# CVE-2026-54256: Winter: Authenticated IDOR in backend FileUpload widget allows cross-user access to attachment metadata

> Authenticated users can read and modify other users' attachment metadata due to missing authorization checks in the FileUpload widget.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 27 Aug 2026
- **Also known as:** GHSA-3277-h8g9-qj5f

## What is CVE-2026-54256?

CVE-2026-54256 is a medium-severity missing authorization vulnerability in wintercms winter, published on 26 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Winter CMS is a content management system built on the Laravel PHP framework. In versions up to and including 1.2.12, the backend FileUpload form widget trusted an attacker-controlled file_id POST parameter when resolving the attachment it operates on, allowing an authenticated backend user to read and modify attachment records belonging to other users or records. The widget's getFileRecord() lookup resolved the posted id against the global system_files table without verifying that the file belonged to the widget's own relation, parent record, or deferred-binding session. Because all attachments share a single File model and table and attachment ids are sequential integers that are easily enumerated, a user reaching any form with a fileupload field, including the built-in My Account avatar field that requires no specific permission, could target arbitrary attachments to modify their title and description via onSaveAttachmentConfig and change their sort order via onSortAttachments, which passed posted ids straight to an unscoped update. CSRF tokens remain enforced, so exploitation requires a valid authenticated backend session with any level of access. This issue is fixed in version 1.2.13.

Weakness: CWE-639 (Authorization Bypass Through User-Controlled Key), CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2026-54256?

2 open source packages ship an affected version, including winter/wn-backend-module.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `winter/wn-backend-module` | Packagist | All versions | 1.2.13 |
| `winter/wn-backend-module` | Packagist | All versions | 1.2.13 |

## How do I fix CVE-2026-54256?

Upgrade winter/wn-backend-module to 1.2.13 or later.

## Is CVE-2026-54256 being exploited?

There is no known exploitation of CVE-2026-54256 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54256 and fix it if it is affected.

What it is: CVE-2026-54256 is a medium-severity missing authorization vulnerability in wintercms winter, published on 26 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-54256 in the wild and no public exploit code.

Affected packages and fixed versions:
- winter/wn-backend-module (Packagist): affected all versions; fixed in 1.2.13
- winter/wn-backend-module (Packagist): affected all versions; fixed in 1.2.13

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54256/ (Markdown: https://insights.sbomify.com/CVE-2026-54256/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54256
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54256
- OSV: https://osv.dev/vulnerability/CVE-2026-54256

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54256/
Summaries are generated and may contain mistakes; the official description above is the record.