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GHSA-F5VF-8PVC-394G

Vulnerability from github – Published: 2026-10-09 00:31 – Updated: 2026-10-09 00:31

Details

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.

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{
  "affected": [],
  "aliases": [
    "CVE-2026-56866"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-10-08T23:17:01Z",
    "severity": null
  },
  "details": "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.",
  "id": "GHSA-f5vf-8pvc-394g",
  "modified": "2026-10-09T00:31:48Z",
  "published": "2026-10-09T00:31:47Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-56866"
    },
    {
      "type": "WEB",
      "url": "https://go.dev/cl/847306"
    },
    {
      "type": "WEB",
      "url": "https://go.dev/issue/81740"
    },
    {
      "type": "WEB",
      "url": "https://groups.google.com/g/golang-announce/c/U2fTuyDJznI"
    },
    {
      "type": "WEB",
      "url": "https://pkg.go.dev/vuln/GO-2026-6605"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}

CVE-2026-56866 (GCVE-0-2026-56866)

Vulnerability from cvelistv5 – Published: 2026-10-08 22:53 – Updated: 2026-10-08 22:53

Title

HTTP/1 client connection desynchronization after CONNECT rejection in net/http

Summary

Severity

No CVSS data available.

CWE

CWE-444 - Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Assigner

Go CNA under the Google root

CNA scorecard E 42/100 over 75 records in the last 180 days details

References

4 references

URL	Tags
https://go.dev/cl/847306
https://go.dev/issue/81740
https://groups.google.com/g/golang-announce/c/U2f…
https://pkg.go.dev/vuln/GO-2026-6605

Impacted products

2 products

Vendor	Product	Version
Go standard library	net/http	Affected: 0 , < 1.26.9 (semver) Affected: 1.27.0-0 , < 1.27.2 (semver)
Go standard library	net/http/httputil	Affected: 0 , < 1.26.9 (semver) Affected: 1.27.0-0 , < 1.27.2 (semver)

Credits

Xclow3n (Rajat Raghav)

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JSON

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{
  "containers": {
    "cna": {
      "affected": [
        {
          "collectionURL": "https://pkg.go.dev",
          "defaultStatus": "unaffected",
          "packageName": "net/http",
          "product": "net/http",
          "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",
          "versions": [
            {
              "lessThan": "1.26.9",
              "status": "affected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThan": "1.27.2",
              "status": "affected",
              "version": "1.27.0-0",
              "versionType": "semver"
            }
          ]
        },
        {
          "collectionURL": "https://pkg.go.dev",
          "defaultStatus": "unaffected",
          "packageName": "net/http/httputil",
          "product": "net/http/httputil",
          "programRoutines": [
            {
              "name": "ReverseProxy.ServeHTTP"
            },
            {
              "name": "DumpRequestOut"
            }
          ],
          "vendor": "Go standard library",
          "versions": [
            {
              "lessThan": "1.26.9",
              "status": "affected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThan": "1.27.2",
              "status": "affected",
              "version": "1.27.0-0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "credits": [
        {
          "lang": "en",
          "value": "Xclow3n (Rajat Raghav)"
        }
      ],
      "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."
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "description": "CWE-444: Inconsistent Interpretation of HTTP Requests (\u0027HTTP Request/Response Smuggling\u0027)",
              "lang": "en"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-08T22:53:58.586Z",
        "orgId": "1bb62c36-49e3-4200-9d77-64a1400537cc",
        "shortName": "Go"
      },
      "references": [
        {
          "url": "https://go.dev/cl/847306"
        },
        {
          "url": "https://go.dev/issue/81740"
        },
        {
          "url": "https://groups.google.com/g/golang-announce/c/U2fTuyDJznI"
        },
        {
          "url": "https://pkg.go.dev/vuln/GO-2026-6605"
        }
      ],
      "title": "HTTP/1 client connection desynchronization after CONNECT rejection in net/http"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "1bb62c36-49e3-4200-9d77-64a1400537cc",
    "assignerShortName": "Go",
    "cveId": "CVE-2026-56866",
    "datePublished": "2026-10-08T22:53:58.586Z",
    "dateReserved": "2026-06-23T15:10:49.354Z",
    "dateUpdated": "2026-10-08T22:53:58.586Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

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Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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GHSA-F5VF-8PVC-394G

CVE-2026-56866 (GCVE-0-2026-56866)

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