gRPC-Go has an authorization bypass via missing leading slash in :path
Date: 2026-03
Affected: go/google.golang.org/grpc
Attack vector: auth-bypass
Category: vulnerability-disclosure
Severity: Critical
CVE: CVE-2026-33186
CWE: CWE-285
CVSS: 9.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N)

### Impact _What kind of vulnerability is it? Who is impacted?_ It is an **Authorization Bypass** resulting from **Improper Input Validation** of the HTTP/2 `:path` pseudo-header. The gRPC-Go server was too lenient in its routing logic, accepting requests where the `:path` omitted the mandatory leading slash (e.g., `Service/Method` instead of `/Service/Method`). While the server successfully routed these requests to the correct handler, authorization interceptors (including the official `grpc/authz` package) evaluated the raw, non-canonical path string. Consequently, "deny" rules defined using canonical paths (starting with `/`) failed to match the incoming request, allowing it to bypass the policy if a fallback "allow" rule was present. **Who is impacted?** This affects gRPC-Go servers that meet both of the following criteria: 1. They use path-based authorization interceptors, such as the official RBAC implementation in `google.golang.org/grpc/authz` or custom interceptors relying on `info.FullMethod` or `grpc.Method(ctx)`. 2. Their security policy contains specific "deny" rules for canonical paths but allows other requests by default (a fallback "allow" rule). The vulnerability is exploitable by an attacker who can send raw HTTP/2 frames with malformed `:path` headers directly to the gRPC server. ### Patches _Has the problem been patched? What versions should users upgrade to?_ Yes, the issue has been patched. The fix ensures that any request with a `:path` that does


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