CVE-2026-32257: Winter Cross-Site Scripting Vulnerability

HIGH
Advisory Identifier: GHSA-v7cf-8gh9-gxmj
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CVSS 8.1
CWE-79
Published
Advisory Summary

CVE-2026-32257 is a HIGH (CVSS 8.1) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Winter libraries in the Packagist ecosystem. The issue involves stored xss through brand settings custom styles. Remediation is available by upgrading affected packages to version 1.2.13.

CVE-2026-32257 at a Glance

CVE Identifier CVE-2026-32257
GHSA Identifier GHSA-v7cf-8gh9-gxmj
Severity HIGH
CVSS v3.1 Base Score 8.1
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Winter
Package Ecosystem Packagist
Published Date
Last Updated

Security Impact

CVSS: 8.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required High (PR:H)
User Interaction Required (UI:R)
Scope Changed (S:C)
Confidentiality High (C:H)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2026-32257

To remediate this vulnerability, upgrade all affected Winter dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

winter/wn-backend-module fixed: 1.2.13 
Upgrade winter/wn-backend-module to 1.2.13
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for winter/wn-backend-module

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-32257, inspect your project dependencies for Winter:

inspect dependencies for winter/wn-backend-module

Analyze the terminal output to locate any version of Winter in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.2.13), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. In Winter, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact Users with the `backend.manage_branding` ("Customize the back-end") permission can provide custom CSS through **Settings → Customize Backend → Styles** that is compiled through the LESS CSS parser and rendered on every backend page. Previously, the compiled output was not sanitized, which could have allowed a stored XSS attack. Although this is a valid security issue, it's important to note that its severity is relatively low. To exploit the vulnerability, an attacker would already need to have trusted access to the Winter CMS backend with a specific administrative permission. The Winter CMS maintainers recommend that the `backend.manage_branding` permission only be granted to trusted administrators and developers. A related issue exists in the custom styles field at **Settings → Editor Settings → Markup Styles** (`backend.manage_editor` permission), which uses the same rendering mechanism and is addressed separately. All users are advised to update to the latest version to ensure their systems remain secure. ### Patches This issue has been patched in v1.2.13. ### Workarounds Apply [https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720](https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720) manually if unable to upgrade to v1.2.13. ### Resources - CVE-2025-61676 (identical vulnerability in October CMS, patched in v3.7.13 and v4.0.12)

CVE-2026-32257 Affected Winter Versions

1 component(s)
Fixed Versions:
1.2.13
Product / Package Ecosystem Affected Versions Fixed Version
winter/wn-backend-module
 Packagist
< 1.2.13
1.2.13

References

GitHub Advisory / Wiki ADVISORY
https://github.com/wintercms/winter/security/advisories/GHSA-v7cf-8gh9-gxmj
Security Fix Commit FIX COMMIT
https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720
Winter Source Repository REPOSITORY
https://github.com/wintercms/winter

Frequently Asked Questions

What is CVE-2026-32257?
CVE-2026-32257 is a security vulnerability in Winter involving Stored XSS through Brand Settings custom styles under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Winter versions are affected?
Affected versions include releases prior to 1.2.13 across affected packages: winter/wn-backend-module.
What version fixes CVE-2026-32257?
Upgrading to 1.2.13 (or the latest patched release for your respective branch) resolves the vulnerability.
Is winter/wn-backend-module affected?
Yes, winter/wn-backend-module is affected in releases prior to 1.2.13. Upgrade your dependency to 1.2.13 or later.
What is the CVSS score of CVE-2026-32257?
CVE-2026-32257 has a CVSS v3.1 base score of 8.1 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:N.
What CWE is associated with CVE-2026-32257?
CVE-2026-32257 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check Packagist dependencies for CVE-2026-32257?
Run the dependency tree command: 'inspect dependencies for winter/wn-backend-module'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v7cf-8gh9-gxmj",
  "modified" : "2026-08-12T14:40:21Z",
  "published" : "2026-08-12T14:40:21Z",
  "aliases" : [ "CVE-2026-32257" ],
  "summary" : "Winter: Stored XSS through Brand Settings custom styles",
  "details" : "### Impact                                                                                                       \n\nUsers with the `backend.manage_branding` (\"Customize the back-end\") permission can provide custom CSS through **Settings → Customize Backend → Styles** that is compiled through the LESS CSS parser and rendered on every backend page. Previously, the compiled output was not sanitized, which could have allowed a stored XSS attack.\n\nAlthough this is a valid security issue, it's important to note that its severity is relatively low. To exploit the vulnerability, an attacker would already need to have trusted access to the Winter CMS backend with a specific administrative permission. The Winter CMS maintainers recommend that the `backend.manage_branding` permission only be granted to trusted administrators and developers.\n\nA related issue exists in the custom styles field at **Settings → Editor Settings → Markup Styles** (`backend.manage_editor` permission), which uses the same rendering mechanism and is addressed separately.\n\nAll users are advised to update to the latest version to ensure their systems remain secure.\n\n### Patches\n\nThis issue has been patched in v1.2.13.\n\n### Workarounds\n\nApply [https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720](https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720) manually if unable to upgrade to v1.2.13.\n\n### Resources\n\n- CVE-2025-61676 (identical vulnerability in October CMS, patched in v3.7.13 and v4.0.12)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:H/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "winter/wn-backend-module"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.2.13"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.2.12"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/wintercms/winter/security/advisories/GHSA-v7cf-8gh9-gxmj"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/wintercms/winter/commit/d28f0b9474af79cfaa80eeb9d691f7a7c4469720"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/wintercms/winter"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-12T14:40:21Z",
    "nvd_published_at" : null
  }
}