Vulnerability-Lookup

FKIE_CVE-2026-84897

Vulnerability from fkie_nvd - Published: 2026-10-07 03:17 - Updated: 2026-10-07 19:17

Severity

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

Summary

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.

References

	URL	Tags
	facts@wolfssl.com	https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22
	facts@wolfssl.com	https://github.com/wolfSSL/wolfssh/pull/1221
	facts@wolfssl.com	https://www.rfc-editor.org/rfc/rfc3526.html
	facts@wolfssl.com	https://www.rfc-editor.org/rfc/rfc4419.html

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "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\u0027s 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\u0027s prime and generator, generates a Diffie-Hellman key pair in the attacker\u0027s 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": "2026-10-07T19:17:44.647",
  "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"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-84897",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-10-07T18:36:45.750446Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "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": "Undergoing Analysis",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-372"
        },
        {
          "lang": "en",
          "value": "CWE-400"
        },
        {
          "lang": "en",
          "value": "CWE-405"
        }
      ],
      "source": "facts@wolfssl.com",
      "type": "Secondary"
    }
  ]
}

CVE-2026-84897 (GCVE-0-2026-84897)

Vulnerability from cvelistv5 – Published: 2026-10-07 02:42 – Updated: 2026-10-07 18:37

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

Summary

Severity

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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-10-07 18:36 UTC

CWE

CWE-372 - Incomplete Internal State Distinction
CWE-405 - Asymmetric Resource Consumption (Amplification)
CWE-400 - Uncontrolled Resource Consumption

Assigner

wolfSSL icscert root Scorecard

References

4 references

URL	Tags
https://github.com/wolfSSL/wolfssh/pull/1221	patch
https://github.com/wolfSSL/wolfssh/commit/a472f1e…	patch
https://www.rfc-editor.org/rfc/rfc4419.html	technical-description
https://www.rfc-editor.org/rfc/rfc3526.html	technical-description

Impacted products

1 product

Vendor	Product	Version
wolfSSL Inc.	wolfSSH	Affected: 1.2.0 , ≤ 1.5.0 (semver)

Credits

Abdullah Al Ishtiaq, Kai Tu, Matthew Carter, Xiaotian Zhou, Ananna Rahman, Yilu Dong, Tianwei Yu, Ali Ranjbar, Syed Rafiul Hussain

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-84897",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-10-07T18:36:45.750446Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-10-07T18:37:00.316Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "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"
            }
          ]
        }
      ],
      "credits": [
        {
          "lang": "en",
          "type": "finder",
          "value": "Abdullah Al Ishtiaq, Kai Tu, Matthew Carter, Xiaotian Zhou, Ananna Rahman, Yilu Dong, Tianwei Yu, Ali Ranjbar, Syed Rafiul Hussain"
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "value": "\u003ccode\u003esrc/internal.c\u003c/code\u003e in wolfSSL wolfSSH through 1.5.0 admits the server-to-client Diffie-Hellman group exchange messages \u003ccode\u003eSSH_MSG_KEX_DH_GEX_GROUP\u003c/code\u003e (31) and \u003ccode\u003eSSH_MSG_KEX_DH_GEX_REPLY\u003c/code\u003e (33) when a server receives them from an unauthenticated client. \u003ccode\u003eIsMessageAllowedServer()\u003c/code\u003e 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\u0027s KEXINIT because nothing sets \u003ccode\u003ehandshake-\u0026gt;expectMsgId\u003c/code\u003e 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 \u003ccode\u003ediffie-hellman-group-exchange-sha256\u003c/code\u003e and then sends message 31 makes the server run the client-side handler \u003ccode\u003eDoKexDhGexGroup()\u003c/code\u003e, which validates the attacker-supplied group with two 8-round Miller-Rabin primality tests, one on \u003ccode\u003ep\u003c/code\u003e and one on \u003ccode\u003e(p-1)/2\u003c/code\u003e, on a value of up to 8192 bits. The handler then returns success: the server stores the attacker\u0027s prime and generator, generates a Diffie-Hellman key pair in the attacker\u0027s group, and sends the client-role message \u003ccode\u003eSSH_MSG_KEX_DH_GEX_INIT\u003c/code\u003e (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 \u003ccode\u003eWOLFSSH_NO_DH_GEX_SHA256\u003c/code\u003e, which is implied by \u003ccode\u003eWOLFSSH_NO_DH\u003c/code\u003e or \u003ccode\u003eNO_SHA256\u003c/code\u003e, are unaffected.\u003cbr\u003e"
            }
          ],
          "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\u0027s 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\u0027s prime and generator, generates a Diffie-Hellman key pair in the attacker\u0027s 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."
        }
      ],
      "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"
            }
          ]
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "Automatable": "YES",
            "Recovery": "NOT_DEFINED",
            "Safety": "NOT_DEFINED",
            "attackComplexity": "LOW",
            "attackRequirements": "NONE",
            "attackVector": "NETWORK",
            "baseScore": 6.9,
            "baseSeverity": "MEDIUM",
            "exploitMaturity": "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/AU:Y",
            "version": "4.0",
            "vulnAvailabilityImpact": "LOW",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "NONE",
            "vulnerabilityResponseEffort": "NOT_DEFINED"
          },
          "format": "CVSS",
          "scenarios": [
            {
              "lang": "en",
              "value": "GENERAL"
            }
          ]
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-372",
              "description": "CWE-372 Incomplete Internal State Distinction",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-405",
              "description": "CWE-405 Asymmetric Resource Consumption (Amplification)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-400",
              "description": "CWE-400 Uncontrolled Resource Consumption",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-07T02:42:28.388Z",
        "orgId": "50d2cd11-d01a-48ed-9441-5bfce9d63b27",
        "shortName": "wolfSSL"
      },
      "references": [
        {
          "name": "Fix PR #1221",
          "tags": [
            "patch"
          ],
          "url": "https://github.com/wolfSSL/wolfssh/pull/1221"
        },
        {
          "name": "Fix commit a472f1ee (reject the KEX replies a server never receives)",
          "tags": [
            "patch"
          ],
          "url": "https://github.com/wolfSSL/wolfssh/commit/a472f1ee2b653f623d85ef2297321733f1dbfd22"
        },
        {
          "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/rfc4419.html"
        },
        {
          "name": "RFC 3526 (published MODP safe primes usable as the worst-case input)",
          "tags": [
            "technical-description"
          ],
          "url": "https://www.rfc-editor.org/rfc/rfc3526.html"
        }
      ],
      "source": {
        "discovery": "EXTERNAL"
      },
      "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",
      "workarounds": [
        {
          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "value": "Build wolfSSH with \u003ccode\u003eWOLFSSH_NO_DH_GEX_SHA256\u003c/code\u003e defined so that \u003ccode\u003ediffie-hellman-group-exchange-sha256\u003c/code\u003e is neither offered nor accepted, which removes the message 31 dispatch path entirely. Where group exchange must stay available, lowering \u003ccode\u003eWOLFSSH_DEFAULT_GEXDH_MAX\u003c/code\u003e 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.\u003cbr\u003e"
            }
          ],
          "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."
        }
      ],
      "x_generator": {
        "engine": "Vulnogram 1.0.5"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "50d2cd11-d01a-48ed-9441-5bfce9d63b27",
    "assignerShortName": "wolfSSL",
    "cveId": "CVE-2026-84897",
    "datePublished": "2026-10-07T02:42:28.388Z",
    "dateReserved": "2026-09-02T15:07:44.223Z",
    "dateUpdated": "2026-10-07T18:37:00.316Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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FKIE_CVE-2026-84897

CVE-2026-84897 (GCVE-0-2026-84897)

Tags

Sightings

Nomenclature

Related by attack behaviour