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Fencer Vulns
medium 5.3

CVE-2026-34230 json

Rack: Quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header

published 2026-04-02 7 mo ago modified 2026-09-10 1 mo ago also EUVD-2026-18378 , GHSA-v569-hp3g-36wr

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 39%
public exploits
0
none known

Summary

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.

Patch / fix

  • rubygems rack 2.2.23, 3.1.21, 3.2.6
  • rack rack 2.2.23, 3.1.21, 3.2.6
  • rack_project rack 2.2.23, 3.1.21, 3.2.6
  • ruby-rack Debian 12 2.2.22-0+deb12u2
  • ruby-rack Debian 13 3.1.20-0+deb13u2
  • ruby-rack Debian 14 3.2.6-2

debian advisories DEBIAN-CVE-2026-34230

  • ruby-rack Ubuntu 24.04 LTS 2.2.7-1ubuntu0.7
  • ruby-rack Ubuntu 25.10 3.1.16-0.1ubuntu0.3
  • ruby-rack Ubuntu 26.04 LTS 3.2.4-1ubuntu1.1
  • ruby-rack Ubuntu Pro 14.04 LTS 1.5.2-3+deb8u3ubuntu1~esm11
  • ruby-rack Ubuntu Pro 16.04 LTS 1.6.4-3ubuntu0.2+esm10
  • ruby-rack Ubuntu Pro 18.04 LTS 1.6.4-4ubuntu0.2+esm10
  • ruby-rack Ubuntu Pro 22.04 LTS 2.1.4-5ubuntu1.2+esm3

ubuntu advisories UBUNTU-CVE-2026-34230

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • rmt-server SLES for SAP applications 16.0, SLES 16.0 3.1.0-160000.1.1
  • rmt-server SLES for SAP 15 SP6, SLES for SAP 15 SP5, SLES 15 SP6-LTSS +5 more 2.27-150500.3.47.1

suse advisories SUSE-SU-2026:23321-1SUSE-SU-2026:2487-1

  • rmt-server openSUSE Leap 16.0 3.1.0-160000.1.1
  • rubygem-rack-2.2 openSUSE Tumbleweed 2.2.23-1.1

opensuse advisories openSUSE-SU-2026:21640-1openSUSE-SU-2026:10508-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-34230 updated cvss 5.3

rack: Rack: Denial of Service via crafted Accept-Encoding header

  • Red Hat Logging Subsystem for Red Hat OpenShift openshift-logging/vector-rhel9, openshift-logging/fluentd-rhel9, openshift-logging/eventrouter-rhel9, openshift-logging/logging-view-plugin-rhel9, openshift-logging/cluster-logging-rhel9-operator, openshift-logging/log-file-metric-exporter-rhel9, +1 more Red Hat 3scale API Management Platform 2 3scale-amp21/zync, 3scale-amp22/zync, 3scale-amp24/zync, 3scale-amp25/zync, 3scale-amp26/zync, 3scale-amp20/system, +19 more Red Hat Enterprise Linux 7 pcs, pcs-snmp, python-clufter Red Hat Enterprise Linux 8 pcs, pcs-snmp Red Hat Enterprise Linux 9 pcs, pcs-snmp, ubi9/ruby-30, ubi9/ruby-33, rhel9/ruby-30, rhel9/ruby-33 Red Hat Enterprise Linux 10 pcs
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.45% chance of exploitation in the next 30 days, higher than 39% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
rubygems rack
<2.2.23
>=3.0.0.beta1, <3.1.21
>=3.2.0, <3.2.6
2.2.23
3.1.21
3.2.6

Affected products

  • rack rack

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption
  • CWE-407 Inefficient Algorithmic Complexity

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.