Attack Chain

1. An authenticated low-privileged user gains access to the Cockpit CMS application.
2. The attacker identifies the vulnerable Bucket file storage API located at the /system/buckets/api endpoint.
3. The attacker crafts an HTTP request to this API, embedding a path traversal sequence such as ../ or its URL-encoded equivalent ..%2f within the supplied bucket name parameter.
4. The server-side api() method within modules/System/Controller/Buckets.php receives the crafted request, attempting to sanitize the bucket name.
5. The preg_replace('/[^a-zA-Z0-9-_\\.]/','', $bucket) sanitization function fails to neutralize the ../ sequences, leaving them intact.
6. The incompletely sanitized bucket name is then used to construct an internal Flysystem path, resulting in a URI-like string such as uploads://buckets/{bucket_with_traversal_sequences}.
7. Flysystem's WhitespacePathNormalizer processes this path, inadvertently resolving the buckets/.. segment to the root of the storage system, effectively bypassing intended access restrictions.
8. The attacker can now perform unauthorized operations, including listing, uploading, and deleting files, across any bucket within the Cockpit CMS environment, potentially leading to data theft or compromise of the application.

Impact

Successful exploitation of CVE-2026-57856 allows an authenticated, low-privileged user to gain unauthorized access and control over all file storage buckets in the Cockpit CMS instance. This includes the ability to list the contents of any bucket, upload arbitrary files, and delete existing files, regardless of ownership or assigned user roles. This could lead to severe data integrity issues, unauthorized data exfiltration of sensitive information, or even remote code execution if malicious files are uploaded and subsequently executed by the server. The broad access afforded to a low-privileged user significantly elevates the risk profile for organizations utilizing Cockpit CMS, potentially exposing all stored data to compromise.

Recommendation

• Patch CVE-2026-57856 on all Cockpit CMS instances immediately to prevent exploitation.
• Deploy the Detect CVE-2026-57856 Exploitation - Cockpit CMS Path Traversal Sigma rule to your SIEM to detect attempts at exploiting the /system/buckets/api endpoint.
• Enable comprehensive web server logging for the webserver category, capturing cs-uri-stem, cs-uri-query, and cs-method, to ensure the log source for the detection rule is available.