GHSA-g53w-w6mj-hrpp The MCP router (ext_proc) exposes an `initialize`-method code path that, when a request carries an `mcp-init-host` header, bypass

Not scored Published 19 May 2026

Unauthenticated attackers can bypass JWT validation and inject authority headers to access backend services via the MCP router's initialize path.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is GHSA-g53w-w6mj-hrpp?

GHSA-g53w-w6mj-hrpp is an authentication bypass vulnerability in github.com/Kuadrant/mcp-gateway, published on 19 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Summary

The MCP router (ext_proc) exposes an initialize-method code path that, when a request carries an mcp-init-host header, bypasses the gateway JWT session validator and rewrites the upstream :authority header to whatever the caller chooses, gated only by a single shared header value (router-key). The shared value is

a literal string (secret-api-key) baked into cmd/mcp-broker-router/main.go as a fall-back default, and
in controller-managed deployments, a SHA-256 truncation of the MCPGatewayExtension UID — a non-secret value visible to anyone with get permission on the resource, and additionally exposed in argv because it is passed to the broker-router container via --mcp-router-key=....

A request that satisfies the trivial header check is forwarded to any backend listener registered with the gateway (including external services such as api.githubcopilot.com when configured), bypassing both the broker (where the signed x-mcp-authorized capability filter is enforced) and the gateway's JWT-based session model.

Which versions are affected by GHSA-g53w-w6mj-hrpp?

3 open source packages ship an affected version, including github.com/Kuadrant/mcp-gateway.

Package	Affected	Fixed in
`github.com/Kuadrant/mcp-gateway` Go	All versions	None
`github.com/Kuadrant/mcp-gateway` Go	All versions	`0.7.0`
`github.com/Kuadrant/mcp-gateway` Go	All versions	`0.7.0`

How do I fix GHSA-g53w-w6mj-hrpp?

Upgrade github.com/Kuadrant/mcp-gateway to 0.7.0 or later.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is GHSA-g53w-w6mj-hrpp being exploited?

There is no known exploitation of GHSA-g53w-w6mj-hrpp in the wild and no public exploit code.

Sources

Also known as GO-2026-5390.