Vulnerability in Coreruleset
CVE-2026-33691
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.
EPSS: 0.036 (88.8th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 6.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:H/A:N.
Affected products
- Coreruleset — versions < 3.3.9, >= 4.0.0-rc1, < 4.25.0
- Owasp Owasp_modsecurity_core_rule_set
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Patch, Mitigation, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC, Issue Tracking)
- security-advisories@github.com (Patch, x_refsource_MISC, Issue Tracking)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Product, x_refsource_MISC, Release Notes)
- security-advisories@github.com (Product, x_refsource_MISC, Release Notes)
- af854a3a-2127-422b-91ae-364da2661108 (Mailing List, Third Party Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Mailing List, Third Party Advisory)
- af854a3a-2127-422b-91ae-364da2661108
Frequently asked questions
- What is CVE-2026-33691?
- CVE-2026-33691 is a medium-severity vulnerability in Coreruleset, classified under Improper Handling of Case Sensitivity. CVSS score: 6.8/10. Published 2026-04-02.
- How severe is CVE-2026-33691?
- Medium severity. CVSS v3 base score is 6.8 out of 10.