Koa is middleware for Node.js using ES2017 async functions. Prior to versions 3.1.2 and 2.16.4, Koa's `ctx.hostname` API performs naive parsing of the HTTP Host header, extracting everything before the first colon without validating the input conforms to RFC 3986 hostname syntax. When a malformed Host header containing a `@` symbol is received, `ctx.hostname` returns `evil[.]com` - an attacker-controlled value. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable to Host header injection attacks. Versions 3.1.2 and 2.16.4 fix the issue.

Affected: npm: koa

## Summary

Koa's `ctx.hostname` API performs naive parsing of the HTTP Host header, extracting everything before the first colon without validating the input conforms to RFC 3986 hostname syntax. When a malformed Host header containing a `@` symbol (e.g., `evil.com:fake@legitimate.com`) is received, `ctx.hostname` returns `evil.com` - an attacker-controlled value. Applications using `ctx.hostname` for URL generation, password reset links, email verification URLs, or routing decisions are vulnerable to Host header injection attacks.

## Details

The vulnerability exists in Koa's hostname getter in `lib/request.js`:

```javascript // Koa 2.16.1 - lib/request.js get hostname() { const host = this.host; if (!host) return ''; if ('[' === host[0]) return this.URL.hostname || ''; // IPv6 literal return host.split(':', 1)[0]; } ```

The `host` getter retrieves the raw header value with HTTP/2 and proxy support:

```javascript // Koa 2.16.1 - lib/request.js get host() { const proxy = this.app.proxy; let host = proxy && this.get('X-Forwarded-Host'); if (!host) { if (this.req.httpVersionMajor >= 2) host = this.get(':authority'); if (!host) host = this.get('Host'); } if (!host) return ''; return host.split(',')[0].trim(); } ```

### The Problem

The parsing logic simply splits on the first `:` and returns the first segment. There is no validation that the resulting string is a valid hostname per RFC 3986 Section 3.2.2.

**RFC 3986 Section 3.2.2** defines the…

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Ein Angreifer kann mehrere Schwachstellen in IBM App Connect Enterprise ausnutzen, um Sicherheitsvorkehrungen zu umgehen, um einen Denial of Service Angriff durchzuführen, um Informationen offenzulegen, um Dateien zu manipulieren, um einen Cross-Site Scripting Angriff durchzuführen, um einen SQL-Injection Angriff durchzuführen, und um beliebigen Programmcode auszuführen.