Bypassing WAF via Charset Confusion: The Story of CVE-2026-21876

Q: What is the root cause of the CVE-2026-21876 vulnerability?

The root cause is a variable overwriting issue in OWASP CRS rule 922110. During multipart request parsing, the WAF writes the Content-Type and charset of each part into the same transaction-scoped variable. Since validation against the whitelist occurs only after processing all parts, only the content-type of the last part is actually validated, leaving preceding parts unchecked.

Q: Why does charset confusion with IBM037 or UTF-7 lead to a WAF bypass?

Attackers can encode their malicious payloads in non-standard charsets like IBM037 (an EBCDIC-based encoding) or UTF-7. The WAF does not decode or inspect these parts because their Content-Type validation is bypassed and the signature rules are written for ASCII/UTF-8. However, backend servers like Undertow or ASP.NET Core automatically support and decode these charsets, restoring the malicious payload to plain text on the backend.

Q: How does the OWASP Core Rule Set remediate this vulnerability?

The vulnerability was patched by rewriting the ruleset. Instead of overwriting a single variable, the new logic initializes a counter, stores the Content-Type of each multipart part in a separate, uniquely named variable, and then iterates through all stored variables to ensure every single one is validated against the whitelist.

Web Application Firewalls (WAFs) are critical components of modern web security. They sit between clients and backend servers, inspecting incoming traffic to detect and block malicious payloads such as SQL Injection (SQLi) and Cross-Site Scripting (XSS). However, a WAF is only as strong as its rules.

In this article, we analyze CVE-2026-21876, a high-severity vulnerability in the OWASP ModSecurity Core Rule Set (CRS). This flaw allowed attackers to completely bypass security checks by exploiting a variable overwriting bug combined with automatic charset decoding on modern application backends.

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Bypassing WAF via Charset Confusion: The Story of CVE-2026-21876

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