CVE-2026-53727: css_parser is a Ruby CSS parser

Q: How do I fix CVE-2026-53727?

css parser: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2026-53727 being exploited?

CVE-2026-53727 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-53727 affect?

CVE-2026-53727 affects css parser. Severity is high (CVSS 8.6). 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

Q: How urgent is CVE-2026-53727?

Priority 51/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

This week

HIGH 8.6REMOTEDOWNGRADE

CVE-2026-53727 is a high-severity flaw (CVSS 8.6) affecting css parser. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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, wh

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.6 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 58%)
CISA exploited list	No
Weakness	CWE-918
Published	2026-07-17
Last modified	2026-10-24

Publisher (CNA): GitHub, Inc. · Vendor: premailer

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-17 21:17 UTCPublished CVE record published
2026-10-24 20:10 UTCUpdated Record last modified

premailer css parser DOWNGRADE

Affected: 2.2.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.2.0

1. Check installed version

apt-cache policy css-parser || rpm -q css-parser || css-parser --version

2. No fixed release: DOWNGRADE to a release before 2.2.0 (Debian / Ubuntu)

apt-cache madison css-parser sudo apt install css-parser=<VERSION_BEFORE_2.2.0> sudo apt-mark hold css-parser

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list css-parser sudo dnf downgrade css-parser-<VERSION_BEFORE_2.2.0> sudo dnf versionlock add css-parser

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

RubyGems · css_parser · FIXED · 3.0.0 · vulnerable: < 3.0.0 · source

bundle update css_parser bundle list | grep css_parser

Fedora 46 · conda · FIXED · 26.9.4-2.fc46 · FEDORA-2026-09931756f1 · source

sudo dnf upgrade --advisory=FEDORA-2026-09931756f1

Debian 12 · ruby-css-parser · NO FIX YET

Debian 13 · ruby-css-parser · NO FIX YET

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-53727?

css parser: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-53727 being exploited?

CVE-2026-53727 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-53727 affect?

CVE-2026-53727 affects css parser. Severity is high (CVSS 8.6). 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

How urgent is CVE-2026-53727?

Priority 51/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Ruby vulnerabilities

References

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