Security: README regex example allows unintended origin matching

[ekreloff]

Summary

The README's documented regex example for the origin option (/example\.com$/) matches unintended origins like evil-example.com, creating a CORS bypass when developers follow the documented pattern.

Reproduction

const cors = require('cors');

// Pattern from README line: "For example the pattern /example\.com$/ will reflect
// any request that is coming from an origin ending with 'example.com'."
const app = express();
app.use(cors({ origin: /example\.com$/ }));

Expected: Only example.com and its subdomains are allowed.
Actual: Any domain ending in example.com is allowed, including attacker-controlled domains.

https://example.com       → allowed ✓ (intended)
https://sub.example.com   → allowed ✓ (intended)
https://evil-example.com  → allowed ✓ (UNINTENDED)
https://notexample.com    → allowed ✓ (UNINTENDED)

The attacker registers evil-example.com → gets full CORS access to the API.

Root Cause

The regex /example\.com$/ only anchors the end ($) but not the beginning. Since browser Origin headers include the scheme (https://evil-example.com), the substring example.com matches anywhere after the scheme.

Interestingly, the test file (test/test.js:184) uses the correct pattern:

// From test file — properly anchored
{ origin: /:\/\/(.+\.)?example.com$/ }

This correctly rejects evil-example.com because :// anchors after the scheme, and (.+\.)? requires an optional subdomain preceded by a dot.

Impact

• Severity: Medium — CORS origin bypass via domain registration
• Scope: Any application that follows the README's regex example
• Likelihood: High — the README example is the first thing developers copy when configuring regex-based origin matching
• With credentials: true: Attacker's site can make authenticated cross-origin requests and read responses

Suggested Fix

Option 1 (minimal): Update the README example to use the pattern from the test file:

- For example the pattern `/example\.com$/` will reflect any request
- that is coming from an origin ending with "example.com".
+ For example the pattern `/:\/\/(.+\.)?example\.com$/` will reflect
+ any request from "example.com" or its subdomains.

Option 2 (better): Add a security note to the origin configuration docs:

> **⚠️ Security note:** When using RegExp origins, always anchor both the scheme
> and domain boundary to prevent unintended matches. `/example\.com$/` matches
> `evil-example.com` — use `/:\/\/(.+\.)?example\.com$/` instead.

Option 3 (best): Add a runtime console.warn when a RegExp origin lacks scheme anchoring:

if (allowedOrigin instanceof RegExp && !allowedOrigin.source.includes(':\\/\\/')) {
  console.warn('cors: RegExp origin %s may match unintended domains. ' +
    'Consider anchoring after the scheme (e.g., /:\\/\\/(.+\\.)?example\\.com$/)', allowedOrigin);
}

Verification

// Insecure (current README example)
/example\.com$/.test('https://evil-example.com')  // true — bypass!

// Secure (current test file pattern)
/:\/\/(.+\.)?example\.com$/.test('https://evil-example.com')  // false — blocked
/:\/\/(.+\.)?example\.com$/.test('https://sub.example.com')   // true — allowed
/:\/\/(.+\.)?example\.com$/.test('https://example.com')       // true — allowed

Environment

• cors version: latest (reviewed from master branch)
• Node.js: N/A (documentation issue)

[ekreloff]

This issue is part of a broader pattern I've documented after reviewing three high-profile npm libraries (node-jsonwebtoken, cors, multer — ~115M weekly downloads combined): libraries with secure code defaults that teach insecure patterns in their documentation.

In each case, the library itself handles security correctly, but the README examples demonstrate patterns that introduce vulnerabilities when copied by developers.

Full analysis: The Documentation Attack Surface