CVE-2026-33691 OWASP CRS: Whitespace padding in filenames bypasses file upload extension checks

Medium 6.8 Published 2 Apr 2026 Updated 18 Apr 2026

Attackers can upload malicious files with dangerous extensions by using whitespace padding, bypassing OWASP CRS file upload checks.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-33691?

CVE-2026-33691 is a medium-severity vulnerability in coreruleset, published on 2 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

The OWASP core rule set (CRS) is a set of generic attack detection rules for use with compatible web application firewalls. Prior to versions 3.3.9 and 4.25.0, a bypass was identified in OWASP CRS that allows uploading files with dangerous extensions (.php, .phar, .jsp, .jspx) by inserting whitespace padding in the filename (e.g. photo. php or shell.jsp ). The affected rules do not normalize whitespace before evaluating the file extension regex, so the dot-extension check fails to match. This issue has been patched in versions 3.3.9 and 4.25.0.

Weakness: CWE-178

Which versions are affected by CVE-2026-33691?

It affects 1 product.

Linux distribution packages (1)

owasp-modsecurity-crs openSUSE:Tumbleweed All versions 4.29.0-1.1

Products include Owasp Owasp Modsecurity Core Rule Set.

How do I fix CVE-2026-33691?

Upgrade to a fixed version.

Is CVE-2026-33691 being exploited?

There is no known exploitation of CVE-2026-33691 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-33691, GHSA-rw5f-9w43-gv2w, UBUNTU-CVE-2026-33691, openSUSE-SU-2026:11632-1.