Coreruleset 4.21.0 - Firewall Bypass

Coreruleset 4.21.0 - Firewall Bypass 🚨

An exploit titled ‘coreruleset 4.21.0 - Firewall Bypass’ has been published, detailing a critical vulnerability tracked as CVE-2026-21876 (EDB-ID: 52558). This web application bypass affects coreruleset versions prior to 4.22.0 and 3.3.8.

The exploit author is Daytrift Newgen, with the report dated 2026-05-13, and testing confirmed on Fedora and MacOS.

Key Components of the Exploit 🔑

• The reported bypass leverages specific handling of request bodies and content types.
• A key component is the _utf7_encode function, which processes text by converting characters to UTF-16-BE bytes, then Base64 encoding them, and appending a + prefix and - suffix to form a modified UTF-7 string.
• Additionally, the _form_urlencoded_to_multipart function is crucial. This function takes an application/x-www-form-urlencoded body and converts it into multipart/form-data.

How the Exploit Works ⚙️

The exploit’s handle function acts as a proxy, intercepting incoming web requests. It specifically modifies requests where the Content-Type header starts with application/x-www-form-urlencoded. In such cases, the request body is transformed using the _form_urlencoded_to_multipart function. This conversion changes the request’s Content-Type header to the new multipart type, while the values within the body are encoded with UTF-7 as previously described. The modified request is then forwarded to the upstream target.

The exploit also defines a set of HOP_BY_HOP_HEADERS which are explicitly excluded when copying headers for the upstream request, including ‘connection’, ‘keep-alive’, ‘proxy-authenticate’, ‘proxy-authorization’, ‘te’, ‘trailer’, ‘transfer-encoding’, and ‘upgrade’.

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