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CVE-2026-35209

Estado: AnalizadaAlta (7.5)—

defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution. A crafted payload containing a `__proto__` key can override intended default values in the merged resul.

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The internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result. Version 6.1.5 replaces `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter.

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).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

Vulnerabilidad en biblioteca de código abierto accesible remotamente sin autenticación (AV:N/PR:N) que permite modificar propiedades a través de prototype pollution, afectando integridad de datos e identidad en aplicaciones web.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Privileged Account ManagementApplication Developer GuidanceActive Directory ConfigurationVulnerability ScanningUser TrainingUser Account Management

Tecnologías afectadas (1)

Unjs — Defu

CWE

Referencias

JSON original (NVD)

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  "id": "CVE-2026-35209",
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          "id": "CVE-2026-35209",
          "role": "CISA Coordinator",
          "options": [
            {
              "exploitation": "none"
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            {
              "automatable": "yes"
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            {
              "technicalImpact": "partial"
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          ],
          "version": "2.0.3",
          "timestamp": "2026-04-06T18:49:21.516737Z"
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          "attackVector": "NETWORK",
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          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
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          "privilegesRequired": "NONE",
          "confidentialityImpact": "NONE"
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      "value": "defu is software that allows uers to assign default properties recursively. Prior to version 6.1.5, applications that pass unsanitized user input (e.g. parsed JSON request bodies, database records, or config files from untrusted sources) as the first argument to `defu()` are vulnerable to prototype pollution. A crafted payload containing a `__proto__` key can override intended default values in the merged resul. The internal `_defu` function used `Object.assign({}, defaults)` to copy the defaults object. `Object.assign` invokes the `__proto__` setter, which replaces the resulting object's `[[Prototype]]` with attacker-controlled values. Properties inherited from the polluted prototype then bypass the existing `__proto__` key guard in the `for...in` loop and land in the final result. Version 6.1.5 replaces `Object.assign({}, defaults)` with object spread (`{ ...defaults }`), which uses `[[DefineOwnProperty]]` and does not invoke the `__proto__` setter."
    },
    {
      "lang": "es",
      "value": "defu es un software que permite a los usuarios asignar propiedades predeterminadas recursivamente. Antes de la versión 6.1.5, las aplicaciones que pasan entrada de usuario no saneada (p. ej., cuerpos de solicitud JSON analizados, registros de base de datos o archivos de configuración de fuentes no confiables) como el primer argumento a 'defu()' son vulnerables a la contaminación de prototipos. Una carga útil manipulada que contiene una clave '__proto__' puede anular los valores predeterminados previstos en el resultado fusionado. La función interna '_defu' usaba 'Object.assign({}, defaults)' para copiar el objeto de valores predeterminados. 'Object.assign' invoca al  setter  '__proto__', que reemplaza el '[[Prototype]]' del objeto resultante con valores controlados por el atacante. Las propiedades heredadas del prototipo contaminado luego eluden la protección de clave '__proto__' existente en el bucle 'for...in' y terminan en el resultado final. La versión 6.1.5 reemplaza 'Object.assign({}, defaults)' con la propagación de objetos ('{ ...defaults }'), que usa '[[DefineOwnProperty]]' y no invoca al  setter  '__proto__'."
    }
  ],
  "lastModified": "2026-07-21T19:10:00.107",
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