GHSA-4GPH-2HHR-5MWG

Envoy AI Proxy - MCP Message Smuggling Vulnerability

This vulnerability makes downstream authorization layers ineffective when Envoy AI Gateway is used as a router. By altering the message content, Envoy effectively acts as a "Confused Deputy" that:

1. Trusted Input: Takes input trusted by the other MCP-compliant actors
2. Malicious Transformation: Converts it into a totally different, but compliant, MCP message. Violating the transparency nature of proxies and altering a valid request into a malicious one
3. Trusted Output: Sends it off as a valid request to the upstream backend

Authentication and Authorization guarantees provided by prior actors or MCP manipulations are nullified because the original message is not the message the target Backend receives.

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1. Strict Protocol Compliance
   Envoy AI Gateway must enforce Strict Case Sensitivity during JSON unmarshalling to comply with JSON-RPC 2.0.

2. Reject Unknown Fields In Protocol Reserved Areas
   Enable DisallowUnknownFields.

3. Preserve Integrity
   If Envoy acts as a transparent router, it should avoid unnecessary re-serialization of the payload body unless modification is explicitly required and safe. If normalization is required, it must be lossless and strictly validated.

It should be highlighted that after our root-cause analysis, github.com/modelcontextprotocol/go-sdk is ultimately responsible for the initial part the issue. As a result, we would expect the maintainers to fix the vulnerability upstream.

As a general recommendation, Envoy AI Gateway should use a JSON RPC parser and subsequent JSON parsing strategies in sonic which are following the SPEC to prevent the differentials, forcing the case sensitive matching on MCP messages.

Doyensec has already reported the Go-SDK violation of the JSON RPC Specification via Hackerone.

1. What is GHSA-4GPH-2HHR-5MWG? GHSA-4GPH-2HHR-5MWG is a medium-severity security vulnerability in github.com/envoyproxy/ai-gateway (go), affecting versions < 0.6.0. It is fixed in 0.6.0.
2. Which versions of github.com/envoyproxy/ai-gateway are affected by GHSA-4GPH-2HHR-5MWG? github.com/envoyproxy/ai-gateway (go) versions < 0.6.0 is affected.
3. Is there a fix for GHSA-4GPH-2HHR-5MWG? Yes. GHSA-4GPH-2HHR-5MWG is fixed in 0.6.0. Upgrade to this version or later.
4. Is GHSA-4GPH-2HHR-5MWG exploitable, and should I be worried? Whether GHSA-4GPH-2HHR-5MWG is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether GHSA-4GPH-2HHR-5MWG is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix GHSA-4GPH-2HHR-5MWG? Upgrade github.com/envoyproxy/ai-gateway to 0.6.0 or later.