CVE-2026-25728 is a critical vulnerability classified under CWE-367 (Time-of-Check Time-of-Use race condition) affecting ClipBucket v5, an open-source video sharing platform. The vulnerability arises from the sequence of operations during avatar and background image uploads: the application moves the uploaded file to a web-accessible directory using move_uploaded_file() before validating the file content with ValidateImage(). If validation fails, the file is deleted via @unlink(). However, the time gap between moving the file and validation creates a race condition window. An attacker can exploit this by replacing or modifying the file after it is moved but before validation completes, enabling execution of arbitrary PHP code on the server. This can lead to remote code execution, full system compromise, and unauthorized access. The vulnerability requires no authentication or user interaction and can be exploited remotely over the network. The CVSS 4.0 vector (AV:N/AC:L/AT:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N) reflects network attack vector, low complexity, no privileges or user interaction needed, and high impact on confidentiality, integrity, and availability. The issue was addressed and fixed in ClipBucket version 5.5.3 - #40. No public exploits are currently known, but the critical nature of the flaw demands immediate remediation for affected deployments.

For European organizations using vulnerable versions of ClipBucket, this vulnerability poses a severe risk of remote code execution, potentially allowing attackers to gain full control over affected servers. This can lead to data breaches, defacement of video sharing platforms, unauthorized access to sensitive user data, and disruption of services. Given the nature of ClipBucket as a media platform, exploitation could also facilitate distribution of malicious content or use of compromised servers for further attacks. The lack of required authentication and user interaction increases the likelihood of automated exploitation attempts. Organizations hosting ClipBucket publicly face increased exposure, especially those with high user engagement or regulatory obligations related to data protection (e.g., GDPR). The impact extends to reputational damage, legal liabilities, and operational downtime.

European organizations should immediately upgrade all ClipBucket installations to version 5.5.3 - #40 or later, where the vulnerability is patched. If immediate upgrade is not feasible, implement temporary mitigations such as restricting file upload functionality to trusted users, disabling avatar and background image uploads, or isolating the upload directory from web-accessible paths using server configuration (e.g., disabling PHP execution in upload directories). Employ web application firewalls (WAFs) to detect and block suspicious upload patterns or attempts to access uploaded files before validation completes. Regularly audit and monitor logs for unusual file upload activity or execution attempts. Conduct thorough security testing post-patch to ensure no residual exposure. Additionally, enforce strict input validation and consider sandboxing file processing to limit potential damage from exploitation.