CVE-2026-56866 Go standard library CVE debrief

Who should care

Defenders managing systems using Go's net/http package, especially those configuring reverse proxies with httputil.ReverseProxy, should assess exposure and prioritize verification and remediation efforts.

Why it matters

CVE-2026-56866 is a vulnerability in Go's net/http package that can lead to cross-user response poisoning in certain configurations. Defenders should prioritize verifying exposure, especially in reverse proxy setups, and update or patch vulnerable Go versions.

• Potential for cross-user response poisoning in shared Transport configurations.
• Possible disruption of service through desynchronized connections.
• Need for verification of affected versions and configurations.
• Priority on updating or patching vulnerable Go versions.

Technical summary

The Go standard library's net/http package does not properly handle CONNECT requests with non-empty Request.Body when the server rejects them with a non-2xx keep-alive response. This can lead to connection desynchronization and potentially allow cross-user response poisoning in certain configurations, such as reverse proxies. The vulnerability arises from how http.Transport handles CONNECT requests with non-empty Request.Body. If a server rejects such a request with a non-2xx keep-alive response, the connection may be returned to the idle pool in a desynchronized state. This can cause subsequent users of the pooled connection to read responses intended for injected requests. Defenders should prioritize

Defensive priority

Defenders should prioritize verifying exposure in systems using Go's net/http package, especially in reverse proxy configurations.

Recommended defensive actions

• Verify and update Go versions to 1.26.9 or 1.27.2, or apply patches as available.
• Review and adjust configurations for reverse proxies using httputil.ReverseProxy.
• Monitor for unusual response patterns that could indicate exploitation attempts.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The vulnerability arises from how http.Transport handles CONNECT requests with non-empty Request.Body. If a server rejects such a request with a non-2xx keep-alive response, the connection may be returned to the idle pool in a desynchronized state. This can cause subsequent users of the pooled connection to read responses intended for injected requests.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.