CVE-2026-33661 is a high-severity (CVSS 8.6) CWE-290 vulnerability in yansongda/pay. O3 Security confirms whether CVE-2026-33661 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
WeChat Pay callback signature verification bypassed when Host header is localhost
Real-World Exposure
yansongda/payReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Pay is an open-source payment SDK extension package for various Chinese payment services. Prior to version 3.7.20, 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. Version 3.7.20 fixes the issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | yansongda/pay | all versions | 3.7.20 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for yansongda/pay. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update yansongda/pay to 3.7.20 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33661 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-33661 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-33661. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-33661 in your dependencies?
O3 detects CVE-2026-33661 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.