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.
CVE-2026-56866
“—” beside the score: it is equal to the CVSS score beside it.
Score 7.5 from GitHub Security Advisory (severity: HIGH) published 2026-10-09. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 7.5; sources differ by 0.0.
- High severity, but no confirmed exploitation yet
A fix is available — apply it.
- CVSS v3
- 7.5
- EG Score
- 7.5HIGHhigh confidence
- EG Risk
- 49EG Risk 49/100
EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).
How it’s computed (value × EchelonGraph’s weight)Severity75% × 45%The EG Score, 7.5 of 10.Exploitation0.3% × 40%FIRST EPSS: the 30-day exploitation probability when EchelonGraph scored this CVE.Automatability100% × 15%CISA's SSVC Automatable value: yes (Vulnrichment).SSVC (CISA's decision table, applied by EchelonGraph): Track at low or medium mission impact; Attend at high (mission-essential systems).Exploitation none (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high. - EPSS PROB
- 0.3%
- EPSS %ILE
- 21st
- KEV
- Not listed
SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Attend at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
Exploitation none (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table
BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; 60 days if it is not.
In CISA KEV no (CISA's KEV catalog) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it
Published
October 8, 2026
Last Modified
October 9, 2026
Advisory Details (4)
Auto-updated Oct 9, 2026net/http: HTTP/1 client connection desynchronization after CONNECT rejection · Issue #81740 · golang/go · GitHub
https://go.dev/issue/81740Vendor Advisories for CVE-2026-56866(1)
These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.
Affected Packages
(6 across 4 ecosystems)
Debian:14(3)
| Package | Vulnerable range | Fix by version range | Dependents |
|---|---|---|---|
| golang-1.25 | 1.25.0-1 ... 1.25.9-1 (20 versions) |
| — |
| golang-1.26 | 1.26.0-1 ... 1.26~rc3-2 (24 versions) |
| — |
| golang-1.27 | 1.27.0-1 ... 1.27~rc3-1 (9 versions) |
| — |
Debian:12(1)
| Package | Vulnerable range | Fix by version range | Dependents |
|---|---|---|---|
| golang-1.19 | 1.19.10-1 ... 1.19.9-1 (12 versions) |
| — |
Debian:13(1)
| Package | Vulnerable range | Fix by version range | Dependents |
|---|---|---|---|
| golang-1.24 | 1.24.12-1 ... 1.24.9-1 (10 versions) |
| — |
Go(1)
| Package | Vulnerable range | Fix by version range | Dependents |
|---|---|---|---|
| stdlib | — |
| — |
Weakness Classification(1)
MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.
Data Freshness Timeline
(refreshed 20× in last 7d / 20× in last 30d)
Each row is a time one of our pipelines fetched, updated or re-scored this CVE. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.
- 2026-10-11 19:37 UTCEG score recompute
- 2026-10-11 17:00 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g · GitHub severity HIGH · GitHub CVSS 7.5
- 2026-10-11 16:53 UTCEG score recompute
- 2026-10-11 14:15 UTCEG score recompute
- 2026-10-11 13:49 UTCEPSS rescoreEPSS 0.3% · 21st percentile
- 2026-10-11 05:01 UTCEG score recompute
- 2026-10-11 05:01 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g · GitHub severity HIGH · GitHub CVSS 7.5
- 2026-10-10 20:00 UTCEPSS rescoreEPSS 0.3% · 21st percentile
- 2026-10-10 16:58 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g · GitHub severity HIGH · GitHub CVSS 7.5
- 2026-10-10 04:57 UTCEG score recompute
- 2026-10-10 04:57 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g · GitHub severity HIGH · GitHub CVSS 7.5
- 2026-10-09 20:07 UTCEPSS rescoreEPSS 0.2% · 5th percentile
- 2026-10-09 16:58 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g
- 2026-10-09 16:34 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g
- 2026-10-09 15:59 UTCEG score recomputeFirst assessed: 7.5 HIGH
- 2026-10-09 15:58 UTCGHSA enrichmentGHSA-f5vf-8pvc-394g
- 2026-10-09 15:58 UTCMITRE cvelistV5CVSS v3 → 7.5 · severity → HIGH
- 2026-10-08 23:20 UTCNVD update
- 2026-10-08 22:58 UTCEG score recompute
- 2026-10-08 22:58 UTCMITRE cvelistV5first tracked
Related CVEs(same CWE)
Frequently asked(5)
What is CVE-2026-56866?
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Dependency Blast Radius
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