CVE-2026-125109 is a medium-severity (CVSS 4.8) CWE-1286 vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.
Rack: Rack::Request#host_with_port (and #base_url / #url) return the raw invalid Host header — incomplete fix of CVE-2026-34835
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
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Description
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 returns http://evil.com/@legit.com. A valid Host together with an invalid X-Forwarded-Host produces the same result, because forwarded headers take priority by default.
Impact
Applications that build absolute URLs, redirects, or origin-dependent checks from req.url, req.base_url, or req.host_with_port, and that do not enforce a host allowlist, may emit URLs containing attacker-chosen characters such as @, /, ?, #, spaces, or angle brackets. Generated links and Location headers can be redirected through userinfo tricks, string-based URL checks can be bypassed, and markup is reflected where the application does not escape the value. For example, redirect in Sinatra behind a proxy that forwards X-Forwarded-Host emits the raw invalid authority in Location.
Rails overrides these methods in Action Dispatch and is not expected to be affected. Rack 3.1 and 3.2 are affected. Rack 2.2 did not receive the CVE-2026-34835 change and returns the raw value from every host method.
Mitigation
- Update to a patched version of Rack, in which
host_with_portreturnsnilfor an invalid authority andbase_urlfalls back to the server's own name. - Enforce a host allowlist with
Rack::Protection::HostAuthorization,config.hostsin Rails, or at the reverse proxy, and configure the proxy to overwriteX-Forwarded-HostandForwarded. - Pin the authority with the
rack.request.config:authoritykey where available.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | rack | ≥ 3.0.0&&< 3.1.23 | 3.1.23bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.2.0&&< 3.2.8 | 3.2.8bundle update rack --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rack to 3.1.23 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-125109 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-125109 in your dependencies?
Find it across RubyGems, including transitive dependencies.