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Css ParserApplication · Premailer

CVE-2026-53727

HIGH · 8.6 CVSS v3.1 Published 2026-07-17
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
css_parser is a Ruby CSS parser. From 2.2.0 until 3.0.0, CssParser::Parser#read_remote_file in lib/css_parser/parser.rb, and therefore load_uri! and the @import-following branch of add_block!, issued HTTP and HTTPS requests against any host, port, and URI without a scheme allowlist, host or IP filtering, or protection against link-local, loopback, or RFC-1918 addresses. Location: redirects were followed recursively back into the same function, which also serviced file:// URIs, so a single attacker-controlled HTTP redirect could upgrade the bug from SSRF to arbitrary local file disclosure. Any consumer of css_parser that hands it attacker-influenced CSS together with a base_uri: option is exposed. This issue is fixed in version 3.0.0.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The css_parser Ruby library versions 2.2.0 through 3.0.0 contains a Server-Side Request Forgery (SSRF) vulnerability in the read_remote_file, load_uri!, and add_block! functions. These methods make HTTP/HTTPS requests without any scheme allowlisting, host/IP filtering, or protection against internal networks (link-local, loopback, RFC-1918). Additionally, redirect following recursively loops back into the same function which also processes file:// URIs, allowing an attacker-controlled HTTP redirect to escalate to arbitrary local file disclosure.

MitigationUpgrade css_parser to version 3.0.0 or later. If upgrading is not immediately possible, avoid passing user-influenced CSS with a base_uri: option to css_parser, as this is the attack vector.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Css ParserApplication
Affected:= 2.2.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify css_parser version
    Run `gem list css_parser` or inspect your Gemfile.lock for the css_parser gem version
    Affected if version is 2.2.0 through 3.0.0 (inclusive)
  2. Locate base_uri usage in code
    Search your codebase for occurrences of `base_uri:` option passed to CssParser::Parser or any CSS parsing method
    Affected if base_uri: option is used and accepts user-controlled or untrusted input
  3. Identify vulnerable method calls
    Search for calls to `load_uri!` or `add_block!` methods in your codebase, particularly where the URI/CSS is derived from external input
    Affected if these methods are called with attacker-influencable URIs or CSS content
  4. Verify input source trust
    Trace the source of CSS/URI input to the css_parser calls and determine if it originates from untrusted sources (user uploads, web requests, external APIs)
    Affected if CSS or URI input can be controlled by an attacker
  5. Check for HTTP/HTTPS to file:// redirection
    Review if the application processes CSS from untrusted sources that could contain URLs redirecting to file:// URIs
    Affected if untrusted CSS sources are processed without sanitization

You are affected if css_parser version 2.2.0-3.0.0 is installed AND your application uses the base_uri: option with attacker-influenced CSS input that could be manipulated to trigger arbitrary HTTP requests or file:// URI access via redirection.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade css_parser to version 3.0.0 or later. If upgrading is not immediately possible, avoid passing user-influenced CSS with a base_uri: option to css_parser, as this is the attack vector.

Recommended fix High confidence

css_parser version 3.0.0 or later

  1. Update the css_parser gem to version 3.0.0 or later
  2. If using Bundler, update your Gemfile: gem 'css_parser', '>= 3.0.0'
  3. Run 'bundle update css_parser' to fetch the fixed version
  4. Verify the installed version with 'gem list css_parser'
  5. Test your application to ensure CSS parsing functionality works correctly after the upgrade
  6. If you maintain a lockfile, ensure it reflects the updated version
Caveat Major version bump from 2.x to 3.x - review the project's changelog for any breaking changes in API or behavior

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Css Parser Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA3.0 h
19.0 hours of engineering $3,380
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-53727 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data