CVE-2026-34831
Rack has Content-Length mismatch in Rack::Files error responses
Impact
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| rubygems | rack | < 2.2.23 | 2.2.23 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Rack | Rack | before 2.2.23 from 3.0.0 before 3.1.21 from 3.2.0 before 3.2.6 |