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

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-2026-34835: 0.20% estimated exploitation activity in the next 30 days; percentile 9.87 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. For example, a check such as req.host.startwith?("myapp.com") can be bypassed with Host: myapp.com@evil.com, and a check such as req.host.endwith?("myapp.com") can be bypassed with Host: evil.com/myapp.com. This can lead to host header poisoning in applications that use req.host, req.url, or req.baseurl for link generation, redirects, or origin validation. Details Rack::Request parses the authority component using logic equivalent to: The character class used for non-IPv6 hosts accepts nearly all printable characters except [ and ]. This includes reserved URI delimiters such as @, /, ?, and #, which are not valid hostname characters under RFC 3986 host syntax. As a result, values…

Published by the source

2 Apr 2026, 20:36 UTC

Last modified by the source

13 May 2026, 16:19 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g2pf-xv49-m2h5

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# Patch Next advisory brief: CVE-2026-34835

Advisory: Rack::Request accepts invalid Host characters, enabling host allowlist bypass
IDs: CVE-2026-34835, GHSA-g2pf-xv49-m2h5
Record: https://patchnext.com/advisories/cve-2026-34835 (revision 1, source checked 26 Oct 2026, 15:52 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 3.2.6, 3.1.21. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected >= 3.0.0.beta1, < 3.1.21; >= 3.2.0, < 3.2.6; fixed in 3.1.21, 3.2.6

## 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-g2pf-xv49-m2h5 (GitHub-reviewed advisory GHSA-g2pf-xv49-m2h5 lists affected ranges: RubyGems rack >=3.0.0.beta1 =3.2.0 <3.2.6)

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