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Medium, CVSS 4.8amended

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

CVE-2026-125109GHSA-2j8w-xcrc-x985

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (rack 3.2.8, 3.1.23), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems rack

    Affected
    >= 3.0.0, < 3.1.23>= 3.2.0, < 3.2.8
    Fixed in
    3.1.23, 3.2.8

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
4.8 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
EPSS
  • CVE-2026-125109: 0.22% estimated exploitation activity in the next 30 days; percentile 12.93 (FIRST EPSS, 2026-11-10). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary `Rack::Request#host_with_port`, and therefore `#base_url` and `#url`, return the raw authority string when it fails validation. An attacker-controlled `Host`, `X-Forwarded-Host`, or `Forwarded: host=` value containing characters that are not permitted in an authority is embedded verbatim in URLs generated by the application. This completes the fix for CVE-2026-34835 (GHSA-g2pf-xv49-m2h5), which made `#host` and `#hostname` return `nil` for an invalid authority but left this path unchanged.

Details `Rack::Request#host_with_port` splits the authority with `split_authority`. When the `AUTHORITY` pattern does not match, no components are returned and `host_with_port` falls back to returning the unvalidated authority string. `base_url` is `"#{scheme}://#{host_with_port}"` and `url` appends the full path, so both inherit the raw value. For a request with `Host: evil.com/@legit.com`, `host` returns `nil` while `host_with_port` returns `evil.com/@legit.com` and `base_url`…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-2j8w-xcrc-x985

  2. Revision 1 · created ·

    Published from ghsa record GHSA-2j8w-xcrc-x985

Agent brief

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# Patch Next advisory brief: CVE-2026-125109

Advisory: Rack: Rack::Request#host_with_port (and #base_url / #url) return the raw invalid Host header — incomplete fix of CVE-2026-34835
IDs: CVE-2026-125109, GHSA-2j8w-xcrc-x985
Record: https://patchnext.com/advisories/cve-2026-125109 (revision 2, source checked 11 Nov 2026, 07:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack 3.2.8, 3.1.23. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected >= 3.0.0, < 3.1.23; >= 3.2.0, < 3.2.8; fixed in 3.1.23, 3.2.8

## Suggested inspection (read-only)
- RubyGems rack: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rack.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2j8w-xcrc-x985 (GitHub-reviewed advisory GHSA-2j8w-xcrc-x985 lists affected ranges: RubyGems rack >=3.0.0 =3.2.0 <3.2.8)
- Verified: https://osv.dev/vulnerability/GHSA-2j8w-xcrc-x985 (OSV record GHSA-2j8w-xcrc-x985 lists affected ranges: RubyGems rack >=3.0.0 =3.2.0 <3.2.8 (fixed))

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