MISP contains a stored cross-site scripting (XSS) vulnerability in the default theme's Galaxies index page. When a MISP instance detects unknown custom or default galaxy clusters during synchronization, it renders sample tag names in an informational notice directed at site administrators. In the default theme, these sample tag names were inserted into the HTML output without HTML-entity encoding. A user with tag-editor privileges could craft a malicious misp-galaxy tag containing arbitrary script. When a site administrator subsequently viewed the Galaxies index page, the crafted tag content was interpreted as executable markup in the administrator's browser session, enabling the attacker to read session data, perform actions on behalf of the administrator, or otherwise compromise the administrator's browser context. The Overmind theme already applied HTML escaping to the same values and was not affected. The vulnerability requires the attacker to possess tag-editor-level access to create the crafted tag and requires the victim (a site administrator) to view the Galaxies index page in the default theme. The impact is limited to the administrator's browser session and does not directly compromise the MISP server process. Version affected: <2.5.47