{"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": [{"type": "text/html", "base64": false, "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.", "supportingMedia": [{"type": "text/html", "base64": false, "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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{"affected": [{"affectedData": [{"defaultStatus": "unaffected", "modules": ["key exchange"], "product": "wolfSSH", "programFiles": ["src/internal.c"], "programRoutines": [{"name": "IsMessageAllowedServer"}, {"name": "DoKexDhGexGroup"}, {"name": "ValidateKexDhGexGroup"}], "repo": "https://github.com/wolfSSL/wolfssh", "vendor": "wolfSSL Inc.", "versions": [{"changes": [{"at": "1.6.0", "status": "unaffected"}], "lessThanOrEqual": "1.5.0", "status": "affected", "version": "1.2.0", "versionType": "semver"}]}], "source": "facts@wolfssl.com"}], "cveTags": [], "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."}], "id": "CVE-2026-84897", "lastModified": "2026-10-07T03:17:00.067", "metrics": {"cvssMetricV40": [{"cvssData": {"Automatable": "YES", "Recovery": "NOT_DEFINED", "Safety": "NOT_DEFINED", "attackComplexity": "LOW", "attackRequirements": "NONE", "attackVector": "NETWORK", "availabilityRequirement": "NOT_DEFINED", "baseScore": 6.9, "baseSeverity": "MEDIUM", "confidentialityRequirement": "NOT_DEFINED", "exploitMaturity": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "privilegesRequired": "NONE", "providerUrgency": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "userInteraction": "NONE", "valueDensity": "NOT_DEFINED", "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/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", "version": "4.0", "vulnAvailabilityImpact": "LOW", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnerabilityResponseEffort": "NOT_DEFINED"}, "source": "facts@wolfssl.com", "type": "Secondary"}]}, "published": "2026-10-07T03:17:00.067", "references": [{"source": "facts@wolfssl.com", "url": "https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22"}, {"source": "facts@wolfssl.com", "url": "https://github.com/wolfSSL/wolfssh/pull/1221"}, {"source": "facts@wolfssl.com", "url": "https://www.rfc-editor.org/rfc/rfc3526.html"}, {"source": "facts@wolfssl.com", "url": "https://www.rfc-editor.org/rfc/rfc4419.html"}], "sourceIdentifier": "facts@wolfssl.com", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-372"}, {"lang": "en", "value": "CWE-400"}, {"lang": "en", "value": "CWE-405"}], "source": "facts@wolfssl.com", "type": "Secondary"}]}

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{}