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

Quick assessment

Affected
Go standard library net/http
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

当 发送带有非空 的 HTTP/1 CONNECT 请求时，它会将请求体直接写入连接，且在请求头之后不进行任何边界封装（framing）。如果服务器以非 2xx 状态码的 keep-alive 响应拒绝该 CONNECT 请求， 会将该连接放回空闲连接池。由于 CONNECT 请求本不应包含请求体，服务器可能会将尾部多余的请求体字节解释为连接上后续的一个管道化 HTTP/1.1 请求，导致已池化的连接状态不同步，并使下一个复用该连接的客户端误读注入请求的响应。在通过共享 转发 CONNECT 请求的反向代理（包括

AI Predicted 7.5 Difficulty: Moderate

Possible ATT&CK Techniques 1 AI

T1102 · Web Service
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I. Basic Information for CVE-2026-56866

Vulnerability Information
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Vulnerability Title
HTTP/1 client connection desynchronization after CONNECT rejection in net/http
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Go standard library net/http 0 ~ 1.26.9 -
Go standard library net/http/httputil 0 ~ 1.26.9 -

II. Public POCs for CVE-2026-56866

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III. Intelligence Information for CVE-2026-56866

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Vendor Advisories for CVE-2026-56866 (2)

Vendor Pages for CVE-2026-56866 (1)

Other References for CVE-2026-56866 (1)

Same Patch Batch · Go standard library · 2026-10-08 · 13 CVEs total

CVE-2026-94439 HTTP/1 server connection desynchronization after 2xx CONNECT response in net/http
CVE-2026-94440 Memory limit bypass when parsing MIME headers in net/textproto, mime/multipart
CVE-2026-94448 Reset context tracking on consecutive template expressions in html/template
CVE-2026-56857 Root.Mkdir(All) can follow junctions out of the root on Windows in os
CVE-2026-78659 HTTP/2 server memory exhaustion due to Trailer headers in net/http
CVE-2026-78660 HTTP/2 transport accepts malformed framing-related headers in net/http
CVE-2026-78663 Double flow control refund on HTTP/2 server streams in net/http
CVE-2026-78667 Lack of limit on size of parsed Range headers in net/http
CVE-2026-78669 Excessive CPU consumption from repeated initial window changes in net/http
CVE-2026-97032 HTTP/2 server crash due to HPACK encoder race in net/http
CVE-2026-97030 Recognize yield as regexp preceder keyword in html/template
CVE-2026-97031 Reject malformed ECH outer extension references in crypto/tls

IV. Related Vulnerabilities

Same product: net/http
Same vendor: Go standard library

V. Comments for CVE-2026-56866

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