CVE-2026-56866
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.
INFO
Published Date :
Oct. 8, 2026, 11:17 p.m.
Last Modified :
Oct. 8, 2026, 11:17 p.m.
Remotely Exploit :
No
Source :
[email protected]
Solution
- Ensure CONNECT requests have an empty request body.
- Update or replace the transport mechanism.
- Use a properly framed request body.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-56866.
| URL | Resource |
|---|---|
| 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 |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-56866 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-56866
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-56866 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-56866 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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New CVE Received by [email protected]
Oct. 08, 2026
Action Type Old Value New Value Added 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. Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/56xxx/CVE-2026-56866.json">CVE-2026-56866</a> Added Reference https://go.dev/cl/847306 Added Reference https://go.dev/issue/81740 Added Reference https://groups.google.com/g/golang-announce/c/U2fTuyDJznI Added Reference https://pkg.go.dev/vuln/GO-2026-6605