Executive brief
Rack is a widely used interface for Ruby web applications that helps manage communication between web servers and application code. A flaw in how it handles error messages for missing files can cause the server to report an incorrect data size to the web browser or proxy. This mismatch can lead to 'response desynchronization,' potentially allowing an attacker to interfere with web traffic or bypass certain security filters in complex network environments.
Technical details
A vulnerability exists in Rack::Files#fail where the Content-Length header is calculated using String#size (character count) instead of String#bytesize (byte count). When a 404 error occurs, Rack reflects the requested path in the response body; if that path contains percent-encoded multibyte UTF-8 characters, the actual byte size of the body will exceed the declared Content-Length. This discrepancy violates HTTP framing standards. An attacker can exploit this by requesting non-existent paths with specific UTF-8 characters to trigger response desynchronization or request smuggling in environments with intermediaries (proxies/load balancers) that rely on the Content-Length header for message delimitation. The issue is patched in versions 2.2.23, 3.1.21, and 3.2.6.
Affected products
- Rack Rack < 2.2.23, >= 3.0.0.beta1 < 3.1.21, >= 3.2.0 < 3.2.6
Timeline
- 2026-04-01: advisory: GitHub Security Advisory published
- 2026-04-02: disclosed: CVE published to NVD
- 2026-04-02: patched: Patched versions released