Executive brief
Rack is a foundational software library used by many Ruby web applications to handle web requests. A flaw in how it processes file uploads allows attackers to submit specially formatted filenames that contain hidden line-break characters. If an application later uses these filenames in its own web responses, it could lead to security issues like session hijacking or the delivery of malicious content to other users.
Technical details
A CRLF injection vulnerability exists in Rack::Multipart::Parser due to improper unfolding of 'obs-fold' sequences in multipart headers. When parsing multipart form data, the library fails to remove embedded carriage return and line feed (CRLF) characters from header parameters such as 'filename' or 'name'. An attacker can exploit this by submitting a multipart request with folded headers containing these sequences. If the resulting parsed values are subsequently used by the application to construct HTTP response headers without further sanitization, it can result in HTTP response splitting or header injection. This issue is patched in version 3.2.6.
Affected products
- Rack Rack >= 3.2.0, < 3.2.6
Timeline
- 2026-04-01: advisory: GitHub advisory published by maintainers
- 2026-04-02: disclosed: CVE published to NVD
- 2026-04-02: patched: Version 3.2.6 released to address the issue