Cockpit File Extension Filter Bypass Enables PHP Execution

Cockpit 2.13.5 and earlier is affected by a misconfiguration within the Bucket component _isFileTypeAllowed function where a specially crafted filename bypasses an extension filter. This allows an authenticated attacker to rename arbitrary files with the .php file extension enabling arbitrary code to be executed on the underlying server.

Casky was already ahead

This CVE exploits attack patterns that Casky's 419matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

Cockpit 2.13.5 and earlier contains a critical misconfiguration in the Bucket component's _isFileTypeAllowed function that fails to properly validate file extensions. An authenticated attacker can craft specially formatted filenames that bypass the extension filter, allowing them to rename arbitrary files with a .php extension. Once a PHP file exists on the server, the attacker can execute arbitrary code with the privileges of the web server process. This vulnerability affects any organization running vulnerable Cockpit versions that expose the Bucket component to authenticated users, making it a significant risk for content management and file storage deployments.

Casky's 419 matched skills detect the attack patterns associated with this vulnerability across MITRE ATT&CK techniques TA0001 (Reconnaissance), TA0043 (Reconnaissance - Infrastructure Recon), and TA0002 (Execution). Practitioners using Casky would see findings related to suspicious file upload attempts with obfuscated or double-extension filenames, authentication logs showing file manipulation operations by accounts with unusual activity patterns, and detection of .php file creation in unexpected directories. Claude's extended reasoning capabilities enable Casky to correlate multiple signals—such as file rename operations following upload attempts, execution of newly created PHP files, and deviations from normal Bucket component usage—to identify exploitation attempts before arbitrary code execution occurs. This multi-technique correlation provides practitioners with actionable intelligence to block attacks at early stages of the kill chain.

Live Threat IntelligenceComing soon

Casky Risk Score

—

EPSS Probability

—

Shodan Exposure

—

GreyNoise Signal

—

Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

419 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-38991.

analyzing-android-malware-with-apktool

malware analysis · medium

Run

analyzing-apt-group-with-mitre-navigator

threat intelligence · low

Run

analyzing-bootkit-and-rootkit-samples

malware analysis · medium

Run

MITRE ATT&CK Techniques

TA0001 TA0043 TA0002

Investigate the attack patterns behind CVE-2026-38991

Casky has 419 skills that investigate the attack patterns behind CVE-2026-38991. Run one and get CVSS-scored findings in 3 minutes.

Run the skill that detects this →

X Instagram LinkedIn