CVE-2026-56866
HTTP/1 client connection desynchronization after CONNECT rejection in net/http
Published Oct 8, 2026 · corrected/updated Oct 16, 2026 · evidence confidence high
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.
Grounded briefings
Catalog interpretations for three audiences. Each brief cites ingested metadata and the original advisories below. They are not independent research and they do not invent exploit steps.
Executive
CVE-2026-56866 is a high public vulnerability record for this product. It is not currently listed in the CISA Known Exploited catalog. Check the linked vendor or CERT advisories for patch status. This summary is a catalog interpretation of the ingested metadata and cites the original advisories below.
System administrator
Identify whether this product is in your estate, then compare installed versions with the affected ranges. Affected packages or ranges on record: golang (0 → 1.26.9); stdlib (0 → 1.26.9). Check the linked vendor or CERT advisories for patch status. Prefer the vendor or distribution advisory linked in Sources before changing production systems. This is a catalog interpretation, not a runbook.
Security analyst
Signals on file: no CVSS/EPSS yet. It is not currently listed in the CISA Known Exploited catalog. Sources contributing to this record: cve_org, osv, osv. Public exploit references, when present, are outbound links only and are not proof of active exploitation. This is a catalog interpretation of stored metadata.
Exploitation and remediation
Exploitation evidence
Not listed as known exploited in CISA KEV. Not in the CISA KEV catalog.
Patch and remediation
No required-action text is stored yet. Use the vendor or CERT links in Sources for patch availability.
Safe detection guidance
Compare installed versions with the affected ranges below and verify the advisory from the original publisher. This site stores metadata and outbound links only; it does not ship exploit code or proof-of-concept payloads.
Code
The project this record applies to. Exploit listings stay in References.
No public source repository is identified for this product.
Timeline
Detection, updates, and resolution for this record.
Oct 8, 2026 · OSV.dev
Detected
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 byt
Oct 16, 2026 · OSV.dev
Fix available
stdlib fixed in 1.26.9
Oct 16, 2026 · OSV.dev
Fix available
stdlib fixed in 1.26.9
Oct 16, 2026 · OSV.dev
Fix available
golang fixed in 1.26.9
Oct 16, 2026 · OSV.dev
Fix available
golang fixed in 1.26.9
Oct 16, 2026 · OSV.dev
Updated
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 byt
Affected
- Bitnami golang0 → 1.26.9
- Go stdlib0 → 1.26.9
Sources
- CVE.orgVulnerability recordCVE-2026-56866
Official CVE identifier
cve.org - OSV.devPackage advisoryBIT-golang-2026-56866
HTTP/1 client connection desynchronization after CONNECT rejection in net/http
osv.dev - OSV.devPackage advisoryGO-2026-6605
HTTP/1 client connection desynchronization after CONNECT rejection in net/http
osv.dev