OWASP CRS <3.3.9 / <4.25.0 File Upload Bypass via Whitespace
CVE-2026-33691 Published on April 2, 2026
OWASP CRS: Whitespace padding in filenames bypasses file upload extension checks
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.
Vulnerability Analysis
CVE-2026-33691 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, a high impact on integrity, and no impact on availability.
Weakness Type
Improper Handling of Case Sensitivity
The software does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.
Affected Versions
coreruleset:- Version < 3.3.9 is affected.
- Version >= 4.0.0-rc1, < 4.25.0 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.