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CVE-2026-77802

Estado: RecibidaMedia (6.3)—

In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value.

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A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them.

CVSS

Probabilidad de explotación (EPSS)

FIRST aún no ha puntuado esta CVE (habitual en CVEs muy recientes o rechazadas).

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 local (AV:L, PR:L) que permite a atacante con privilegios bajos manipular solicitudes HTTP en Fiddler para envenenar respuestas a otros usuarios, modificar datos en tránsito y acceder a respuestas destinadas a terceros mediante desincronización de Content-Length.

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

Data Loss PreventionThreat Intelligence ProgramNetwork Intrusion PreventionExecution PreventionEncrypt Sensitive InformationApplication Isolation and Sandboxing

Tecnologías afectadas (1)

⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.

Progress — Fiddler ClassicAI

CWE

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-77802",
  "cveTags": [],
  "metrics": {
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "security@progress.com",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 6.3,
          "attackVector": "LOCAL",
          "baseSeverity": "MEDIUM",
          "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "HIGH",
          "availabilityImpact": "NONE",
          "privilegesRequired": "LOW",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.2,
        "exploitabilityScore": 1
      }
    ]
  },
  "affected": [
    {
      "source": "security@progress.com",
      "affectedData": [
        {
          "vendor": "Progress Software",
          "product": "Progress® Telerik® Fiddler® Classic",
          "versions": [
            {
              "status": "affected",
              "version": "1.0.0",
              "lessThan": "6.0.20262.10021",
              "versionType": "custom"
            }
          ],
          "platforms": [
            "Windows"
          ],
          "defaultStatus": "unaffected"
        }
      ]
    }
  ],
  "published": "2026-10-05T13:16:54.293",
  "references": [
    {
      "url": "https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802",
      "source": "security@progress.com"
    }
  ],
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "security@progress.com",
      "description": [
        {
          "lang": "en",
          "value": "CWE-444"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them."
    }
  ],
  "lastModified": "2026-10-05T13:16:54.293",
  "sourceIdentifier": "security@progress.com"
}