SA-CONTRIB-2026-279: Critical severity composer/drupal/s3fs vulnerability
S3 File System (s3fs) provides an additional file system to your Drupal site that stores files in Amazon's Simple Storage Service (S3) storage. The module doesn't sufficiently isolate upload context between separate object(file) operations in the same request resulting in the potential for objects to be uploaded with incorrect access control list (ACL) entries or corruption through incorrect operation metadata. This vulnerability is mitigated by the fact that an attacker must be able to trigger multiple file operations on S3FS. Some vectors are mitigated by:
Not enabling private:// takeover ($settings['s3fs.uses3forprivate'] = TRUE;) or only enabled with $settings['s3fs.uploadasprivate'] = TRUE; The deployment does not utilize hooks3fsstreamopenparamsalter(), hooks3fsuploadparamsalter(), hooks3fscopyparamsalter(), or hooks3fscommandparamsalter()
If both points above have always been true, all currently known vectors are mitigated.
Credit
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
composer/drupal/s3fsto a version that resolves this vulnerability.Fixed in 8.x-3.12 - Configuration
Do not enable private:// takeover; if s3fs.use_s3_for_private is enabled, only enable it when s3fs.upload_as_private is TRUE.
Drupal S3 File System (s3fs) s3fs.use_s3_for_private = FALSE
Event History
Frequently Asked Questions
What must an attacker be able to do to exploit this issue?
An attacker must be able to trigger multiple file operations through S3FS within the same request.
Which deployments have all currently known attack vectors mitigated?
All currently known vectors are mitigated if private:// takeover has never been enabled, or it has only been enabled together with $settings['s3fs.upload_as_private'] = TRUE, and the deployment does not use hook_s3fs_stream_open_params_alter(), hook_s3fs_upload_params_alter(), hook_s3fs_copy_params_alter(), or hook_s3fs_command_params_alter().
What temporary configuration constraints can reduce exposure if remediation cannot be applied immediately?
Avoid enabling private:// takeover with $settings['s3fs.use_s3_for_private'] = TRUE unless $settings['s3fs.upload_as_private'] = TRUE is also enabled. Do not use the listed S3FS parameter-alter hooks; these conditions mitigate some vectors, and together mitigate all currently known vectors.