{"id":"CVE-2026-84897","summary":"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","details":"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-\u003eexpectMsgId 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.","modified":"2026-10-09T07:06:05.861053280Z","published":"2026-10-07T02:42:28.388Z","database_specific":{"cna_assigner":"wolfSSL","cwe_ids":["CWE-372","CWE-400","CWE-405"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84897.json","unresolved_ranges":[{"source":"AFFECTED_FIELD","extracted_events":[{"introduced":"1.2.0"},{"fixed":"1.6.0"}]},{"extracted_events":[{"introduced":"30"},{"fixed":"34"}],"source":"DESCRIPTION"}]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84897.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-84897"},{"type":"FIX","url":"https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22"},{"type":"FIX","url":"https://github.com/wolfSSL/wolfssh/pull/1221"},{"type":"PACKAGE","url":"https://github.com/wolfSSL/wolfssh"},{"type":"ARTICLE","url":"https://www.rfc-editor.org/rfc/rfc3526.html"},{"type":"ARTICLE","url":"https://www.rfc-editor.org/rfc/rfc4419.html"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/wolfssl/wolfssh","events":[{"introduced":"0"},{"fixed":"8643d7be841184f766374e3b0ed68ced6391543c"},{"introduced":"815d2919d5785a89714e91b49c84b1fcd8bb4235"},{"fixed":"7d4829843625eb7f59216136a9f21e1f986a8c2e"},{"fixed":"a472f1ee2b653f623d85ef2297321733f1dbfd22"}],"database_specific":{"source":["DESCRIPTION","REFERENCES"],"extracted_events":[{"introduced":"0"},{"fixed":"1.5.0"},{"introduced":"1.2.0"},{"fixed":"1.4.22"}]}}],"versions":["v1.4.21-stable","v1.4.20-stable","v1.4.19-stable","v1.4.18-stable","v1.4.17-stable","v1.4.15-stable","v1.4.16","v1.4.14-stable","v1.4.13-stable","v1.4.12-stable","v1.4.11-stable","v1.4.10-stable","v1.4.9","v1.4.8-stable","v1.4.7-stable","v1.4.6-stable","v1.4.5-stable","v1.4.4-stable","v1.4.3-stable","v1.4.2-stable","v1.4.0-stable","v1.3.0-stable","v1.2.2","v1.2.0-stable"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-84897.json","vanir_signatures":[{"signature_type":"Function","signature_version":"v1","source":"https://github.com/wolfssl/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22","target":{"file":"src/internal.c","function":"IsMessageAllowedServer"},"deprecated":false,"digest":{"function_hash":"332597429388241481065802417261341409358","length":2367},"id":"CVE-2026-84897-00ddc3b5"},{"target":{"file":"tests/regress.c","function":"main"},"deprecated":false,"digest":{"length":5650,"function_hash":"128697116366816681178397962724568946588"},"id":"CVE-2026-84897-474b24ff","signature_type":"Function","signature_version":"v1","source":"https://github.com/wolfssl/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22"},{"id":"CVE-2026-84897-63bf690c","signature_type":"Line","signature_version":"v1","source":"https://github.com/wolfssl/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22","target":{"file":"tests/regress.c"},"deprecated":false,"digest":{"line_hashes":["118513238373630913010649926792335052979","249562867172711739799557390989647643018","325921520442702441106489839726675749863","76289822188778458399863853977056367152","52564300302719386044077990138923678774","321929433519072388629500615019388727461","94056652126704302590815800423611593456"],"threshold":0.9}},{"signature_type":"Line","signature_version":"v1","source":"https://github.com/wolfssl/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22","target":{"file":"src/internal.c"},"deprecated":false,"digest":{"line_hashes":["185246754689052513008561199853287132380","137609406753648681267639826552501788982","149705089427072683951731583736537310304","150271123820429758589687398548444471965"],"threshold":0.9},"id":"CVE-2026-84897-ea0e1f8e"}],"vanir_signatures_modified":"2026-10-09T07:06:05Z"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}