A critical-severity vulnerability in the open source help desk and shared mailbox solution FreeScout can be exploited in zero-click remote code execution (RCE) attacks, Ox Security warns.
Tracked as CVE-2026-28289 (CVSS score of 10/10), the security defect is a patch bypass for CVE-2026-27636, a recently fixed high-severity authenticated RCE bug.
The initial issue, described as a missing .htaccess in the file upload restriction list, could allow an authenticated attacker to upload a .htaccess file, tampering with file processing and achieving RCE.
The patch for the initial CVE, Ox discovered, can be bypassed using a zero-width space character, which is invisible and passes the dot-check, resulting in a valid .htaccess filename being saved to disk.
According to the FreeScout maintainers, CVE-2026-28289 is a Time-of-Check to Time-of-Use (TOCTOU) issue in the filename sanitization function, “where the dot-prefix check occurs before sanitization removes invisible characters”.
The fix for CVE-2026-27636 attempted to block filenames that have restricted file extensions or that begin with a period (“.”) by appending an underscore to the file extension.
To bypass the patch, an attacker prepends a zero-width space character (Unicode U+200B) to the filename. Because the character is not treated as visible content, the filename bypasses validation, the U+200B character is stripped, and the file is saved as a true dotfile.
“The attack works by sending a malicious email from any address to a mailbox configured in FreeScout. Importantly, this requires no authentication and no user interaction. The malicious payload is written to disk on the FreeScout server and can then be leveraged to execute commands remotely,” Ox explains.
An attacker, the cybersecurity firm notes, can predict where the file will be saved on the disk, which allows them to access the payload and execute commands on the server.
Successful exploitation of the new vulnerability could allow attackers to take full control of vulnerable servers, exfiltrate helpdesk tickets, mailbox content, and other sensitive data from FreeScout, and potentially move laterally to other systems on the network.
“All FreeScout 1.8.206 installations are affected when running on Apache with AllowOverride All enabled (a common configuration),” FreeScout’s maintainers explain.
CVE-2026-28289 was resolved in FreeScout version 1.8.207. Users are advised to update their deployments as soon as possible.
Related: VMware Aria Operations Vulnerability Exploited in the Wild
Related: Honeywell, Researcher Clash Over Impact of Building Controller Vulnerability
Related: Vulnerability in MS-Agent AI Framework Can Allow Full System Compromise
Related: Vulnerability Allowed Hijacking Chrome’s Gemini Live AI Assistant
