Skip to content

fix: reject requests with missing Origin header in origin validation middleware (CWE-346) #1161

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
sebastionoss wants to merge 2 commits into from
+11 −3
Closed

fix: reject requests with missing Origin header in origin validation middleware (CWE-346) #1161

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
ea34c17
fix: reject requests with missing Origin header in origin validation …
sebastionoss Mar 27, 2026
5bb0342
fix: distinguish missing vs invalid Origin, and document the fail-clo…
cliffhall Aug 19, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 2 additions & 0 deletions README.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -237,6 +237,8 @@ To prevent DNS rebinding attacks, the MCP Inspector validates the `Origin` heade
ALLOWED_ORIGINS=http://localhost:6274,http://localhost:8000 npm start
```

Validation fails closed: a request whose `Origin` header is **missing** is rejected with `403` just like one whose `Origin` is not on the allow list. Browsers always send `Origin` on the cross-origin requests the Inspector client makes, so normal use is unaffected — but a non-browser client (curl, Postman, a CI script) driving the proxy API directly must send the header explicitly, for example `-H "Origin: http://localhost:6274"`. The unauthenticated `/health` endpoint is not origin-checked, so container health checks are unaffected.

### Configuration

The MCP Inspector supports the following configuration settings. To change them, click on the `Configuration` button in the MCP Inspector UI:
Expand Down
12 changes: 9 additions & 3 deletions server/src/index.ts
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -203,12 +203,18 @@ const originValidationMiddleware = (
defaultOrigin,
];

if (origin && !allowedOrigins.includes(origin)) {
console.error(`Invalid origin: ${origin}`);
if (!origin || !allowedOrigins.includes(origin)) {
Comment thread
cliffhall marked this conversation as resolved.
// Distinguish the two rejection causes so operators can tell a genuine
// cross-origin attempt from a non-browser client that sent no Origin at all.
console.error(
origin
? `Invalid origin: ${origin}`
: "Missing origin header - request rejected",
);
res.status(403).json({
error: "Forbidden - invalid origin",
message:
"Request blocked to prevent DNS rebinding attacks. Configure allowed origins via environment variable.",
"Request blocked to prevent DNS rebinding attacks. Requests must send an Origin header matching an allowed origin; configure allowed origins via the ALLOWED_ORIGINS environment variable.",
});
return;
}
Expand Down