CVE-2026-77929 in ClipBucket
Summary
by MITRE • 09/18/2026
ClipBucket v5 before 5.5.3-#182 contains a file upload vulnerability that allows authenticated users to achieve remote code execution by uploading a PHP file with valid image magic bytes through the photo upload endpoint. The FileUpload::manageFile() function in fileupload.class.php fails to update the file extension after MIME validation, allowing an attacker-controlled .php extension to persist on disk and execute as PHP via PHP-FPM when the uploaded file is retrieved.
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Analysis
by VulDB Data Team • 09/18/2026
ClipBucket versions prior to 5.5.3-#182 are susceptible to a critical remote code execution vulnerability stemming from improper handling of file uploads within the platform's photo management subsystem. This flaw specifically affects authenticated users who possess the ability to upload media content through the designated photo upload endpoint. The core technical deficiency resides in the FileUpload::manageFile() function located in the fileupload.class.php module, which exhibits a logic error during the validation and storage process. While the system performs MIME type validation by inspecting the magic bytes of the uploaded file to ensure it matches an expected image format such as JPEG or PNG, this check is decoupled from the subsequent file extension handling routine. Consequently, although the content may be verified as a valid image, the original filename provided by the attacker retains its executable PHP extension without being sanitized or renamed to a safe alternative like .jpg or .png.
This architectural oversight allows an attacker to bypass standard web server and application-level restrictions that typically rely on file extensions for execution control. By crafting a malicious payload that contains both valid image magic bytes in its header and PHP code within the body, the user can upload this disguised script through the legitimate photo upload interface. Because the system validates the MIME type based on content but fails to enforce extension consistency or strip executable suffixes, the file is saved to the server's public web directory with a .php extension intact. When another user or an automated process retrieves and displays the image, or when the web server processes requests for this specific path, PHP-FPM interprets the file as executable code rather than static media data. This results in arbitrary command execution on the underlying host system under the context of the web server process, granting the attacker full control over the compromised environment.
The operational impact of this vulnerability is severe, as it facilitates complete compromise of the ClipBucket installation and potentially the entire backend infrastructure hosting the application. An authenticated adversary can leverage this remote code execution to exfiltrate sensitive database credentials, manipulate user data, install persistent backdoors, or pivot further into internal networks if the server has broader connectivity. This scenario aligns with CWE-434, which describes the unrestricted upload of files with dangerous types, and specifically highlights a failure in input validation where content-based checks are not synchronized with filename sanitization protocols. From an offensive security perspective, this technique is consistent with ATT&CK tactic T1505, known as Server Software Component, where attackers establish persistence or execute commands through legitimate software components like web servers or application frameworks that have been manipulated to run malicious code.
Mitigation strategies must prioritize immediate patching of the ClipBucket installation to version 5.5.3-#182 or later, which addresses this logic flaw by ensuring that file extensions are either stripped entirely and replaced with safe equivalents derived from MIME validation results, or strictly validated against a whitelist of allowed non-executable image formats regardless of user input. In environments where immediate patching is not feasible, administrators should implement strict web server configuration rules to disable PHP execution in the specific upload directories used by ClipBucket. This can be achieved through Nginx location blocks or Apache .htaccess directives that deny access to any file with a php extension within those paths. Additionally, deploying a Web Application Firewall capable of detecting anomalous file uploads and enforcing strict MIME-to-extension consistency checks can provide an additional layer of defense against exploitation attempts targeting this specific vulnerability vector.