High 7.5Published Oct 8, 2026

CVE-2026-56866HTTP/1 client connection desynchronization after CONNECT rejection in net/http

Updated under an hour ago (2026-10-19 06:30 UTC)

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It affects Go standard library net/http and 1 other product. Add what you run to find out whether that includes you.

EPSS puts the chance of exploitation in the next 30 days at 0.31%, higher than 22% of scored CVEs.

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Every CVE for Go standard library net/http

Exploited in the last 30 days

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: 7.5 HIGH (v3.1, CISA ADP)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Weakness (CWE): CWE-444
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): 0.31% (percentile 22)
Assigned by: Go
NVD analysis: Awaiting Analysis
Record last updated: Oct 9, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Go standard library net/http go_standard_library:net_http (from the name)	before 1.26.9; from 1.27.0-0 before 1.27.2	CNA record
Go standard library net/http/httputil go_standard_library:net_http_httputil (from the name)	before 1.26.9; from 1.27.0-0 before 1.27.2	CNA record

References

https://go.dev/cl/847306
https://go.dev/issue/81740
https://groups.google.com/g/golang-announce/c/U2fTuyDJznI
https://pkg.go.dev/vuln/GO-2026-6605

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.