Security Digest · Vulnerability record
CVE-2026-56866: HTTP/1 client connection desynchronization after CONNECT rejection in net/http
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.
CVSS 7.5 High EPSS 0.3% · p22 Patch this week
Should you act?
Normal patch cycle
No exploitation signal; fix it in your next routine update if you are affected. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.3%, percentile 22: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 22% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Patch this week. Go October 2026 Security Releases: 16 Stdlib Fixes — Rebuild and redeploy Go services with Go 1.27.3 or 1.26.10 this week, starting with internet-facing HTTP/2 servers and reverse proxies; none of these bugs is known to be exploited.
What it is
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.
Weakness type: CWE-444.
Product: Go standard library net/http · Assigned by: Go · Published: 2026-10-08
Affected and fixed versions
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| Go standard library | net/http | < 1.26.9, 1.27.0-0 < 1.27.2 |
| Go standard library | net/http/httputil | < 1.26.9, 1.27.0-0 < 1.27.2 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| CISA-ADP | 3.1 | 7.5 High | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: none
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none, automatable yes, technical impact partial.
- FIRST EPSS (model of 2026-10-21): 0.31% probability of exploitation activity in the next 30 days; percentile 22.
Timeline
- — CVE record published by Go
- — erroneo.us brief: Go October 2026 Security Releases: 16 Stdlib Fixes
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vulnerability records
- CVE.org record CVE-2026-56866 cve.org
- NVD entry CVE-2026-56866 nvd.nist.gov
Other references from the CVE record and advisory
- go.dev go.dev
- go.dev go.dev
- groups.google.com groups.google.com
- pkg.go.dev pkg.go.dev