CVE-2026-1527

ImpactWhen an application passes user-controlled input to the upgrade option of client.request(), an attacker can inject CRLF sequences (\r\n) to:

  *  Inject arbitrary HTTP headers
  *  Terminate the HTTP request prematurely and smuggle raw data to non-HTTP services (Redis, Memcached, Elasticsearch)
The vulnerability exists because undici writes the upgrade value directly to the socket without validating for invalid header characters:

// lib/dispatcher/client-h1.js:1121
if (upgrade) {
  header += `connection: upgrade\r\nupgrade: ${upgrade}\r\n`
}