eventsource-encoder encodes events as well-formed EventSource/Server Sent Event (SSE) messages. Prior to 1.0.2, eventsource-encoder does not sanitize the event or id fields of an EventSourceMessage before serializing them. An attacker who controls either field can inject arbitrary Server-Sent Events line terminators (\n, \r, or \r\n) and thereby forge additional SSE fields or entire messages on the stream. This vulnerability is fixed in 1.0.2.

Affected: npm: eventsource-encoder

### Summary

`eventsource-encoder` does not sanitize the `event` or `id` fields of an `EventSourceMessage` before serializing them. An attacker who controls either field can inject arbitrary Server-Sent Events line terminators (`\n`, `\r`, or `\r\n`) and thereby forge additional SSE fields or entire messages on the stream. This is similar in spirit to [GHSA-4hxc-9384-m385](https://github.com/advisories/GHSA-4hxc-9384-m385) (h3), but the vulnerable fields are `event`/`id` rather than `data`/`comment`. These are less likely to be user-controllable, but should still be sanitized.

### Details

In `src/encode.ts`, `encodeMessage` interpolates `event` and `id` into the output without inspecting them for line terminators:

```ts if (message.event) { output += `event: ${message.event}\n` } // ... if (typeof message.id === 'string' || typeof message.id === 'number') { output += `id: ${message.id}\n` } ```

The SSE specification treats `\r`, `\n`, and `\r\n` as line terminators. A `\n` (or `\r`) embedded in either field is rendered as the end of that field, allowing the rest of the input to be interpreted by the client as new SSE fields.

By contrast, `data` and `comment` already normalize all three line-terminator forms via `NEWLINES_RE = /(\r\n|\r|\n)/g`, so they are not affected.

### Proof of concept

```js import {encode} from 'eventsource-encoder'

// Attacker-controlled value flows into `event` const userSuppliedTopic = 'message\nevent: admin\ndata: {"role":"admin"}'

cons…