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 (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:

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

import {encode} from 'eventsource-encoder'

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

console.log(encode({event: userSuppliedTopic, data: 'hello'}))

Output:

event: message
event: admin
data: {"role":"admin"}
data: hello

The browser sees two events: a forged admin event with attacker-chosen payload, followed by the legitimate message event. The same primitive works through id for any string id value.

Impact

If untrusted input is passed into the event or id field of a message, an attacker can:

• Spoof events of arbitrary type (rerouting payloads to handlers the attacker chooses)
• Inject additional SSE fields (data:, id:, retry:) into the stream
• Split a single encode() call into multiple distinct browser events
• Override the client's Last-Event-ID via injected id: lines

The vulnerability requires that an application places attacker-controlled data into event or id. Applications that only put trusted, statically-defined values into these fields are not affected.

Patches

Fixed in eventsource-encoder@1.0.2. The event and string id fields are now validated; any value containing \r or \n causes the encoder to throw a TypeError rather than emit a malformed stream.

Workarounds

If users cannot upgrade, validate or strip line terminators from any untrusted value before passing it to encode / encodeMessage:

function safeSingleLine(value) {
  if (/[\r\n]/.test(value)) throw new Error('SSE field must be single-line')
  return value
}

encode({event: safeSingleLine(topic), id: safeSingleLine(id), data})

Resources

• Related advisory (different package, same class): GHSA-4hxc-9384-m385
• SSE spec, line terminators: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream

Credit

Discovered while reviewing in light of GHSA-4hxc-9384-m385.

References

• GHSA-m9g3-3g99-mhpx
• https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream
• https://nvd.nist.gov/vuln/detail/CVE-2026-44214