# CVE-2026-125109: Rack: Rack::Request#host_with_port (and #base_url / #url) return the raw invalid Host header — incomplete fix of CVE-2026-34835

Status: published. Published: 2026-11-09. Updated: 2026-11-09.

Rack is a modular Ruby web server interface. From 3.0 until 3.1.23 and 3.2.8, Rack::Request#host_with_port, #base_url, and #url can return an attacker-controlled invalid authority from Host, X-Forwarded-Host, or Forwarded: host= verbatim. Applications without an independent host allowlist that use these methods for absolute URLs, redirects, or origin-dependent checks can emit attacker-chosen delimiters, enabling redirect manipulation, string-based allowlist bypass, or unescaped markup reflection. Rails overrides these methods and is not expected to be affected. This issue is fixed in versions 3.1.23 and 3.2.8.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- rack / rack: >= 3.0, < 3.1.23 (affected); >= 3.2, < 3.2.8 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 3.1: 4.8 (medium); CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-1286

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/rack/rack/security/advisories/GHSA-2j8w-xcrc-x985 (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/rack/rack/commit/4380381a56dc52556a436d6d794c8e18dc8b0565 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/rack/rack/commit/49386f460286b7346c7003b69a839de782cc8a39 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/rack/rack/releases/tag/v3.1.23 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/rack/rack/releases/tag/v3.2.8 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-125109; retrieved 2026-11-09; source updated 2026-11-09

Revision: 6b0013c0d1201c27ee3631cc3bf20e5430ad1fe46cc4738aff111359a10a420c

Canonical: https://y2.dev/cve/CVE-2026-125109

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