What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 3.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-02 · updated 2026-07-24
• Weakness: CWE-178

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.

Affected products

• coreruleset: before 3.3.9; from 4.0.0 before 4.25.0 — fixed in 3.3.9, 4.25.0

How to fix

Upgrade to a fixed version: coreruleset 3.3.9, 4.25.0. Check the vendor advisory for mitigations.

References

• https://github.com/coreruleset/coreruleset/commit/2a8c63512811c5dd74472becebb79a783e68ff02
• https://github.com/coreruleset/coreruleset/pull/4546
• https://github.com/coreruleset/coreruleset/pull/4547
• https://github.com/coreruleset/coreruleset/pull/4548
• https://github.com/coreruleset/coreruleset/releases/tag/v3.3.9
• https://github.com/coreruleset/coreruleset/releases/tag/v4.25.0
• https://github.com/coreruleset/coreruleset/security/advisories/GHSA-rw5f-9w43-gv2w
• http://seclists.org/fulldisclosure/2026/Apr/0
• http://www.openwall.com/lists/oss-security/2026/03/29/2
• http://www.openwall.com/lists/oss-security/2026/04/18/4

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