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CVE.org (no NVD analysis yet)· CNA Go

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

HighCVSS 7.5 · v3.1—No exploit Fix available

Noroxi assessment

Priority: Monitor (action score 30/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.60% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: Go standard library net/http 1.26.9, 1.27.2; Go standard library net/http/httputil 1.26.9, 1.27.2; azl3 golang 1.26.10-1 on Azure Linux 3.0 Release Notes.

Low priority for now. Upgrade to Go standard library net/http 1.26.9, 1.27.2; Go standard library net/http/httputil 1.26.9, 1.27.2; azl3 golang 1.26.10-1 on Azure Linux 3.0 Release Notes or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

Published
Oct 8, 2026
Updated
Oct 9, 2026
EPSS
0.6% · 49th percentile

Report tools

JSON

Action score

30

Monitor

Low priority for now.

CVSS
30 / 40 · 7.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.6%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-56866

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Go). Independent of NVD's CPE analysis and usually ahead of it.

  • Go standard library net/http

    • before 1.26.9affected · semver
    • 1.27.0-0 and later · before 1.27.2affected · semver

  • Go standard library net/http/httputil

    • before 1.26.9affected · semver
    • 1.27.0-0 and later · before 1.27.2affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:10go-toolsetbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golangbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-binbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-docsbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-miscbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-racebefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-srcbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:10golang-testsbefore 1.26.10-1.el10_2.alma.11.26.10-1.el10_2.alma.1
AlmaLinux:8delvebefore 1.26.1-1.module_el8.10.0+4307+6e4cbc601.26.1-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8go-toolsetbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golangbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-binbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-docsbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-miscbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-racebefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-srcbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:8golang-testsbefore 1.26.10-1.module_el8.10.0+4307+6e4cbc601.26.10-1.module_el8.10.0+4307+6e4cbc60
AlmaLinux:9go-toolsetbefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golangbefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golang-binbefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golang-docsbefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golang-miscbefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golang-racebefore 1.26.10-1.el9_81.26.10-1.el9_8
AlmaLinux:9golang-srcbefore 1.26.10-1.el9_81.26.10-1.el9_8

+19

Same product

go standard library: all records

Other highest-scoring records for the same primary product.

  • CVE-2023-45288HTTP/2 CONTINUATION flood in net/http
    57Plan
  • CVE-2026-78663Double flow control refund on HTTP/2 server streams in net/http
    36Monitor
  • CVE-2026-97032HTTP/2 server crash due to HPACK encoder race in net/http
    30Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2025-58186Oct 29, 2025 · Lack of limit when parsing cookies can cause memory exhaustion in net/http
    21Monitor
  • ← EarlierCVE-2026-56853Aug 13, 2026 · Apply ReadHeaderTimeout when doing unencrypted HTTP/2 check in net/http
    30Monitor
  • Later →CVE-2026-78659Oct 8, 2026 · HTTP/2 server memory exhaustion due to Trailer headers in net/http
    30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Go standard library net/http1.26.9Vendor (CNA)
Go standard library net/http1.27.2Vendor (CNA)
Go standard library net/http/httputil1.26.9Vendor (CNA)
Go standard library net/http/httputil1.27.2Vendor (CNA)
azl3 golang 1.26.10-1 on Azure Linux 3.0Release NotesMicrosoft (MSRC)
azl3 golang 1.26.7-1 on Azure Linux 3.0Release NotesMicrosoft (MSRC)
azl3 golang 1.27.1-1 on Azure Linux 3.0Release NotesMicrosoft (MSRC)
azl3 golang 1.27.3-1 on Azure Linux 3.0Release NotesMicrosoft (MSRC)
almalinux:delve1.26.1-1.module_el8.10.0+4307+6e4cbc60 · AlmaLinux:8Package registry (OSV)
almalinux:go-toolset1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-bin1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-docs1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-misc1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-race1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-src1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
almalinux:golang-tests1.26.10-1.el10_2.alma.1 · AlmaLinux:10Package registry (OSV)
alpine:go1.26.9-r0 · Alpine:v3.24Package registry (OSV)
debian:golang-1.261.26.9-1 · Debian:14Package registry (OSV)
debian:golang-1.271.27.2-1 · Debian:14Package registry (OSV)
Go:stdlib1.26.9Package registry (OSV)
Go:stdlib1.27.2Package registry (OSV)
opensuse:go1.261.26.9-1.1 · openSUSE:TumbleweedPackage registry (OSV)
opensuse:go1.271.27.2-1.1 · openSUSE:TumbleweedPackage registry (OSV)
opensuse:rclone1.75.2-1.1 · openSUSE:TumbleweedPackage registry (OSV)
suse:go1.261.26.9-150000.1.33.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7Package registry (OSV)
suse:go1.261.26.9-160000.1.1 · SUSE:Linux Enterprise Server 16.0Package registry (OSV)
suse:go1.26-openssl1.26.10-150600.13.18.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7Package registry (OSV)
suse:go1.271.27.2-150000.1.12.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7Package registry (OSV)
suse:go1.271.27.2-160000.1.1 · SUSE:Linux Enterprise Server 16.0Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Published
  2. Fix recordedsame day
  3. TodayNov 12, 2026+35 d
EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-786600 days apartHTTP/2 transport accepts malformed framing-related headers in net/http
    30Monitor
  • CVE-2026-944390 days apartHTTP/1 server connection desynchronization after 2xx CONNECT response in net/http
    30Monitor

Not on record for this CVE: patch commit or PR link · public bounty program.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
none
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
None
Availability impact
None

Weakness class (CWE)

CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

ATT&CK techniques

—

Change log

  1. CVSS— → 7.5
  2. Severity— → high
  3. SSVC exploitation— → none
  4. Exploit maturity— → none
  5. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records