{
    "dataType": "CVE_RECORD",
    "dataVersion": "5.2",
    "cveMetadata": {
        "cveId": "CVE-2026-84897",
        "assignerOrgId": "50d2cd11-d01a-48ed-9441-5bfce9d63b27",
        "state": "PUBLISHED",
        "assignerShortName": "wolfSSL",
        "dateReserved": "2026-09-02T15:07:44.223Z",
        "datePublished": "2026-10-07T02:42:28.388Z",
        "dateUpdated": "2026-10-07T02:42:28.388Z"
    },
    "containers": {
        "cna": {
            "providerMetadata": {
                "orgId": "50d2cd11-d01a-48ed-9441-5bfce9d63b27",
                "shortName": "wolfSSL",
                "dateUpdated": "2026-10-07T02:42:28.388Z"
            },
            "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",
            "problemTypes": [
                {
                    "descriptions": [
                        {
                            "lang": "en",
                            "cweId": "CWE-372",
                            "description": "CWE-372 Incomplete Internal State Distinction",
                            "type": "CWE"
                        }
                    ]
                },
                {
                    "descriptions": [
                        {
                            "lang": "en",
                            "cweId": "CWE-405",
                            "description": "CWE-405 Asymmetric Resource Consumption (Amplification)",
                            "type": "CWE"
                        }
                    ]
                },
                {
                    "descriptions": [
                        {
                            "lang": "en",
                            "cweId": "CWE-400",
                            "description": "CWE-400 Uncontrolled Resource Consumption",
                            "type": "CWE"
                        }
                    ]
                }
            ],
            "impacts": [
                {
                    "descriptions": [
                        {
                            "lang": "en",
                            "value": "Pre-authentication CPU exhaustion with a high work-per-byte ratio, plus key exchange role confusion. One unauthenticated packet of roughly 1 KB costs a wolfSSH server about 0.48 seconds of single-core CPU when it carries the 4096-bit RFC 3526 safe prime, and about 5.8 seconds when it carries the 8192-bit one, measured on a 64-bit desktop core; a single-threaded or embedded server is unavailable for that whole interval, and the cost repeats on every connection. The server then stores the attacker-chosen group, generates a Diffie-Hellman key pair in it, and sends the client-role message SSH_MSG_KEX_DH_GEX_INIT back to the attacker, so the key exchange continues in the wrong role until it fails. Reaching the 8192-bit case requires a wolfSSL math configuration that can represent an 8192-bit integer; a default build tops out near 4096 bits and caps the burn at roughly half a second."
                        }
                    ]
                },
                {
                    "capecId": "CAPEC-227",
                    "descriptions": [
                        {
                            "lang": "en",
                            "value": "CAPEC-227 Sustained Client Engagement"
                        }
                    ]
                }
            ],
            "affected": [
                {
                    "vendor": "wolfSSL Inc.",
                    "product": "wolfSSH",
                    "repo": "https://github.com/wolfSSL/wolfssh",
                    "modules": [
                        "key exchange"
                    ],
                    "programFiles": [
                        "src/internal.c"
                    ],
                    "programRoutines": [
                        {
                            "name": "IsMessageAllowedServer"
                        },
                        {
                            "name": "DoKexDhGexGroup"
                        },
                        {
                            "name": "ValidateKexDhGexGroup"
                        }
                    ],
                    "versions": [
                        {
                            "status": "affected",
                            "version": "1.2.0",
                            "lessThanOrEqual": "1.5.0",
                            "changes": [
                                {
                                    "at": "1.6.0",
                                    "status": "unaffected"
                                }
                            ],
                            "versionType": "semver"
                        }
                    ],
                    "defaultStatus": "unaffected"
                }
            ],
            "descriptions": [
                {
                    "lang": "en",
                    "value": "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.",
                    "supportingMedia": [
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                            "value": "<code>src/internal.c</code> in wolfSSL wolfSSH through 1.5.0 admits the server-to-client Diffie-Hellman group exchange messages <code>SSH_MSG_KEX_DH_GEX_GROUP</code> (31) and <code>SSH_MSG_KEX_DH_GEX_REPLY</code> (33) when a server receives them from an unauthenticated client. <code>IsMessageAllowedServer()</code> 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 <code>handshake-&gt;expectMsgId</code> 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 <code>diffie-hellman-group-exchange-sha256</code> and then sends message 31 makes the server run the client-side handler <code>DoKexDhGexGroup()</code>, which validates the attacker-supplied group with two 8-round Miller-Rabin primality tests, one on <code>p</code> and one on <code>(p-1)/2</code>, 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 <code>SSH_MSG_KEX_DH_GEX_INIT</code> (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 <code>WOLFSSH_NO_DH_GEX_SHA256</code>, which is implied by <code>WOLFSSH_NO_DH</code> or <code>NO_SHA256</code>, are unaffected.<br>"
                        }
                    ]
                }
            ],
            "references": [
                {
                    "url": "https://github.com/wolfSSL/wolfssh/pull/1221",
                    "name": "Fix PR #1221",
                    "tags": [
                        "patch"
                    ]
                },
                {
                    "url": "https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22",
                    "name": "Fix commit a472f1ee (reject the KEX replies a server never receives)",
                    "tags": [
                        "patch"
                    ]
                },
                {
                    "url": "https://www.rfc-editor.org/rfc/rfc4419.html",
                    "name": "RFC 4419 sections 3 and 5 (SSH_MSG_KEX_DH_GEX_GROUP and SSH_MSG_KEX_DH_GEX_REPLY are sent by the server)",
                    "tags": [
                        "technical-description"
                    ]
                },
                {
                    "url": "https://www.rfc-editor.org/rfc/rfc3526.html",
                    "name": "RFC 3526 (published MODP safe primes usable as the worst-case input)",
                    "tags": [
                        "technical-description"
                    ]
                }
            ],
            "metrics": [
                {
                    "format": "CVSS",
                    "scenarios": [
                        {
                            "lang": "en",
                            "value": "GENERAL"
                        }
                    ],
                    "cvssV4_0": {
                        "attackVector": "NETWORK",
                        "attackComplexity": "LOW",
                        "attackRequirements": "NONE",
                        "privilegesRequired": "NONE",
                        "userInteraction": "NONE",
                        "vulnConfidentialityImpact": "NONE",
                        "subConfidentialityImpact": "NONE",
                        "vulnIntegrityImpact": "NONE",
                        "subIntegrityImpact": "NONE",
                        "vulnAvailabilityImpact": "LOW",
                        "subAvailabilityImpact": "NONE",
                        "exploitMaturity": "NOT_DEFINED",
                        "Safety": "NOT_DEFINED",
                        "Automatable": "YES",
                        "Recovery": "NOT_DEFINED",
                        "valueDensity": "NOT_DEFINED",
                        "vulnerabilityResponseEffort": "NOT_DEFINED",
                        "providerUrgency": "NOT_DEFINED",
                        "version": "4.0",
                        "baseSeverity": "MEDIUM",
                        "baseScore": 6.9,
                        "vectorString": "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"
                    }
                }
            ],
            "workarounds": [
                {
                    "lang": "en",
                    "value": "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.",
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                            "value": "Build wolfSSH with <code>WOLFSSH_NO_DH_GEX_SHA256</code> defined so that <code>diffie-hellman-group-exchange-sha256</code> is neither offered nor accepted, which removes the message 31 dispatch path entirely. Where group exchange must stay available, lowering <code>WOLFSSH_DEFAULT_GEXDH_MAX</code> 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.<br>"
                        }
                    ]
                }
            ],
            "credits": [
                {
                    "lang": "en",
                    "value": "Abdullah Al Ishtiaq, Kai Tu, Matthew Carter, Xiaotian Zhou, Ananna Rahman, Yilu Dong, Tianwei Yu, Ali Ranjbar, Syed Rafiul Hussain",
                    "type": "finder"
                }
            ],
            "source": {
                "discovery": "EXTERNAL"
            },
            "x_generator": {
                "engine": "Vulnogram 1.0.5"
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    }
}