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CVE-2026-44040

Estado: ModificadaMedia (6.5)—

UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID).

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An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.

CVSS

Probabilidad de explotación (EPSS)

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

Tecnologías afectadas (1)

Uvnc — Ultravnc

CWE

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-44040",
  "cveTags": [],
  "metrics": {
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-44040",
          "role": "CISA Coordinator",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "version": "2.0.3",
          "timestamp": "2026-07-01T12:44:56.012420Z"
        }
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
        "cvssData": {
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          "attackVector": "NETWORK",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "integrityImpact": "LOW",
          "userInteraction": "NONE",
          "attackComplexity": "HIGH",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "LOW"
        },
        "impactScore": 2.5,
        "exploitabilityScore": 2.2
      },
      {
        "type": "Primary",
        "source": "nvd@nist.gov",
        "cvssData": {
          "scope": "UNCHANGED",
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          "attackVector": "NETWORK",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "integrityImpact": "LOW",
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            "vncviewer"
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          "product": "UltraVNC",
          "versions": [
            {
              "status": "affected",
              "version": "0",
              "versionType": "custom",
              "lessThanOrEqual": "1.8.2.2"
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          "defaultStatus": "unaffected"
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  ],
  "published": "2026-07-01T05:16:20.897",
  "references": [
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      "url": "https://github.com/ultravnc/UltraVNC",
      "tags": [
        "Product"
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      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"
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    {
      "url": "https://uvnc.com/",
      "tags": [
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        "Release Notes"
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      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"
    },
    {
      "url": "https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc",
      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"
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  ],
  "vulnStatus": "Modified",
  "weaknesses": [
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      "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
      "description": [
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          "value": "CWE-338"
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  "descriptions": [
    {
      "lang": "en",
      "value": "UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation."
    }
  ],
  "lastModified": "2026-07-09T06:16:20.777",
  "configurations": [
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              "versionEndIncluding": "1.8.2.2"
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          "operator": "OR"
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  "sourceIdentifier": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"
}