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A vulnerability was identified in OWASP CRS where whitespace padding
in filenames can bypass file upload extension checks, allowing uploads
of dangerous files such as .php, .phar, .jsp, and .jspx. This issue
has been assigned CVE‑2026‑33691.

Impact: Attackers may evade CRS protections and upload web shells
disguised with whitespace‑padded extensions. Exploitation is most
practical on Windows backends that normalize whitespace in filenames
before execution, In linux harder because it require a backend that
use like `.strip()` and `.trim()` and other whitespace trimming
methods depending on the language here vulnerable to that or the
webserver strip whitespaces or the backend on general, If not they not
vulnerable to that.

Fix: Patched in CRS v3.3.9, v4.25.x LTS, and v4.8.x. Security fixes
are always backported to supported branches.

References:

Full advisory: https://github.com/coreruleset/coreruleset/security/advisories/GHSA-rw5f-9w43-gv2w

Credits: Reported by RelunSec (aka @HackingRepo on Github).
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• [CVE-2026-33691] OWASP CRS whitespace padding bypass vulnerability cyber security (Apr 02)