CVE-2026-55872 in GYM One
Summary
by MITRE • 10/15/2026
GYM One is open-source gym management software for fitness centers, personal trainers, and sports clubs. Prior to 1.4.1, GYM One's admin/users/index.php renders the stored firstname, lastname, and email profile fields into the administrator user list without HTML output encoding. An attacker who can store values in those fields can cause content to execute in an administrator's browser when the administrator visits /admin/users/. The persistent script can access administrator-session data or perform actions with the administrator's privileges until the stored record is removed. This issue is fixed in version 1.4.1.
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Analysis
by VulDB Data Team • 10/15/2026
GYM One, a widely used open-source platform designed for managing fitness centers and sports clubs, suffered from a critical security flaw prior to version 1.4.1 that exposed administrative interfaces to persistent threats. The vulnerability resided within the admin users index page, specifically in how the application handled user profile data during rendering. When administrators accessed the /admin/users/ endpoint to review registered members or staff, the system retrieved stored values for firstname, lastname, and email fields from the database and injected them directly into the HTML response without applying any form of output encoding or sanitization. This architectural oversight created a classic vector for persistent cross-site scripting attacks within an administrative context, where trust in data integrity is paramount but was not enforced by the application logic at the point of display.
The technical nature of this flaw aligns with CWE-79, commonly known as Improper Neutralization of Input During Web Page Generation or Cross-site Scripting (XSS). By failing to escape special characters such as angle brackets and quotes before rendering user-supplied data in an HTML context, the application allowed malicious scripts to be embedded within legitimate-looking profile entries. An attacker who possesses the ability to register a new account or modify existing profiles can inject arbitrary JavaScript code into these fields. Unlike reflected XSS which requires tricking a victim into clicking a crafted link, this stored variant persists on the server side. Consequently, every time an administrator loads the user list page, the malicious script executes automatically in their browser session, leveraging the administrative privileges associated with that specific account.
The operational impact of this vulnerability is severe due to the high-privilege nature of the targeted users. Once executed within an administrator's browser context, the persistent script can access sensitive session data, including authentication tokens and cookies, which may facilitate session hijacking or privilege escalation attacks. Furthermore, the malicious code can perform actions on behalf of the administrator without their knowledge or consent, such as creating new admin accounts, modifying system configurations, deleting user records, or exfiltrating confidential member information to an external server controlled by the attacker. This capability effectively compromises the confidentiality and integrity of the entire platform, allowing for long-term unauthorized access that persists until the malicious profile entry is manually identified and removed from the database.
From a threat intelligence perspective, this vulnerability maps directly to MITRE ATT&CK technique T1059, Command and Scripting Interpreter, specifically through browser-based execution via JavaScript. The attack lifecycle involves initial access through account creation or modification, followed by persistence as long as the malicious profile remains in the system. It also relates to credential harvesting techniques where session tokens are stolen to maintain ongoing access even if passwords are changed later. Organizations relying on GYM One prior to version 1.4.1 were at significant risk of such multi-stage attacks that could lead to complete compromise of their fitness management infrastructure and associated customer data.
To mitigate this vulnerability, immediate action is required for all systems running versions earlier than 1.4.1. The primary remediation step is to upgrade the GYM One installation to version 1.4.1 or later, where the developers have implemented proper output encoding mechanisms to neutralize special characters before they are rendered in HTML contexts. For organizations unable to patch immediately due to operational constraints, a temporary workaround involves implementing input validation on the server side to reject any profile fields containing script-like content and applying context-specific escaping for all user-supplied data displayed in administrative views. Additionally, deploying a Web Application Firewall with rules capable of detecting and blocking cross-site scripting payloads can provide an additional layer of defense against exploitation attempts while permanent fixes are being deployed.