How severe is CVE-2026-33661?

CVE-2026-33661 has a CVSS score of 8.6/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-33661?

CVE-2026-33661 affects the following packages: yansongda/pay (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-33661?

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.

How do I detect CVE-2026-33661 in my Packagist dependencies?

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.

How do I mitigate CVE-2026-33661 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2026-33661?

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.

Is CVE-2026-33661 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-33661 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-33661?

CVE-2026-33661 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 39th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-33661?

CVE-2026-33661 is classified as CWE-290 (CWE-290). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-33661 published, and has it been updated?

CVE-2026-33661 was published on March 26, 2026 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘 Packagist

CVE-2026-33661

HIGH

WeChat Pay callback signature verification bypassed when Host header is localhost

Also known asGHSA-q938-ghwv-8gvc

Published

Mar 26, 2026

Updated

Apr 10, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk39th percentile+0.49%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

1 pkg affected

🐘yansongda/pay

Real-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

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`yansongda/pay`	all versions	3.7.20

Detection & mitigation playbook

Open-source dependency

Detect
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

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

O3 Security · Impact-Aware SCA

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.

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