critical CVSS 10

CVE-2026-28289·PHP vulnerability

FreeScout is a free help desk and shared inbox built with PHP's Laravel framework. A patch bypass vulnerability for CVE-2026-27636 in FreeScout 1.8.206 and earlier allows any authenticated user with file upload permissions to achieve Remote Code Execution (RCE) on the server by uploading a malicious .htaccess file using a zero-width space character prefix to bypass the security check. The vulnerability exists in the sanitizeUploadedFileName() function in app/Http/Helper.php. The function contains a Time-of-Check to Time-of-Use (TOCTOU) flaw where the dot-prefix check occurs before sanitization removes invisible characters. This vulnerability is fixed in 1.8.207.

Severity
critical
Software
PHP
Fixed in
1.8.207
Published
2026-03-03

Affected versions

Until: 1.8.207

Fixed in: 1.8.207

How to fix this CVE

Upgrade FreeScout to version 1.8.207 or later to patch the file upload validation bypass that allows remote code execution. This update fixes a critical flaw in the sanitizeUploadedFileName() function that failed to remove invisible characters before checking for dangerous file extensions. Organizations running FreeScout 1.8.206 or earlier should prioritize this update immediately given the critical severity rating.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST requests to file upload endpoints with multipart form data containing filenames with Unicode zero-width space characters (U+200B, U+200C, U+200D, U+FEFF) followed by .htaccess or .php extensions; pattern: regex '(\u200b|\u200c|\u200d|\ufeff)\.(htaccess|php)'

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Block or sanitize HTTP POST requests to file upload endpoints where the filename parameter contains zero-width Unicode characters (U+200B, U+200C, U+200D, U+FEFF); additionally restrict .htaccess file uploads entirely at the WAF layer regardless of filename encoding

How to check if you are affected

  1. Step 1: Check FreeScout version with: grep -r 'VERSION\|version' /path/to/freescout/config/ | head -5 or inspect the application dashboard for version information
  2. Step 2: Verify file upload functionality is enabled by checking if the 'Attachments' or 'File Upload' feature is active in FreeScout admin settings
  3. Step 3: Search web server logs for suspicious .htaccess upload attempts using: grep -i '\.htaccess' /var/log/apache2/access.log or /var/log/nginx/access.log
  4. Step 4: Confirm the patch by checking the app/Http/Helper.php file for the updated sanitizeUploadedFileName() function that properly strips zero-width characters before extension validation

Indicators of compromise

  • HTTP POST to /api/upload or /conversation/*/upload with zero-width space in filename
  • Creation of .htaccess files in web-accessible directories with execute permissions
  • Sudden appearance of .htaccess files in storage/app/uploads or similar directories with timestamps matching upload requests

FAQ

What is CVE-2026-28289?

This vulnerability is a bypass of a prior security patch in FreeScout that allows authenticated file upload users to upload malicious .htaccess files by prepending zero-width space characters, circumventing filename validation and enabling remote code execution on the server.

Is CVE-2026-28289 being actively exploited?

According to CISA, this vulnerability is not currently listed in the Known Exploited Vulnerabilities catalog and no public exploit code is available, though the attack method is relatively straightforward for authenticated attackers.

What versions of FreeScout are affected by CVE-2026-28289?

FreeScout versions 1.8.206 and earlier are affected. Version 1.8.207 and later include the security fix.

How do I check if my FreeScout installation is vulnerable?

Run: grep -i 'freescout' /path/to/application/version.php or check the admin dashboard for the version number; any version below 1.8.207 is vulnerable.

Does Defensia detect CVE-2026-28289?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If FreeScout or PHP is installed on a monitored server, CVE-2026-28289 will appear in your dashboard with remediation steps.

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References

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