CVE-2026-34230: Rack

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Utils.select_best_encoding processes Accept-Encoding values with quadratic time complexity when the header contains many wildcard (*) entries. Because this method is used by Rack::Deflater to choose a response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header and cause disproportionate CPU consumption on the compression middleware path. This results in a denial of service condition for applications using Rack::Deflater. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

Affected products

• Rack Rack: before 2.2.23 (fixed in 2.2.23); from 3.0.0, before 3.1.21 (fixed in 3.1.21); from 3.2.0, before 3.2.6 (fixed in 3.2.6)

Published 2026-04-02. Last modified 2026-07-24.