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Fencer Intel
medium 4.8

CVE-2026-34835 json

Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.

published 7mo ago modified last mo. also GHSA-g2pf-xv49-m2h5 , EUVD-2026-18478

cvss
4.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 9%
public exploits
0
none known

Summary

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed. This can lead to host header poisoning in applications that use req.host, req.url, or req.base_url for link generation, redirects, or origin validation. This issue has been patched in versions 3.1.21 and 3.2.6.

Patch / fix

  • rubygems rack 3.1.21, 3.2.6
  • rack rack 3.1.21, 3.2.6
  • ruby-rack Debian 13 3.1.20-0+deb13u2
  • ruby-rack Debian 14 3.2.6-2

debian advisories DEBIAN-CVE-2026-34835

  • ruby-rack Ubuntu 25.10 3.1.16-0.1ubuntu0.3
  • ruby-rack Ubuntu 26.04 LTS 3.2.4-1ubuntu1.1

ubuntu advisories UBUNTU-CVE-2026-34835

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

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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-2026-34835 updated cvss 4.8

rack: Rack: Host header poisoning due to malformed Host header bypasses validation

  • 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
no
Technical impact
partial
Assessed
7mo ago

Exploit prediction

0.20% chance of exploitation in the next 30 days, higher than 9% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • github.com https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 4.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
GitHub 3.1 4.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected packages

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

Affected products

  • rack rack

Classification

Weakness
  • CWE-1286 Improper Validation of Syntactic Correctness of Input

References

Sources

Fencer Intel 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.