Rack has an unsafe default in Rack::QueryParser allows params_limit bypass via semicolon-separated parameters

Exploitation

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

CVSS

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

EPSS

• CVE-2025-59830: 0.57% estimated exploitation activity in the next 30 days; percentile 47.41 (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::QueryParser in version < 2.2.18 enforces its paramslimit only for parameters separated by &, while still splitting on both & and ;. As a result, attackers could use ; separators to bypass the parameter count limit and submit more parameters than intended. Details The issue arises because Rack::QueryParser#checkquerystring counts only & characters when determining the number of parameters, but the default separator regex DEFAULTSEP = /[&;] /n splits on both & and ;. This mismatch means that queries using ; separators were not included in the parameter count, allowing paramslimit to be bypassed. Other safeguards (bytesizelimit and keyspacelimit) still applied, but did not prevent this particular bypass. Impact Applications or middleware that directly invoke Rack::QueryParser with its default configuration (no explicit delimiter) could be exposed to increased CPU and memory consumption. This can be abused as a limited denial-of-service vector. Rack::Request, the primary…

Published by the source

25 Sep 2025, 16:39 UTC

Last modified by the source

25 Sep 2025, 16:39 UTC

Source last checked

26 Oct 2026, 15:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-625h-95r8-8xpm

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# Patch Next advisory brief: CVE-2025-59830

Advisory: Rack has an unsafe default in Rack::QueryParser allows params_limit bypass via semicolon-separated parameters
IDs: CVE-2025-59830, GHSA-625h-95r8-8xpm
Record: https://patchnext.com/advisories/cve-2025-59830 (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: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rack 2.2.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems rack: affected < 2.2.18; fixed in 2.2.18

## 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-625h-95r8-8xpm (GitHub-reviewed advisory GHSA-625h-95r8-8xpm lists affected ranges: RubyGems rack >=0 <2.2.18)

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