SA-CONTRIB-2026-337: Critical severity composer/drupal/s3fs_cors vulnerability
The S3 File System CORS Upload module lets users upload files straight from the browser to Amazon S3 (or an S3-compatible service), using a file widget on entity forms. The module doesn't sufficiently restrict the upload permission it gives the browser. A user who can upload through the widget can overwrite other files already stored in the bucket, including files they couldn't otherwise replace. This vulnerability is mitigated by the fact that an attacker must have access to a form that uses the S3 File System CORS Upload widget.
Credit
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
composer/drupal/s3fs_corsto a version that resolves this vulnerability.Fixed in 8.x-1.4
Event History
Frequently Asked Questions
Who is exposed to this issue?
Sites using composer/drupal/s3fs_cors are exposed where an entity form includes the S3 File System CORS Upload widget and users can access that form. The issue is mitigated because an attacker must be able to use such a form.
What access does an attacker need?
An attacker needs access to a form that uses the S3 File System CORS Upload widget and the ability to upload through that widget. No other prerequisites are provided.
What could an attacker do after exploiting it?
A user able to upload through the widget can overwrite files already stored in the associated Amazon S3 or S3-compatible bucket, including files they would not normally be permitted to replace.