CVE-2025-59833: Flagforge Information Disclosure Flaw

CVE-2025-59833 Overview

CVE-2025-59833 is an information disclosure vulnerability affecting Flag Forge, a popular Capture The Flag (CTF) platform. The vulnerability exists in the API endpoint GET /api/problems/:id, which returns challenge hints in plaintext within the question object regardless of whether the user has unlocked them through the intended point deduction mechanism. This business logic flaw allows users to view all hints for free, bypassing the platform's economic model and compromising the integrity of the challenge system.

Critical Impact

Users can access all challenge hints without paying the required points, undermining the competitive fairness and economic model of the CTF platform.

Affected Products

• Flag Forge versions 2.1.0 to 2.2.x
• Flag Forge CTF Platform API (/api/problems/:id endpoint)
• All installations running vulnerable versions prior to 2.3.0

Discovery Timeline

• 2025-09-24 - CVE-2025-59833 published to NVD
• 2025-10-08 - Last updated in NVD database

Technical Details for CVE-2025-59833

Vulnerability Analysis

This vulnerability represents a classic business logic flaw (CWE-200: Exposure of Sensitive Information to an Unauthorized Actor) in the Flag Forge CTF platform. The core issue lies in the API's response structure when fetching problem details. When a user requests challenge information via the GET /api/problems/:id endpoint, the server returns the complete question object, including all associated hints in plaintext form.

The design flaw stems from the server-side response not filtering out hint content based on the user's unlock status. While the frontend application may conditionally display hints based on whether points have been deducted, the raw API response exposes all hint data unconditionally. This allows technically savvy users to intercept API responses or directly query the endpoint to obtain hints without any point expenditure.

Root Cause

The root cause is improper access control at the API layer. The backend fails to implement server-side authorization checks that would verify whether a user has legitimately unlocked specific hints before including them in the API response. Instead, the authorization logic appears to exist only at the presentation layer (frontend), which can be trivially bypassed by inspecting network traffic or making direct API calls.

Attack Vector

The attack vector is network-based and requires no special privileges or user interaction. An attacker can exploit this vulnerability by:

1. Authenticating to the Flag Forge platform with any valid user account
2. Navigating to a challenge or directly calling the GET /api/problems/:id endpoint
3. Inspecting the JSON response using browser developer tools or an HTTP proxy
4. Extracting the plaintext hints from the question object without any point deduction

This vulnerability is straightforward to exploit, as the sensitive data is directly exposed in standard API responses without obfuscation or encryption.

Detection Methods for CVE-2025-59833

Indicators of Compromise

• Unusual patterns of API requests to /api/problems/:id endpoints from single users
• Users accessing hints without corresponding point deductions in the database
• High volume of challenge completions without expected hint unlock transactions
• Direct API calls bypassing the standard web interface

Detection Strategies

• Monitor API access logs for patterns of systematic hint retrieval without associated point transactions
• Implement correlation rules comparing hint access events with point deduction records
• Deploy web application firewalls to flag unusual patterns of /api/problems/ endpoint access
• Audit user activity for anomalies such as rapid challenge completion without hint purchases

Monitoring Recommendations

• Enable detailed logging on all /api/problems/:id endpoint requests
• Implement alerting for users who complete challenges at statistically improbable rates
• Review access logs periodically for users making direct API calls outside normal application flow
• Consider implementing rate limiting on problem detail endpoints

How to Mitigate CVE-2025-59833

Immediate Actions Required

• Upgrade Flag Forge to version 2.3.0 or later immediately
• Review user activity logs to identify potential abuse of the vulnerability
• Consider resetting competition scores if exploitation is detected during active CTF events
• Audit any custom API integrations for similar access control issues

Patch Information

Flag Forge version 2.3.0 addresses this vulnerability by implementing proper server-side authorization checks on the /api/problems/:id endpoint. The patched version filters hint content from API responses unless the requesting user has legitimately unlocked the hints through point deduction. For detailed patch information, refer to the GitHub Security Advisory GHSA-hm85-2j65-j8j2.

Workarounds

• If immediate upgrade is not possible, implement a reverse proxy rule to filter hint data from API responses
• Deploy network-level controls to restrict direct API access and force traffic through the web interface
• Temporarily disable hint functionality until the patch can be applied
• Monitor and audit all hint access activity during the interim period

# Upgrade Flag Forge to patched version
git pull origin main
git checkout v2.3.0
npm install
npm run build
npm run start