fix(mcp): fail closed on DB outage in BYOK credential check

`_check_byok_credential` previously returned silently when `prisma_client`
was None, bypassing BYOK ownership validation during database-outage
windows. Any proxy-authenticated user could invoke BYOK-protected MCP
tools without a stored credential during the outage window.

Now raises HTTP 503 with a structured error so the flow fails closed.

Regression test asserts 503 is raised when `prisma_client` is None.

Reported by @brodmart in GHSA-6762-2m23-5mxp.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

litellm/proxy/_experimental/mcp_server/server.py

@@ -1952,7 +1952,18 @@ if MCP_AVAILABLE:
         from litellm.proxy.proxy_server import prisma_client
 
         if prisma_client is None:
-            return
+            # Fail closed on DB unavailability: returning here previously
+            # bypassed the ownership check and let any proxy-authenticated
+            # caller invoke BYOK tools during outage windows.
+            raise HTTPException(
+                status_code=503,
+                detail={
+                    "error": "byok_auth_unavailable",
+                    "server_id": mcp_server.server_id,
+                    "server_name": mcp_server.server_name or mcp_server.name,
+                    "message": "BYOK credential check requires a database connection.",
+                },
+            )
 
         credential = await get_user_credential(
             prisma_client=prisma_client,

tests/test_litellm/proxy/_experimental/mcp_server/test_byok_oauth_endpoints.py

@@ -564,6 +564,35 @@ async def test_check_byok_credential_has_credential():
         await _check_byok_credential(server, user_auth)
 
 
+@pytest.mark.asyncio
+async def test_check_byok_credential_db_unavailable_fails_closed():
+    """BYOK server with no prisma_client → 503, not silent pass.
+
+    Regression for GHSA-6762: previously returned silently, bypassing the
+    ownership check during DB outage windows.
+    """
+    from litellm.proxy._experimental.mcp_server.server import _check_byok_credential
+    from litellm.proxy._types import UserAPIKeyAuth
+    from litellm.types.mcp_server.mcp_server_manager import MCPServer
+
+    server = MCPServer(
+        server_id="byok-4",
+        name="byok-server",
+        transport=MCPTransport.http,
+        is_byok=True,
+    )
+    user_auth = UserAPIKeyAuth(user_id="user-55", api_key="sk-test")
+
+    with patch("litellm.proxy.proxy_server.prisma_client", None):
+        with pytest.raises(HTTPException) as exc_info:
+            await _check_byok_credential(server, user_auth)
+
+    assert exc_info.value.status_code == 503
+    detail: Any = exc_info.value.detail
+    assert detail["error"] == "byok_auth_unavailable"
+    assert detail["server_id"] == "byok-4"
+
+
 # ---------------------------------------------------------------------------
 # Security regression tests for AO2kf_-9 / GHSA-jg3h:
 # Unauthenticated /v1/mcp/oauth/authorize previously allowed an attacker to