For complete license details review LICENSE.txt.

GeoServer is free software and is licensed under the GNU General Public License:

    GeoServer, open geospatial information server
    Copyright (C) 2014-2020 Open Source Geospatial Foundation.
    Copyright (C) 2001-2014 OpenPlans
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version (collectively, "GPL").
    As an exception to the terms of the GPL, you may copy, modify,
    propagate, and distribute a work formed by combining GeoServer with the
    EMF and XSD Libraries, or a work derivative of such a combination, even if
    such copying, modification, propagation, or distribution would otherwise
    violate the terms of the GPL. Nothing in this exception exempts you from
    complying with the GPL in all respects for all of the code used other
    than the EMF and XSD Libraries. You may include this exception and its grant
    of permissions when you distribute GeoServer.  Inclusion of this notice
    with such a distribution constitutes a grant of such permissions.  If
    you do not wish to grant these permissions, remove this paragraph from
    your distribution. "GeoServer" means the GeoServer software licensed
    under version 2 or any later version of the GPL, or a work based on such
    software and licensed under the GPL. "EMF and XSD Libraries" means 
    Eclipse Modeling Framework Project and XML Schema Definition software
    distributed by the Eclipse Foundation, all licensed 
    under the Eclipse Public License Version 1.0 ("EPL"), or a work based on 
    such software and licensed under the EPL.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    GNU General Public License for more details.
    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335  USA

This product includes software developed by the Apache Software Foundation ( licensed under the Apache License Version 2.0 and Apache License Version 1.1.

This product includes software developed by the Eclipse Software Foundation under the Eclipse Public License.