Resource exhaustion in Rack

CVE-2026-34230

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.

Vulnerability class: DoS (Denial of Service)

EPSS: 0.004 (35.3th percentile) — read the EPSS interpretation.

CVSS v3 metric

CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.

Affected products

  • Rack — versions < 2.2.23, >= 3.0.0.beta1, < 3.1.21, >= 3.2.0, < 3.2.6

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-34230?
CVE-2026-34230 is a medium-severity vulnerability in Rack, classified under Uncontrolled Resource Consumption. CVSS score: 5.3/10. Published 2026-04-02.
How severe is CVE-2026-34230?
Medium severity. CVSS v3 base score is 5.3 out of 10.