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CVE-2026-56866

Status: Awaiting AnalysisHigh (7.5)—

When http.Transport sends an HTTP/1 CONNECT request with a non-empty Request.Body, it writes the body directly to the connection without framing after the request headers. If the server rejects the CONNECT request with a non-2xx keep-alive response, Transport returns the connection to the idle pool. Because CONNECT requests do not have a request body, the server may interpret the trailing body bytes as a subsequent pipelined HTTP/1.1 request on the connection, leaving the pooled connection desynchronized and causing the next caller that reuses it to read the response to the injected request. In reverse proxies (including httputil.ReverseProxy) that forward CONNECT requests through a shared Transport, this can lead to cross-user response poisoning.

CVSS

Exploitation probability (EPSS)

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

🎯 ATT&CK techniques

How this vulnerability is exploited and what the attacker gains, in MITRE ATT&CK terms.

Vulnerabilidad de red sin autenticación (PR:N) en servicio remoto HTTP/1 que permite lectura de respuestas ajenas (C:H) mediante desincronización de conexiones reutilizadas en proxies inversos.

Inferred by our analysis agent from the official description, CVSS vector and CWE, and checked by a supervisor. May contain errors.

🛡️ ATT&CK mitigations that cover these techniques

Network SegmentationLimit Access to Resource Over NetworkFilter Network TrafficVulnerability ScanningUser TrainingPrivileged Account Management

Affected technologies (1)

⚠ AI-inferred from the description — NVD hasn't analyzed this CVE yet, these aren't verified CPEs.

Golang — GOAI

CWEs

References

Raw JSON (NVD)

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{
  "id": "CVE-2026-56866",
  "cveTags": [],
  "metrics": {
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-56866",
          "role": "CISA Coordinator",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "version": "2.0.3",
          "timestamp": "2026-10-09T15:53:00.591143Z"
        }
      }
    ],
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 7.5,
          "attackVector": "NETWORK",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N",
          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 3.6,
        "exploitabilityScore": 3.9
      }
    ]
  },
  "affected": [
    {
      "source": "security@golang.org",
      "affectedData": [
        {
          "vendor": "Go standard library",
          "product": "net/http",
          "versions": [
            {
              "status": "affected",
              "version": "0",
              "lessThan": "1.26.9",
              "versionType": "semver"
            },
            {
              "status": "affected",
              "version": "1.27.0-0",
              "lessThan": "1.27.2",
              "versionType": "semver"
            }
          ],
          "packageName": "net/http",
          "collectionURL": "https://pkg.go.dev",
          "defaultStatus": "unaffected",
          "programRoutines": [
            {
              "name": "persistConn.readLoop"
            },
            {
              "name": "Client.CloseIdleConnections"
            },
            {
              "name": "Client.Do"
            },
            {
              "name": "Client.Get"
            },
            {
              "name": "Client.Head"
            },
            {
              "name": "Client.Post"
            },
            {
              "name": "Client.PostForm"
            },
            {
              "name": "ClientConn.Close"
            },
            {
              "name": "ClientConn.RoundTrip"
            },
            {
              "name": "Get"
            },
            {
              "name": "Head"
            },
            {
              "name": "Post"
            },
            {
              "name": "PostForm"
            },
            {
              "name": "Transport.CloseIdleConnections"
            },
            {
              "name": "Transport.NewClientConn"
            },
            {
              "name": "Transport.RoundTrip"
            },
            {
              "name": "http1ClientConn.Close"
            },
            {
              "name": "http1ClientConn.RoundTrip"
            }
          ]
        },
        {
          "vendor": "Go standard library",
          "product": "net/http/httputil",
          "versions": [
            {
              "status": "affected",
              "version": "0",
              "lessThan": "1.26.9",
              "versionType": "semver"
            },
            {
              "status": "affected",
              "version": "1.27.0-0",
              "lessThan": "1.27.2",
              "versionType": "semver"
            }
          ],
          "packageName": "net/http/httputil",
          "collectionURL": "https://pkg.go.dev",
          "defaultStatus": "unaffected",
          "programRoutines": [
            {
              "name": "ReverseProxy.ServeHTTP"
            },
            {
              "name": "DumpRequestOut"
            }
          ]
        }
      ]
    }
  ],
  "published": "2026-10-08T23:17:01.620",
  "references": [
    {
      "url": "https://go.dev/cl/847306",
      "source": "security@golang.org"
    },
    {
      "url": "https://go.dev/issue/81740",
      "source": "security@golang.org"
    },
    {
      "url": "https://groups.google.com/g/golang-announce/c/U2fTuyDJznI",
      "source": "security@golang.org"
    },
    {
      "url": "https://pkg.go.dev/vuln/GO-2026-6605",
      "source": "security@golang.org"
    }
  ],
  "vulnStatus": "Awaiting Analysis",
  "weaknesses": [
    {
      "type": "Secondary",
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "description": [
        {
          "lang": "en",
          "value": "CWE-444"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "When http.Transport sends an HTTP/1 CONNECT request with a non-empty Request.Body, it writes the body directly to the connection without framing after the request headers. If the server rejects the CONNECT request with a non-2xx keep-alive response, Transport returns the connection to the idle pool. Because CONNECT requests do not have a request body, the server may interpret the trailing body bytes as a subsequent pipelined HTTP/1.1 request on the connection, leaving the pooled connection desynchronized and causing the next caller that reuses it to read the response to the injected request. In reverse proxies (including httputil.ReverseProxy) that forward CONNECT requests through a shared Transport, this can lead to cross-user response poisoning."
    }
  ],
  "lastModified": "2026-10-09T16:35:35.900",
  "sourceIdentifier": "security@golang.org"
}