{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.0.1"
      },
      "package": {
        "ecosystem": "npm",
        "name": "eventsource-encoder"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.0.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44214"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-113",
      "CWE-93"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-08T20:49:40Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\n\n`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.\n\n### Details\n\nIn `src/encode.ts`, `encodeMessage` interpolates `event` and `id` into the output without inspecting them for line terminators:\n\n```ts\nif (message.event) {\n  output += `event: ${message.event}\\n`\n}\n// ...\nif (typeof message.id === \u0027string\u0027 || typeof message.id === \u0027number\u0027) {\n  output += `id: ${message.id}\\n`\n}\n```\n\nThe 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.\n\nBy contrast, `data` and `comment` already normalize all three line-terminator forms via `NEWLINES_RE = /(\\r\\n|\\r|\\n)/g`, so they are not affected.\n\n### Proof of concept\n\n```js\nimport {encode} from \u0027eventsource-encoder\u0027\n\n// Attacker-controlled value flows into `event`\nconst userSuppliedTopic = \u0027message\\nevent: admin\\ndata: {\"role\":\"admin\"}\u0027\n\nconsole.log(encode({event: userSuppliedTopic, data: \u0027hello\u0027}))\n```\n\nOutput:\n\n```\nevent: message\nevent: admin\ndata: {\"role\":\"admin\"}\ndata: hello\n\n```\n\nThe 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.\n\n### Impact\n\nIf untrusted input is passed into the `event` or `id` field of a message, an attacker can:\n\n- Spoof events of arbitrary type (rerouting payloads to handlers the attacker chooses)\n- Inject additional SSE fields (`data:`, `id:`, `retry:`) into the stream\n- Split a single `encode()` call into multiple distinct browser events\n- Override the client\u0027s `Last-Event-ID` via injected `id:` lines\n\nThe 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.\n\n### Patches\n\nFixed 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.\n\n### Workarounds\n\nIf users cannot upgrade, validate or strip line terminators from any untrusted value before passing it to `encode` / `encodeMessage`:\n\n```js\nfunction safeSingleLine(value) {\n  if (/[\\r\\n]/.test(value)) throw new Error(\u0027SSE field must be single-line\u0027)\n  return value\n}\n\nencode({event: safeSingleLine(topic), id: safeSingleLine(id), data})\n```\n\n### Resources\n\n- Related advisory (different package, same class): https://github.com/advisories/GHSA-4hxc-9384-m385\n- SSE spec, line terminators: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream\n\n### Credit\n\nDiscovered while reviewing in light of GHSA-4hxc-9384-m385.",
  "id": "GHSA-m9g3-3g99-mhpx",
  "modified": "2026-05-08T20:49:40Z",
  "published": "2026-05-08T20:49:40Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/rexxars/eventsource-encoder/security/advisories/GHSA-m9g3-3g99-mhpx"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/rexxars/eventsource-encoder"
    },
    {
      "type": "WEB",
      "url": "https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "eventsource-encoder vulnerable to SSE event injection via unsanitized `event` and `id` fields"
}