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Echo: Rejected WebSocket upgrades can bypass Echo route-level access controls — github.com/labstack/echo/v4

Inconsistent Interpretation of HTTP Requests ('HTTP Request Smuggling')

Impact

Echo's Proxy middleware selected a raw tunnel when the client requested a WebSocket upgrade, before validating whether the configured upstream accepted it. If the upstream returned an ordinary HTTP response and kept the connection alive, subsequent HTTP requests on that connection were forwarded directly to the same upstream without passing through Echo's router or route-specific middleware.

This can bypass authentication or authorization enforced only at Echo's routes. Exploitation requires an accessible proxy route, an upstream that rejects the upgrade without closing the connection, and a target resource on that upstream without independent authorization. The issue does not provide arbitrary-host proxying or bypass the upstream's own authentication.

Affected versions and patch

The raw proxy implementation is present throughout the v4 and v5 release lines up to v4.16.0 and v5.4.0. Runtime reproductions confirmed v4.15.0, v4.16.0, v5.0.0 and v5.4.0; source history also shows the missing upgrade-response validation in v4.0.0.

Upgrade to v4.16.1 or v5.4.1. The patch routes WebSocket upgrades through Go's HTTP reverse proxy and enters a tunnel only after a valid upstream upgrade response. Rejected upgrades remain ordinary HTTP requests.

Fix commits: v4, v5.

Workarounds

Reject WebSocket upgrade requests before they reach affected Echo proxy routes, or enforce authorization independently at every upstream resource. Merely returning a non-101 response from the upstream does not prevent the vulnerable raw tunnel.

Credits

Reported by @Duodum.

  • Published: Oct 23, 2026
  • Updated: Oct 29, 2026
  • GHSA: GHSA-cxf3-9h37-7rr8
  • Severity: Critical
  • Exploit:
  • CISA KEV:

No technical information available.

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A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

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Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

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