CVE-2021-21274

Estado: ModificadaMedia (6.5)—

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Puntuación base: 6.5

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 2.16%
Percentil entre todas las CVEs puntuadas: 82
Fecha de la puntuación: 6/10/2026

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 (2)

Fedoraproject — Fedora Matrix — Synapse

CWE

CWE-400
CWE-770

Referencias

JSON original (NVD)

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{
  "id": "CVE-2021-21274",
  "cveTags": [],
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        "source": "nvd@nist.gov",
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          "baseScore": 4.3,
          "accessVector": "NETWORK",
          "vectorString": "AV:N/AC:M/Au:N/C:N/I:N/A:P",
          "authentication": "NONE",
          "integrityImpact": "NONE",
          "accessComplexity": "MEDIUM",
          "availabilityImpact": "PARTIAL",
          "confidentialityImpact": "NONE"
        },
        "acInsufInfo": false,
        "impactScore": 2.9,
        "baseSeverity": "MEDIUM",
        "obtainAllPrivilege": false,
        "exploitabilityScore": 8.6,
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        "obtainOtherPrivilege": false,
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          "integrityImpact": "NONE",
          "userInteraction": "REQUIRED",
          "attackComplexity": "LOW",
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  "published": "2021-02-26T18:15:12.237",
  "references": [
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  "vulnStatus": "Modified",
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    {
      "lang": "en",
      "value": "Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation."
    },
    {
      "lang": "es",
      "value": "Synapse es un servidor doméstico de referencia de Matrix escrito en python (paquete pypi matrix-synapse).&#xa0;Matrix es un ecosistema para VoIP y mensajería instantánea federada abierta.&#xa0;En Synapse versiones anteriores a 1.25.0, un servidor doméstico malicioso podía redireccionar las peticiones para su archivo .well-known a un archivo grande.&#xa0;Esto puede conllevar a un ataque de denegación de servicio en el que los servidores domésticos consumirán significativamente más recursos al solicitar el archivo .well-known de un servidor doméstico malicioso.&#xa0;Esto afecta a cualquier servidor que acepte peticiones de federación desde servidores que no sean confiables.&#xa0;El problema es resuelto en versión 1.25.0.&#xa0;Como solución alternativa, el ajuste de \"federation_domain_whitelist\" puede ser usado para restringir los servidores domésticos que se comunican con la federación"
    }
  ],
  "lastModified": "2026-06-17T03:35:10.330",
  "configurations": [
    {
      "nodes": [
        {
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}