H3 is a minimal H(TTP) framework. In versions prior to 1.15.6 and between 2.0.0 through 2.0.1-rc.14, createEventStream is vulnerable to Server-Sent Events (SSE) injection due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). An attacker who controls any part of an SSE message field (id, event, data, or comment) can inject arbitrary SSE events to connected clients. This issue is fixed in versions 1.15.6 and 2.0.1-rc.15.

Affected: npm: h3

## Summary

`createEventStream` in h3 is vulnerable to Server-Sent Events (SSE) injection due to missing newline sanitization in `formatEventStreamMessage()` and `formatEventStreamComment()`. An attacker who controls any part of an SSE message field (`id`, `event`, `data`, or comment) can inject arbitrary SSE events to connected clients.

## Details

The vulnerability exists in `src/utils/internal/event-stream.ts`, lines [170](https://github.com/h3js/h3/blob/52c82e18bb643d124b8b9ec3b1f62b081f044611/src/utils/internal/event-stream.ts#L170)-[187](https://github.com/h3js/h3/blob/52c82e18bb643d124b8b9ec3b1f62b081f044611/src/utils/internal/event-stream.ts#L187):

```typescript export function formatEventStreamComment(comment: string): string { return `: ${comment}\n\n`; }

export function formatEventStreamMessage(message: EventStreamMessage): string { let result = ""; if (message.id) { result += `id: ${message.id}\n`; } if (message.event) { result += `event: ${message.event}\n`; } if (typeof message.retry === "number" && Number.isInteger(message.retry)) { result += `retry: ${message.retry}\n`; } result += `data: ${message.data}\n\n`; return result; } ```

The SSE protocol (defined in the [WHATWG HTML spec](https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation)) uses newline characters (`\n`) as field delimiters and double newlines (`\n\n`) as event separators.

None of the fields (`id`, `event`, `data`, comment)…