CVE-2026-7830

{
  "id": "CVE-2026-7830",
  "sourceIdentifier": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
  "published": "2026-07-01T05:16:24.210",
  "lastModified": "2026-10-25T17:10:00.197",
  "vulnStatus": "Modified",
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "UltraVNC through 1.8.2.2 uses inadequate cryptography in the MS-Logon II authentication scheme (rfbUltraVNC_MsLogonIIAuth). In rfb/dh.cpp the Diffie-Hellman key exchange is performed with parameters that fit in an unsigned 64-bit integer (DH_MAX_BITS controls the prime size). A 64-bit DH key can be broken by Pollard's rho algorithm in under one second on current hardware. Additionally, the private exponent is generated by the rng() function, which multiplies three libc rand() values seeded from time(NULL). With approximately 31 bits of internal state and a time-based seed, the private exponent is recoverable in under a minute by a passive observer. A network attacker who can observe the MS-Logon II handshake (via sniffing, recording, or man-in-the-middle) can derive the shared DH key and decrypt the encapsulated username and password, resulting in full credential disclosure. This affects legacy MS-Logon II connections; MS-Logon III (X25519 + AES-256-GCM) is unaffected."
    },
    {
      "lang": "es",
      "value": "UltraVNC hasta la versión 1.8.2.2 utiliza criptografía inadecuada en el esquema de autenticación MS-Logon II (rfbUltraVNC_MsLogonIIAuth). En rfb/dh.cpp, el intercambio de claves Diffie-Hellman se realiza con parámetros que caben en un entero sin signo de 64 bits (DH_MAX_BITS controla el tamaño del número primo). Una clave DH de 64 bits puede ser rota por el algoritmo rho de Pollard en menos de un segundo en hardware actual. Además, el exponente privado es generado por la función rng(), que multiplica tres valores de libc rand() sembrados desde time(NULL). Con aproximadamente 31 bits de estado interno y una semilla basada en el tiempo, el exponente privado es recuperable en menos de un minuto por un observador pasivo. Un atacante de red que puede observar el handshake de MS-Logon II (mediante sniffing, grabación o man-in-the-middle) puede derivar la clave DH compartida y descifrar el nombre de usuario y la contraseña encapsulados, lo que resulta en la divulgación completa de credenciales. Esto afecta a las conexiones MS-Logon II heredadas; MS-Logon III (X25519 + AES-256-GCM) no se ve afectado."
    }
  ],
  "affected": [
    {
      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
      "affectedData": [
        {
          "vendor": "uvnc",
          "product": "UltraVNC",
          "defaultStatus": "unaffected",
          "modules": [
            "winvnc",
            "ms-logon"
          ],
          "versions": [
            {
              "version": "0",
              "lessThanOrEqual": "1.8.2.2",
              "versionType": "custom",
              "status": "affected"
            }
          ]
        }
      ]
    }
  ],
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        "type": "Secondary",
        "cvssData": {
          "version": "3.1",
          "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
          "baseScore": 7.4,
          "baseSeverity": "HIGH",
          "attackVector": "NETWORK",
          "attackComplexity": "HIGH",
          "privilegesRequired": "NONE",
          "userInteraction": "NONE",
          "scope": "UNCHANGED",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "availabilityImpact": "NONE"
        },
        "exploitabilityScore": 2.2,
        "impactScore": 5.2
      }
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    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "timestamp": "2026-07-01T13:13:10.627838Z",
          "id": "CVE-2026-7830",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "total"
            }
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          "role": "CISA Coordinator",
          "version": "2.0.3"
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  "weaknesses": [
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      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
      "type": "Secondary",
      "description": [
        {
          "lang": "en",
          "value": "CWE-326"
        },
        {
          "lang": "en",
          "value": "CWE-338"
        }
      ]
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          "cpeMatch": [
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  "references": [
    {
      "url": "https://github.com/ultravnc/UltraVNC",
      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
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      "tags": [
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        "Release Notes"
      ]
    },
    {
      "url": "https://www.securin.io/zero-days/cve-2026-7830-31-bit-dh-weak-rng-ms-logon-credential-interception-ultravnc",
      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"
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}