{"api_version":"1","generated_at":"2026-10-22T04:55:41+00:00","cve":"CVE-2026-56866","urls":{"html":"https://cve.report/CVE-2026-56866","api":"https://cve.report/api/cve/CVE-2026-56866.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2026-56866","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-56866"},"summary":{"title":"HTTP/1 client connection desynchronization after CONNECT rejection in net/http","description":"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.","state":"PUBLISHED","assigner":"Go","published_at":"2026-10-08 23:17:01","updated_at":"2026-10-09 16:35:35"},"problem_types":["CWE-444","CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')","CWE-444 CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')"],"metrics":[{"version":"3.1","source":"ADP","type":"DECLARED","score":"7.5","severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","data":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":7.5,"baseSeverity":"HIGH","confidentialityImpact":"HIGH","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","version":"3.1"}},{"version":"3.1","source":"134c704f-9b21-4f2e-91b3-4a467353bcc0","type":"Secondary","score":"7.5","severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","data":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","baseScore":7.5,"baseSeverity":"HIGH","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"HIGH","integrityImpact":"NONE","availabilityImpact":"NONE"}}],"references":[{"url":"https://groups.google.com/g/golang-announce/c/U2fTuyDJznI","name":"https://groups.google.com/g/golang-announce/c/U2fTuyDJznI","refsource":"security@golang.org","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://go.dev/issue/81740","name":"https://go.dev/issue/81740","refsource":"security@golang.org","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://go.dev/cl/847306","name":"https://go.dev/cl/847306","refsource":"security@golang.org","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://pkg.go.dev/vuln/GO-2026-6605","name":"https://pkg.go.dev/vuln/GO-2026-6605","refsource":"security@golang.org","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-56866","name":"CVE Program record","refsource":"CVE.ORG","tags":["canonical"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-56866","name":"NVD vulnerability detail","refsource":"NVD","tags":["canonical","analysis"]}],"affected":[{"source":"CNA","vendor":"Go standard library","product":"net/http","version":"affected 1.26.9 semver","platforms":[]},{"source":"CNA","vendor":"Go standard library","product":"net/http","version":"affected 1.27.0-0 1.27.2 semver","platforms":[]},{"source":"CNA","vendor":"Go standard library","product":"net/http/httputil","version":"affected 1.26.9 semver","platforms":[]},{"source":"CNA","vendor":"Go standard library","product":"net/http/httputil","version":"affected 1.27.0-0 1.27.2 semver","platforms":[]}],"timeline":[],"solutions":[],"workarounds":[],"exploits":[],"credits":[{"source":"CNA","value":"Xclow3n (Rajat Raghav)","lang":"en"}],"nvd_cpes":[],"vendor_comments":[],"enrichments":{"kev":null,"epss":{"cve_year":"2026","cve_id":"56866","cve":"CVE-2026-56866","epss":"0.003090000","percentile":"0.218250000","score_date":"2026-10-11","updated_at":"2026-10-12 00:13:21"},"legacy_qids":[]},"source_records":{"cve_program":{"containers":{"adp":[{"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":7.5,"baseSeverity":"HIGH","confidentialityImpact":"HIGH","integrityImpact":"NONE","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","version":"3.1"}},{"other":{"content":{"id":"CVE-2026-56866","options":[{"Exploitation":"none"},{"Automatable":"yes"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","timestamp":"2026-10-09T15:53:00.591143Z","version":"2.0.3"},"type":"ssvc"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-444","description":"CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-10-09T15:54:59.009Z","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP"},"title":"CISA ADP Vulnrichment"}],"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 ('HTTP Request/Response Smuggling')","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":"2026-10-09T15:54:59.009Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"},"nvd":{"publishedDate":"2026-10-08 23:17:01","lastModifiedDate":"2026-10-09 16:35:35","problem_types":["CWE-444","CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')","CWE-444 CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')"],"metrics":{"cvssMetricV31":[{"source":"134c704f-9b21-4f2e-91b3-4a467353bcc0","type":"Secondary","cvssData":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","baseScore":7.5,"baseSeverity":"HIGH","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"HIGH","integrityImpact":"NONE","availabilityImpact":"NONE"},"exploitabilityScore":3.9,"impactScore":3.6}],"ssvcV203":[{"source":"134c704f-9b21-4f2e-91b3-4a467353bcc0","ssvcData":{"timestamp":"2026-10-09T15:53:00.591143Z","id":"CVE-2026-56866","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}]},"configurations":[]},"legacy_mitre":{"record":{"CveYear":"2026","CveId":"56866","Ordinal":"1","Title":"HTTP/1 client connection desynchronization after CONNECT rejecti","CVE":"CVE-2026-56866","Year":"2026"},"notes":[{"CveYear":"2026","CveId":"56866","Ordinal":"1","NoteData":"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.","Type":"Description","Title":"HTTP/1 client connection desynchronization after CONNECT rejecti"}]}}}