External risk intelligence

FlagForge Session Invalidation Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2025-59841

Flag Forge is a web application platform. Web applications are commonly deployed as internet-facing services to allow users to interact with the platform, making the application's API endpoints and web interface typically reachable from the public internet in standard deployment scenarios.

Cross-site Request Forgery

Flagforge

2.2 to before 2.3.1

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in the Flag Forge CTF platform allows authenticated users to access protected areas and perform unauthorized actions even after logging out, due to improper session handling and lingering CSRF tokens. This critical issue, affecting specific versions of the application, could potentially expose sensitive information and system integrity if exploited.

Users can stay logged in after logout.
Session errors risk unauthorized access and actions.
Confirm relevance; consider upgrade status.

Attack Path

How an attacker could exploit the issue

An attacker could leverage this vulnerability by exploiting a flaw in how the Flag Forge web application manages user sessions after logout. Specifically, even after a user logs out, their session might remain valid, allowing access to protected parts of the application. This could enable an attacker to perform unauthorized actions using the still-valid session, potentially leading to a complete compromise of the application's integrity and confidentiality.

Requires no prior authentication.
Triggers by accessing protected endpoints post-logout.
Risk of unauthorized actions and data compromise.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, authenticated users could retain access to protected endpoints and perform unauthorized actions even after logging out of the Flag Forge web application. This occurs due to improper session invalidation and valid CSRF tokens post-logout.

User session data and profile information.
Unauthorized actions via lingering session tokens.
Continued unauthorized access and potential data manipulation.

Operational Fix

Recommended remediation, mitigation, and detection steps

Determine ownership by identifying the application owner responsible for Flag Forge. First, confirm where this platform is deployed and assess its business criticality and exposure. Once ownership and risk are understood, plan remediation activities, potentially coordinating with the vendor for updates during a scheduled maintenance window.

Application owners should manage the issue.
Verify platform exposure and criticality first.
Plan remediation based on confirmed risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Flag Forge?

Flag Forge is a software platform designed for hosting Capture The Flag (CTF) cybersecurity competitions. It manages user authentication, team registration, and the presentation of various technical challenges. Organizations use it to facilitate interactive learning and skill testing for security enthusiasts.

What does CWE-613 mean for CVE-2025-59841?

CVE-2025-59841 involves CWE-613, which is the weakness class for insufficient session expiration. In plain terms, the application fails to properly destroy a user's digital 'key' when they click logout. Because the system does not invalidate the session or the associated CSRF security tokens, the account remains active and accessible to anyone who possesses those old credentials.

How is this session vulnerability triggered?

An attacker triggers this flaw by interacting with the application's protected API endpoints or web pages after a logout event has occurred. Crucially, this does not require a new login; the system simply accepts the existing, theoretically expired credentials. Simply visiting the application without a prior authenticated session does not trigger the bug, as it relies on the existence of a lingering, post-logout session state.

Why does Halo Surface Signal categorize this as external?

Halo Surface Signal labels this CVE as external because Flag Forge is a web application typically hosted on public servers to allow participants to compete from anywhere. Since these API endpoints are often reachable from the open internet, the service is exposed to remote users, increasing the likelihood that a session flaw could be reached without needing local network access.

How do I fix the Flag Forge session issue?

The primary response is to update your Flag Forge instance to version 2.3.1 or later. Before applying the update, identify who manages your specific deployment and review your current version to confirm if it falls within the 2.2.0 to 2.3.0 range. Planning this update during a scheduled maintenance window will help ensure a smooth transition while securing your platform's session management.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.