:37:00.316+00:00","affected":[{"ecosystem":"Vendor product","name":"wolfSSH","ranges":[],"versions":[],"rawVersionEvidence":"{\n                    \"vendor\": \"wolfSSL Inc.\",\n                    \"product\": \"wolfSSH\",\n                    \"repo\": \"https://github.com/wolfSSL/wolfssh\",\n                    \"modules\": [\n                        \"key exchange\"\n                    ],\n                    \"programFiles\": [\n                        \"src/internal.c\"\n                    ],\n                    \"programRoutines\": [\n                        {\n                            \"name\": \"IsMessageAllowedServer\"\n                        },\n                        {\n                            \"name\": \"DoKexDhGexGroup\"\n                        },\n                        {\n                            \"name\": \"ValidateKexDhGexGroup\"\n                        }\n                    ],\n                    \"versions\": [\n                        {\n                            \"status\": \"affected\",\n                            \"version\": \"1.2.0\",\n                            \"lessThanOrEqual\": \"1.5.0\",\n                            \"changes\": [\n                                {\n                                    \"at\": \"1.6.0\",\n                                    \"status\": \"unaffected\"\n                                }\n                            ],\n                            \"versionType\": \"semver\"\n                        }\n                    ],\n                    \"defaultStatus\": \"unaffected\"\n                }"}],"metrics":[{"system":"CVSS 4.0","vector":"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","score":6.9,"severity":"MEDIUM"}],"references":["https://github.com/wolfSSL/wolfssh/pull/1221","https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22","https://www.rfc-editor.org/rfc/rfc4419.html","https://www.rfc-editor.org/rfc/rfc3526.html"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-84897","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-84897","identifiers":["CVE-2026-84897"]},"retained":false,"historicalAffected":[]}],"coverage":[]}