Rack:: Static header_rules bypass via URL-encoded paths

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS

• CVE-2026-34786: 0.20% estimated exploitation activity in the next 30 days; percentile 10.24 (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::Static#applicablerules evaluates several headerrules types against the raw URL-encoded PATHINFO, while the underlying file-serving path is decoded before the file is served. As a result, a request for a URL-encoded variant of a static path can serve the same file without the headers that headerrules were intended to apply. In deployments that rely on Rack::Static to attach security-relevant response headers to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the path. Details Rack::Static#applicablerules matches rule types such as :fonts, Array, and Regexp directly against the incoming PATHINFO. For example: These checks operate on the raw request path. If the request contains encoded characters such as %2E in place of ., the rule may fail to match even though the file path is later decoded and served successfully by the static file server. For example, both of the following requests may resolve to the same file on…

Published by the source

2 Apr 2026, 18:44 UTC

Last modified by the source

13 May 2026, 15:20 UTC

Source last checked

26 Oct 2026, 15:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-q4qf-9j86-f5mh

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# Patch Next advisory brief: CVE-2026-34786

Advisory: Rack:: Static header_rules bypass via URL-encoded paths
IDs: CVE-2026-34786, GHSA-q4qf-9j86-f5mh
Record: https://patchnext.com/advisories/cve-2026-34786 (revision 1, source checked 26 Oct 2026, 15:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
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, 2.2.23. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected < 2.2.23; >= 3.0.0.beta1, < 3.1.21; >= 3.2.0, < 3.2.6; fixed in 3.1.21, 2.2.23, 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-q4qf-9j86-f5mh (GitHub-reviewed advisory GHSA-q4qf-9j86-f5mh lists affected ranges: RubyGems rack >=0 =3.0.0.beta1 =3.2.0 <3.2.6)

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