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Vulnerabilities

Vulnerability · CVE-2026-56866

HTTP/1 client connection desynchronization after CONNECT rejection in net/http

Published Oct 8, 2026 · catalog updated Oct 16, 2026 · HTTP/1 client connection desynchronization

Check if you run it

No confirmed exploitation is on record for CVE-2026-56866. EPSS is 0.31%. Act only if a listed version of HTTP/1 client connection desynchronization is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (HTTP/1 client connection desynchronization).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High—
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.31%
Remediation
Not established
Affected products
Bitnami golang · Go stdlib

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • Bitnami golang0 → 1.26.9
  • Go stdlib0 → 1.26.9

How to fix CVE-2026-56866

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-56866. EPSS is 0.31%. Act only if a listed version of HTTP/1 client connection desynchronization is installed.

Which versions are affected?

Compare what you have installed with 0 → 1.26.9. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

No weakness class is stored. EPSS 0.31%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

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.

  1. Oct 8, 2026 · OpenSSF OSV / OSV Scanner

    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

  2. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    stdlib fixed in 1.26.9

  3. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    stdlib fixed in 1.26.9

  4. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    golang fixed in 1.26.9

  5. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    Fix available

    golang fixed in 1.26.9

  6. Oct 16, 2026 · OpenSSF OSV / OSV Scanner

    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

Sources

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.