Executive Summary

CVE-2026-8406 is an Insecure Direct Object Reference (IDOR) vulnerability found in openSIS Classic version 9.3, specifically in its messaging module.

This vulnerability allows any authenticated user to access sent message details by manipulating the mail_id parameter in the SentMail.php file without proper authorization checks.

Because mail_id values are sequential, an attacker can enumerate and retrieve messages belonging to other users, including administrative users.

Additionally, the DownloadWindow.php file allows unauthorized downloading of attachments linked to these messages.

Useful 0 Not Useful 0

Compliance Impact

The vulnerability in openSIS Classic 9.3 allows authenticated users to access sent messages and attachments belonging to other users without proper authorization. This leads to a confidentiality breach exposing sensitive internal messaging data, including message content, sender/recipient details, timestamps, and file attachments.

Such unauthorized access to personal and potentially sensitive information can negatively impact compliance with data protection regulations like GDPR and HIPAA, which require strict controls on access to personal and health-related data to ensure confidentiality and privacy.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can lead to a serious confidentiality breach.

• Low-privileged authenticated users can access sensitive internal messaging data belonging to other users.
• Attackers can view message content, sender and recipient details, timestamps, and download file attachments without authorization.
• This unauthorized access can expose private communications and sensitive information, potentially leading to data leaks or misuse.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability can be detected by attempting to access the messaging module's SentMail.php endpoint with different mail_id values to see if sent-message details from other users are returned without proper authorization.

Since mail_id values are sequential integers, an attacker can enumerate these IDs to retrieve messages belonging to other users.

A simple detection command could be a curl request to the SentMail.php endpoint with varying mail_id parameters while authenticated as a low-privileged user.

• curl -b cookies.txt 'https://target-system/modules/messaging/SentMail.php?mail_id=1'
• curl -b cookies.txt 'https://target-system/modules/messaging/SentMail.php?mail_id=2'

If responses return message details for mail_id values not belonging to the authenticated user, the vulnerability is present.

Useful 0 Not Useful 0

Mitigation Strategies

Immediate mitigation steps include applying the security patch that implements proper authorization checks and input sanitization in the messaging module.

Specifically, update the openSIS Classic application to include the changes from commit c45d431 which restrict access to sent messages and attachments based on user profile and ID.

• Ensure only authorized users can view specific emails by enforcing user-specific filtering in SentMail.php.
• Restrict attachment downloads so non-admin users can only download their own files.
• Improve input sanitization to prevent directory traversal and encoding attacks.

If patching immediately is not possible, restrict access to the messaging module to trusted users only and monitor for suspicious access patterns.

Useful 0 Not Useful 0