CVE-2026-21626: Forum Post ACL Information Disclosure

CVE-2026-21626 Overview

CVE-2026-21626 is an Information Disclosure vulnerability affecting forum software where access control settings for forum post custom fields are not properly applied to the JSON output type. This Access Control List (ACL) bypass allows unauthorized users to access sensitive custom field data that should be restricted, potentially exposing confidential information stored in forum posts.

Critical Impact

Unauthenticated remote attackers can bypass access controls to retrieve sensitive custom field data from forum posts via JSON API endpoints, leading to widespread information disclosure.

Affected Products

• StackIdeas EasyDiscuss (affected versions not specified)

Discovery Timeline

• 2026-02-06 - CVE CVE-2026-21626 published to NVD
• 2026-02-06 - Last updated in NVD database

Technical Details for CVE-2026-21626

Vulnerability Analysis

This vulnerability (CWE-200: Exposure of Sensitive Information to an Unauthorized Actor) stems from inconsistent access control enforcement across different output formats. While the application correctly enforces ACL restrictions when rendering custom field data in standard HTML views, it fails to apply these same restrictions when serving data through JSON API endpoints.

The root issue lies in the separation of access control logic between the presentation layer and the API layer. Custom fields in forum posts may contain sensitive information such as personal details, internal notes, or other confidential data that administrators have intentionally restricted to specific user roles. When these fields are requested via the JSON output type, the access control checks are bypassed entirely, exposing the restricted data to any requester.

Root Cause

The vulnerability exists because the JSON serialization pathway does not invoke the same access control validation routines that protect the HTML rendering path. When forum post data is serialized to JSON format for API responses, the custom field filtering logic that enforces ACL rules is either missing or incorrectly implemented. This creates a situation where the same data is protected in one context (HTML) but fully exposed in another (JSON), representing a fundamental architectural oversight in the access control implementation.

Attack Vector

An attacker can exploit this vulnerability by making direct requests to the JSON API endpoints that serve forum post data. Since the vulnerability requires no authentication and can be exploited over the network with low complexity, attackers can systematically enumerate and extract custom field data from all forum posts by simply requesting the JSON representation instead of the HTML view.

The attack flow typically involves:

1. Identifying forum posts with custom fields through normal browsing
2. Constructing API requests that return JSON-formatted responses
3. Extracting sensitive custom field values that would normally be hidden from the attacker's access level

This vulnerability is particularly dangerous because it can be automated to extract large volumes of sensitive data without triggering typical security monitoring that looks for authentication failures or brute force attempts.

Detection Methods for CVE-2026-21626

Indicators of Compromise

• Unusual volume of JSON API requests targeting forum post endpoints
• API requests from unauthenticated sessions attempting to access custom field data
• Access patterns showing systematic enumeration of post IDs through JSON endpoints
• Log entries indicating custom field data retrieval for users without appropriate permissions

Detection Strategies

• Monitor API access logs for patterns of JSON format requests that significantly exceed normal usage patterns
• Implement rate limiting and anomaly detection on JSON API endpoints serving forum post data
• Compare access control enforcement between HTML and JSON responses in security testing
• Deploy web application firewall (WAF) rules to detect and alert on suspicious API enumeration patterns

Monitoring Recommendations

• Enable detailed logging for all JSON API requests, including requested fields and response sizes
• Set up alerts for bulk data extraction patterns targeting forum post endpoints
• Monitor for automated scraping tools accessing JSON endpoints
• Track and alert on access attempts from untrusted IP ranges or geographic locations

How to Mitigate CVE-2026-21626

Immediate Actions Required

• Review and audit all JSON API endpoints that expose forum post data with custom fields
• Implement access control checks at the API layer to match restrictions enforced in the HTML presentation layer
• Consider temporarily disabling JSON output for custom fields until a proper fix is deployed
• Apply vendor patches as soon as they become available

Patch Information

Organizations should monitor the vendor's official channels for security updates. Refer to the StackIdeas EasyDiscuss product page for the latest security advisories and patch information. As this is a critical severity vulnerability with a network-based attack vector requiring no user interaction or privileges, patching should be prioritized.

Workarounds

• Disable or restrict access to JSON API endpoints until the vulnerability is patched
• Implement network-level access controls to limit JSON API access to trusted internal networks only
• Remove sensitive data from custom fields temporarily if the API cannot be disabled
• Deploy a reverse proxy or WAF rule to block or sanitize JSON responses containing custom field data

# Example: Restrict JSON API access at the web server level (Apache)
# Add to .htaccess or virtual host configuration
<LocationMatch "\.json$">
    Order deny,allow
    Deny from all
    Allow from 10.0.0.0/8
    Allow from 192.168.0.0/16
</LocationMatch>