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GHSA-m3x9-938q-5767

Loofah: CSS url() sanitization bypass via escaped parentheses

Products / packages flavorjones loofah · loofah

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Severity and exploitation

Not in CISA KEVSource: MODERATE

KEV coverage does not rule out exploitation.

KEV checked 15/11/2026, 03:35:31 UTC
OSV · CVSS:3.1 · 4.7
Assessment vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N
GitHub_M · CVSS:3.1 · 4.7
Assessment vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N
The headline uses the highest imported catalog score. Assessments can differ by source and CVSS version.

Severity

medium4.7

CVSS measures technical severity, not the likelihood of an attack.

Known exploitation

Not listed in the checked CISA KEV catalog

Absence from CISA KEV does not rule out attacks.

Public exploit

Not established by this record

Missing evidence does not mean no public exploit exists.

Remediation

Package fixes recorded

Fix versions are recorded for 1 of 1 mapped packages. Check each release branch and any separate product guidance.

What this means

Loofah::HTML5::Scrub.scrub_css rejected any property whose top-level Crass tokens included a :url or :bad_url, and left url out of the allowed CSS functions. Both checks ran only against the first parse of the style attribute. When scrub_css rebuilt each property it emitted identifiers with their CSS escapes decoded, so parentheses written as escapes — url\28 …\29 — hid the url token from the checks, and the rebuilt value was re-parsed into a live url() that nothing checked again. An escaped property name, six-digit escapes, escaped comments and escaped quotes all reached the same place.

Source excerpt · OSV

Attack conditions

No attack-condition excerpt has been identified here. Review the full source description and references for authentication, network access, and configuration requirements.

Review supporting sources →

Affected versions

Check the exact model, release, and any platform requirements against your installation.

flavorjones · GitHub_M

loofah

1 range
GitHub_M version evidence

Compare with this publisher’s ranges. This does not inspect your installation.

affected

< 2.25.3

unaffected

unknown

Other versions: unknown

Packages

Compare each installed version with the publisher’s affected ranges. Where a fix is recorded, choose the appropriate release branch and test compatibility before upgrading.

loofah RubyGems
Affected range Recorded fix
Earlier releases before 2.25.3Ecosystem versions2.25.3

Version evidence: OSV · GHSA-m3x9-938q-5767

View original version evidence (65 explicit versions)

What to do

Compare your installed product and version with the affected versions above. A matching product name alone does not confirm exposure.

Use the recorded fix for your release branch. Review the publisher’s instructions and test compatibility before upgrading.

cve.new guidance unless attributed to a publisher. This page does not inspect your installation.

Public exploit evidence

No public exploit reference is identified in the imported evidence. This is not a claim that no exploit exists.

Original source description

Read the full publisher description

Summary

Loofah::HTML5::Scrub.scrub_css rejected any property whose top-level Crass tokens included a :url or :bad_url, and left url out of the allowed CSS functions. Both checks ran only against the first parse of the style attribute. When scrub_css rebuilt each property it emitted identifiers with their CSS escapes decoded, so parentheses written as escapes — url\28 …\29 — hid the url token from the checks, and the rebuilt value was re-parsed into a live url() that nothing checked again. An escaped property name, six-digit escapes, escaped comments and escaped quotes all reached the same place.

Impact

A sanitized style attribute could make a viewer's browser issue a request to an attacker-chosen URL. list-style fetches on render with no interaction, cursor on hover. That leaks the viewer's IP address, user agent and access timing, and can reach hosts routable only from the viewer's network. No script survives sanitization and the attacker cannot read the response, so the request itself is the whole impact.

Applications that sanitize user-supplied HTML with Loofah's default allowlist and display it to other people are affected.

Mitigation

Upgrade to Loofah >= 2.25.3.

Credit

Reported by NotAFlightRisk through GitHub's private vulnerability reporting.

References

6 sources
flavorjones/loofah · security/advisories/GHSA-m3x9-938q-5767
Security advisoryGitHub_M
flavorjones/loofah · pull request #324
PatchGitHub_M
flavorjones/loofah · commit b198a36
PatchGitHub_M
flavorjones/loofah · repository
Source reference
flavorjones/loofah · releases/tag/v2.25.3
Release notesGitHub_M
rubysec/ruby-advisory-db · GHSA-m3x9-938q-5767.yml
Source reference

Inside the record

Evidence available

Package identity
Version evidence
Remediation guidance · not supplied
Weakness classification
Additional publishers

Describes the evidence in this view, not whether the product is safe.

Original records

CVE-2026-126207

View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.

cve.new JSON →

Underlying weaknesses

The source’s classification of what caused the vulnerability.

CWE-180 ↗

CWE-180: Incorrect Behavior Order: Validate Before Canonicalize

GitHub_M

Related →
CWE-184 ↗

CWE-184: Incomplete List of Disallowed Inputs

GitHub_M

Related →
Contributing organizations · 2
GitHub_M

Assigning organization

2026-11-12
CISA-ADP

Additional data publisher

2026-11-13

Timeline

All times UTC

  1. Advisory published

    Publication date supplied by the source.

  2. Source updated

    Latest modification date supplied by the source.

  3. Last imported by cve.new

    Latest catalog import, not the date we first saw this advisory.

What changed

Follow changes
  1. CVE · CVE-2026-126207

    Observed 2026-11-13T20:27:24.846Z · upstream updated 2026-11-13T19:43:00.397Z

    Prioritization updated

    before

    []

    after

    [
  {
    "source": "CISA-ADP",
    "type": "SSVC",
    "role": "CISA Coordinator",
    "timestamp": "2026-11-13T19:42:04.755994Z",
    "decisions": [
      {
        "name": "Exploitation",
        "value": "none"
      },
      {
        "name": "Automatable",
        "value": "no"
      },
      {
        "name": "Technical Impact",
        "value": "partial"
      }
    ]
  }
]

Large field values are marked as truncated previews. Original source snapshots remain retained.

Showing up to 50 most recent observed changes.