Ruby CSS Parser: SSRF and Local File Disclosure in CssParser::Parser#read_remote_file

Exploitation

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

CVSS 4.0

8.9 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N · score source

EPSS

• CVE-2026-53727: 0.51% estimated exploitation activity in the next 30 days; percentile 43.28 (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 CssParser::Parser#readremotefile (and therefore loaduri!, and the @import-following branch of addblock!) issues HTTP/HTTPS requests against any host, port and URI it is handed, with no scheme allowlist, no host / IP filtering, and no protection against link-local, loopback or RFC‑1918 addresses. Location: redirects are followed recursively back into the same function, which also services file:// URIs, so a single attacker-controlled HTTP redirect upgrades the bug from SSRF to arbitrary local file disclosure. In practice, any consumer of cssparser that hands it attacker‑influenced CSS together with a baseuri: option — Premailer being the canonical example — is exposed. The attacker only needs the ability to land one @import url(...) in the CSS that the host application parses. Vulnerable code lib/cssparser/parser.rb#L613-L687: There is no validation of uri.host, uri.scheme, or the resolved IP address — neither before the HTTP request, nor before the recursive call on the…

Published by the source

9 Jul 2026, 13:43 UTC

Last modified by the source

9 Jul 2026, 13:43 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-9pmc-p236-855h

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# Patch Next advisory brief: CVE-2026-53727

Advisory: Ruby CSS Parser: SSRF and Local File Disclosure in `CssParser::Parser#read_remote_file`
IDs: CVE-2026-53727, GHSA-9pmc-p236-855h
Record: https://patchnext.com/advisories/cve-2026-53727 (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: High (CVSS 8.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: css_parser 3.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems css_parser: affected < 3.0.0; fixed in 3.0.0

## Suggested inspection (read-only)
- RubyGems css_parser: 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 css_parser.
- 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-9pmc-p236-855h (GitHub-reviewed advisory GHSA-9pmc-p236-855h lists affected ranges: RubyGems css_parser >=0 <3.0.0)

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