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<?xml version="1.0" encoding="UTF-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file distributed with
this work for additional information regarding copyright ownership.
The ASF licenses this file to You under the Apache License, Version 2.0
(the "License"); you may not use this file except in compliance with
the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<!DOCTYPE document [
<!ENTITY project SYSTEM "project.xml">
]>
<document url="security-howto.html">
&project;
<properties>
<title>Security Considerations</title>
</properties>
<body>
<section name="Table of Contents">
<toc/>
</section>
<section name="Introduction">
<p>Tomcat is configured to be reasonably secure for most use cases by
default. Some environments may require more, or less, secure configurations.
This page is to provide a single point of reference for configuration
options that may impact security and to offer some commentary on the
expected impact of changing those options. The intention is to provide a
list of configuration options that should be considered when assessing the
security of a Tomcat installation.</p>
<p><strong>Note</strong>: Reading this page is not a substitute for reading
and understanding the detailed configuration documentation. Fuller
descriptions of these attributes may be found in the relevant documentation
pages.</p>
</section>
<section name="Non-Tomcat settings">
<p>Tomcat configuration should not be the only line of defense. The other
components in the system (operating system, network, database, etc.) should
also be secured.</p>
<p>Tomcat should not be run under the root user. Create a dedicated user for
the Tomcat process and provide that user with the minimum necessary
permissions for the operating system. For example, it should not be possible
to log on remotely using the Tomcat user.</p>
<p>File permissions should also be suitably restricted. In the
<code>.tar.gz</code> distribution, files and directories are not world
readable and the group does not have write access. On Unix like operating
systems, Tomcat runs with a default umask of <code>0027</code> to maintain
these permissions for files created while Tomcat is running (e.g. log files,
expanded WARs, etc.).</p>
<p>Taking the Tomcat instances at the ASF as an example (where
auto-deployment is disabled and web applications are deployed as exploded
directories), the standard configuration is to have all Tomcat files owned
by root with group Tomcat and whilst owner has read/write privileges, group
only has read and world has no permissions. The exceptions are the logs,
temp and work directory that are owned by the Tomcat user rather than root.
This means that even if an attacker compromises the Tomcat process, they
can't change the Tomcat configuration, deploy new web applications or
modify existing web applications. The Tomcat process runs with a umask of
007 to maintain these permissions.</p>
<p>At the network level, consider using a firewall to limit both incoming
and outgoing connections to only those connections you expect to be
present.</p>
<subsection name="JMX">
<p>The security of the JMX connection is dependent on the implementation
provided by the JRE and therefore falls outside the control of Tomcat.</p>
<p>Typically, access control is very limited (either read-only to
everything or read-write to everything). Tomcat exposes a large amount
of internal information and control via JMX to aid debugging, monitoring
and management. Given the limited access control available, JMX access
should be treated as equivalent to local root/admin access and restricted
accordingly.</p>
<p>The JMX access control provided by most (all?) JRE vendors does not
log failed authentication attempts, nor does it provide an account
lock-out feature after repeated failed authentications. This makes a
brute force attack easy to mount and difficult to detect.</p>
<p>Given all of the above, care should be taken to ensure that, if used,
the JMX interface is appropriately secured. Options you may wish to
consider to secure the JMX interface include:</p>
<ul>
<li>configuring a strong password for all JMX users;</li>
<li>binding the JMX listener only to an internal network;</li>
<li>limiting network access to the JMX port to trusted clients; and</li>
<li>providing an application specific health page for use by external
monitoring systems.</li>
</ul>
</subsection>
</section>
<section name="Default web applications">
<subsection name="General">
<p>Tomcat ships with a number of web applications that are enabled by
default. Vulnerabilities have been discovered in these applications in the
past. Applications that are not required should be removed so the system
will not be at risk if another vulnerability is discovered.</p>
</subsection>
<subsection name="ROOT">
<p>The ROOT web application presents a very low security risk but it does
include the version of Tomcat that is being used. The ROOT web application
should normally be removed from a publicly accessible Tomcat instance, not
for security reasons, but so that a more appropriate default page is shown
to users.</p>
</subsection>
<subsection name="Documentation">
<p>The documentation web application presents a very low security risk but
it does identify the version of Tomcat that is being used. It should
normally be removed from a publicly accessible Tomcat instance.</p>
</subsection>
<subsection name="Examples">
<p>The examples web application should always be removed from any security
sensitive installation. While the examples web application does not
contain any known vulnerabilities, it is known to contain features
(particularly the cookie examples that display the contents of all
received and allow new cookies to be set) that may be used by an attacker
in conjunction with a vulnerability in another application deployed on the
Tomcat instance to obtain additional information that would otherwise be
unavailable.</p>
</subsection>
<subsection name="Manager">
<p>The Manager application allows the remote deployment of web
applications and is frequently targeted by attackers due to the widespread
use of weak passwords and publicly accessible Tomcat instances with the
Manager application enabled. The Manager application is not accessible by
default as no users are configured with the necessary access. If the
Manager application is enabled then guidance in the section
<strong>Securing Management Applications</strong> section should be
followed.</p>
</subsection>
<subsection name="Host Manager">
<p>The Host Manager application allows the creation and management of
virtual hosts - including the enabling of the Manager application for a
virtual host. The Host Manager application is not accessible by default
as no users are configured with the necessary access. If the Host Manager
application is enabled then guidance in the section <strong>Securing
Management Applications</strong> section should be followed.</p>
</subsection>
<subsection name="Securing Management Applications">
<p>When deploying a web application that provides management functions for
the Tomcat instance, the following guidelines should be followed:</p>
<ul>
<li>Ensure that any users permitted to access the management application
have strong passwords.</li>
<li>Do not remove the use of the <a
href="config/realm.html#LockOut_Realm_-_org.apache.catalina.realm.LockOutRealm">LockOutRealm</a>
which prevents brute force attacks against user passwords.</li>
<li>Configure the <a href="config/valve.html#Remote_Address_Valve">RemoteAddrValve</a>
in the <a href="config/context.html">context.xml</a> file for the
management application which limits access to localhost by default.
If remote access is required, limit it to specific IP addresses using
this valve.</li>
</ul>
</subsection>
</section>
<section name="Security manager">
<p>Enabling the security manager causes web applications to be run in a
sandbox, significantly limiting a web application's ability to perform
malicious actions such as calling System.exit(), establishing network
connections or accessing the file system outside of the web application's
root and temporary directories. However, it should be noted that there are
some malicious actions, such as triggering high CPU consumption via an
infinite loop, that the security manager cannot prevent.</p>
<p>Enabling the security manager is usually done to limit the potential
impact, should an attacker find a way to compromise a trusted web
application . A security manager may also be used to reduce the risks of
running untrusted web applications (e.g. in hosting environments) but it
should be noted that the security manager only reduces the risks of
running untrusted web applications, it does not eliminate them. If running
multiple untrusted web applications, it is recommended that each web
application is deployed to a separate Tomcat instance (and ideally separate
hosts) to reduce the ability of a malicious web application impacting the
availability of other applications.</p>
<p>Tomcat is tested with the security manager enabled; but the majority of
Tomcat users do not run with a security manager, so Tomcat is not as well
user-tested in this configuration. There have been, and continue to be,
bugs reported that are triggered by running under a security manager.</p>
<p>The restrictions imposed by a security manager are likely to break most
applications if the security manager is enabled. The security manager should
not be used without extensive testing. Ideally, the use of a security
manager should be introduced at the start of the development cycle as it can
be time-consuming to track down and fix issues caused by enabling a security
manager for a mature application.</p>
<p>Enabling the security manager changes the defaults for the following
settings:</p>
<ul>
<li>The default value for the <strong>deployXML</strong> attribute of the
<strong>Host</strong> element is changed to <code>false</code>.</li>
</ul>
</section>
<section name="server.xml">
<subsection name="General">
<p>The default server.xml contains a large number of comments, including
some example component definitions that are commented out. Removing these
comments makes it considerably easier to read and comprehend
server.xml.</p>
<p>If a component type is not listed, then there are no settings for that
type that directly impact security.</p>
</subsection>
<subsection name="Server">
<p>Setting the <strong>port</strong> attribute to <code>-1</code> disables
the shutdown port.</p>
<p>If the shutdown port is not disabled, a strong password should be
configured for <strong>shutdown</strong>.</p>
</subsection>
<subsection name="Listeners">
<p>The APR Lifecycle Listener is not stable if compiled on Solaris using
gcc. If using the APR/native connector on Solaris, compile it with the
Sun Studio compiler.</p>
<p>The JNI Library Loading Listener may be used to load native code. It should
only be used to load trusted libraries.</p>
<p>The Security Lifecycle Listener should be enabled and configured as appropriate.
</p>
</subsection>
<subsection name="Connectors">
<p>By default, a non-TLS, HTTP/1.1 connector is configured on port 8080.
Connectors that will not be used should be removed from server.xml.</p>
<p>AJP Connectors should only be used on trusted networks or be
appropriately secured with a suitable <code>secret</code> attribute.</p>
<p>AJP Connectors block forwarded requests with unknown request
attributes. Known safe and/or expected attributes may be allowed by
configuration an appropriate regular expression for the
<code>allowedRequestAttributesPattern</code> attribute.</p>
<p>The <strong>address</strong> attribute may be used to control which IP
address a connector listens on for connections. By default, a connector
listens on all configured IP addresses.</p>
<p>The <strong>allowTrace</strong> attribute may be used to enable TRACE
requests which can be useful for debugging. Due to the way some browsers
handle the response from a TRACE request (which exposes the browser to an
XSS attack), support for TRACE requests is disabled by default.</p>
<p>The <strong>discardFacades</strong> attribute set to <code>true</code>
will cause a new facade object to be created for each request. This
reduces the chances of a bug in an application exposing data from one
request to another.</p>
<p>The <strong>encodedSolidusHandling</strong> attribute allows
non-standard parsing of the request URI. Setting this attribute to a
non-default value when behind a reverse proxy may enable an attacker to
bypass any security constraints enforced by the proxy.</p>
<p>The <strong>maxPostSize</strong> attribute controls the maximum size
of a POST request that will be parsed for parameters. The parameters are
cached for the duration of the request so this is limited to 2MB by
default to reduce exposure to a DOS attack.</p>
<p>The <strong>maxSavePostSize</strong> attribute controls the saving of
the request body during FORM and CLIENT-CERT authentication and HTTP/1.1
upgrade. For FORM authentication, the request body is cached for the
duration of the authentication (which may be many minutes) so this is
limited to 4KB by default to reduce exposure to a DOS attack.</p>
<p>The <strong>maxParameterCount</strong> attribute controls the
maximum number of parameter and value pairs (GET plus POST) that can
be parsed and stored in the request. Excessive parameters are ignored.
If you want to reject such requests, configure a
<a href="config/filter.html">FailedRequestFilter</a>.</p>
<p>The <strong>xpoweredBy</strong> attribute controls whether or not the
X-Powered-By HTTP header is sent with each request. If sent, the value of
the header contains the Servlet and JSP specification versions, the full
Tomcat version (e.g. Apache Tomcat/<version-major-minor/>), the name of
the JVM vendor and
the version of the JVM. This header is disabled by default. This header
can provide useful information to both legitimate clients and attackers.
</p>
<p>The <strong>server</strong> attribute controls the value of the Server
HTTP header. The default value of this header for Tomcat 4.1.x to
8.0.x is Apache-Coyote/1.1. From 8.5.x onwards this header is not set by
default. This header can provide limited information to both legitimate
clients and attackers.</p>
<p>The <strong>SSLEnabled</strong>, <strong>scheme</strong> and
<strong>secure</strong> attributes may all be independently set. These are
normally used when Tomcat is located behind a reverse proxy and the proxy
is connecting to Tomcat via HTTP or HTTPS. They allow Tomcat to see the
SSL attributes of the connections between the client and the proxy rather
than the proxy and Tomcat. For example, the client may connect to the
proxy over HTTPS but the proxy connects to Tomcat using HTTP. If it is
necessary for Tomcat to be able to distinguish between secure and
non-secure connections received by a proxy, the proxy must use separate
connectors to pass secure and non-secure requests to Tomcat. If the
proxy uses AJP then the SSL attributes of the client connection are
passed via the AJP protocol and separate connectors are not needed.</p>
<p>The <strong>tomcatAuthentication</strong> and
<strong>tomcatAuthorization</strong> attributes are used with the
AJP connectors to determine if Tomcat should handle all authentication and
authorisation or if authentication should be delegated to the reverse
proxy (the authenticated user name is passed to Tomcat as part of the AJP
protocol) with the option for Tomcat to still perform authorization.</p>
<p>The <strong>requiredSecret</strong> attribute in AJP connectors
configures shared secret between Tomcat and reverse proxy in front of
Tomcat. It is used to prevent unauthorized connections over AJP protocol.</p>
</subsection>
<subsection name="Host">
<p>The host element controls deployment. Automatic deployment allows for
simpler management but also makes it easier for an attacker to deploy a
malicious application. Automatic deployment is controlled by the
<strong>autoDeploy</strong> and <strong>deployOnStartup</strong>
attributes. If both are <code>false</code>, only Contexts defined in
server.xml will be deployed and any changes will require a Tomcat restart.
</p>
<p>In a hosted environment where web applications may not be trusted, set
the <strong>deployXML</strong> attribute to <code>false</code> to ignore
any context.xml packaged with the web application that may try to assign
increased privileges to the web application. Note that if the security
manager is enabled that the <strong>deployXML</strong> attribute will
default to <code>false</code>.</p>
</subsection>
<subsection name="Context">
<p>This applies to <a href="config/context.html">Context</a>
elements in all places where they can be defined:
<code>server.xml</code> file,
default <code>context.xml</code> file,
per-host <code>context.xml.default</code> file,
web application context file in per-host configuration directory
or inside the web application.</p>
<p>The <strong>crossContext</strong> attribute controls if a context is
allowed to access the resources of another context. It is
<code>false</code> by default and should only be changed for trusted web
applications.</p>
<p>The <strong>privileged</strong> attribute controls if a context is
allowed to use container provided servlets like the Manager servlet. It is
<code>false</code> by default and should only be changed for trusted web
applications.</p>
<p>The <strong>allowLinking</strong> attribute of a nested
<a href="config/resources.html">Resources</a> element controls if a context
is allowed to use linked files. If enabled and the context is undeployed,
the links will be followed when deleting the context resources. Changing
this setting from the default of <code>false</code> on case insensitive
operating systems (this includes Windows) will disable a number of
security measures and allow, among other things, direct access to the
WEB-INF directory.</p>
<p>The <strong>sessionCookiePathUsesTrailingSlash</strong> can be used to
work around a bug in a number of browsers (Internet Explorer, Safari and
Edge) to prevent session cookies being exposed across applications when
applications share a common path prefix. However, enabling this option
can create problems for applications with Servlets mapped to
<code>/*</code>. It should also be noted the RFC6265 section 8.5 makes it
clear that different paths should not be considered sufficient to isolate
cookies from other applications.</p>
</subsection>
<subsection name="Valves">
<p>It is strongly recommended that an AccessLogValve is configured. The
default Tomcat configuration includes an AccessLogValve. These are
normally configured per host but may also be configured per engine or per
context as required.</p>
<p>Any administrative application should be protected by a
RemoteAddrValve (this Valve is also available as a Filter).
The <strong>allow</strong> attribute should be used to limit access to a
set of known trusted hosts.</p>
<p>The default ErrorReportValve includes the Tomcat version number in the
response sent to clients. To avoid this, custom error handling can be
configured within each web application. Alternatively, you can explicitly
configure an <a href="config/valve.html">ErrorReportValve</a> and set its
<strong>showServerInfo</strong> attribute to <code>false</code>.
Alternatively, the version number can be changed by creating the file
CATALINA_BASE/lib/org/apache/catalina/util/ServerInfo.properties with
content as follows:</p>
<source>server.info=Apache Tomcat/<version-major-minor/>.x</source>
<p>Modify the values as required. Note that this will also change the version
number reported in some of the management tools and may make it harder to
determine the real version installed. The CATALINA_HOME/bin/version.bat|sh
script will still report the correct version number.</p>
<p>The default ErrorReportValve can display stack traces and/or JSP
source code to clients when an error occurs. To avoid this, custom error
handling can be configured within each web application. Alternatively, you
can explicitly configure an <a href="config/valve.html">ErrorReportValve</a>
and set its <strong>showReport</strong> attribute to <code>false</code>.</p>
<p>The RewriteValve uses regular expressions and poorly formed regex
patterns may be vulnerable to "catastrophic backtracking" or "ReDoS". See
<a href="rewrite.html">Rewrite docs</a> for more details.</p>
</subsection>
<subsection name="Realms">
<p>The MemoryRealm is not intended for production use as any changes to
tomcat-users.xml require a restart of Tomcat to take effect.</p>
<p>The JDBCRealm is not recommended for production use as it is single
threaded for all authentication and authorization options. Use the
DataSourceRealm instead.</p>
<p>The UserDatabaseRealm is not intended for large-scale installations. It
is intended for small-scale, relatively static environments.</p>
<p>The JAASRealm is not widely used and therefore the code is not as
mature as the other realms. Additional testing is recommended before using
this realm.</p>
<p>By default, the realms do not implement any form of account lock-out.
This means that brute force attacks can be successful. To prevent a brute
force attack, the chosen realm should be wrapped in a LockOutRealm.</p>
</subsection>
<subsection name="Manager">
<p>The manager component is used to generate session IDs.</p>
<p>The class used to generate random session IDs may be changed with
the <strong>randomClass</strong> attribute.</p>
<p>The length of the session ID may be changed with the
<strong>sessionIdLength</strong> attribute.</p>
<p>The <strong>persistAuthentication</strong> controls whether the
authenticated Principal associated with the session (if any) is included
when the session is persisted during a restart or to a Store.</p>
<p>When using the <strong>JDBCStore</strong>, the session store should be
secured (dedicated credentials, appropriate permissions) such that only
the <strong>JDBCStore</strong> is able to access the persisted session
data. In particular, the <strong>JDBCStore</strong> should not be
accessible via any credentials available to a web application.</p>
</subsection>
<subsection name="Cluster">
<p>The cluster implementation is written on the basis that a secure,
trusted network is used for all of the cluster related network traffic. It
is not safe to run a cluster on a insecure, untrusted network.</p>
<p>If you require confidentiality and/or integrity protection then you can
use the
<a href="config/cluster-interceptor.html#org.apache.catalina.tribes.group.interceptors.EncryptInterceptor_Attributes">EncryptInterceptor</a>
to encrypt traffic between nodes. This interceptor does not protect
against all the risks of running on an untrusted network, particularly
DoS attacks.</p>
</subsection>
</section>
<section name="System Properties">
<p>The <strong>
org.apache.catalina.connector.CoyoteAdapter.ALLOW_BACKSLASH</strong> and
<strong>org.apache.tomcat.util.buf.UDecoder.ALLOW_ENCODED_SLASH</strong>
system properties allow non-standard parsing of the request URI. Using
these options when behind a reverse proxy may enable an attacker to bypass
any security constraints enforced by the proxy.</p>
<p>The <strong>
org.apache.catalina.connector.Response.ENFORCE_ENCODING_IN_GET_WRITER
</strong> system property has security implications if disabled. Many user
agents, in breach of RFC2616, try to guess the character encoding of text
media types when the specification-mandated default of ISO-8859-1 should be
used. Some browsers will interpret as UTF-7 a response containing characters
that are safe for ISO-8859-1 but trigger an XSS vulnerability if interpreted
as UTF-7.</p>
</section>
<section name="web.xml">
<p>This applies to the default <code>conf/web.xml</code> file, the
<code>/WEB-INF/tomcat-web.xml</code> and the <code>/WEB-INF/web.xml</code>
files in web applications if they define the components mentioned here.</p>
<p>The <a href="default-servlet.html">DefaultServlet</a> is configured
with <strong>readonly</strong> set to
<code>true</code>. Changing this to <code>false</code> allows clients to
delete or modify static resources on the server and to upload new
resources. This should not normally be changed without requiring
authentication.</p>
<p>The DefaultServlet is configured with <strong>listings</strong> set to
<code>false</code>. This isn't because allowing directory listings is
considered unsafe but because generating listings of directories with
thousands of files can consume significant CPU leading to a DOS attack.
</p>
<p>The DefaultServlet is configured with <strong>showServerInfo</strong>
set to <code>true</code>. When the directory listings is enabled the Tomcat
version number is included in the response sent to clients. To avoid this,
you can explicitly configure a DefaultServlet and set its
<strong>showServerInfo</strong> attribute to false.
Alternatively, the version number can be changed by creating the file
CATALINA_BASE/lib/org/apache/catalina/util/ServerInfo.properties with
content as follows:</p>
<source>server.info=Apache Tomcat/<version-major-minor/>.x</source>
<p>Modify the values as required. Note that this will also change the version
number reported in some of the management tools and may make it harder to
determine the real version installed. The CATALINA_HOME/bin/version.bat|sh
script will still report the correct version number.
</p>
<p>The CGI Servlet is disabled by default. If enabled, the debug
initialisation parameter should not be set to <code>10</code> or higher on a
production system because the debug page is not secure.</p>
<p>When using the CGI Servlet on Windows with
<code>enableCmdLineArguments</code> enabled, review the setting of
<code>cmdLineArgumentsDecoded</code> carefully and ensure that it is
appropriate for your environment. The default value is secure. Insecure
configurations may expose the server to remote code execution. Further
information on the potential risks and mitigations may be found by
following the links in the <a href="cgi-howto.html">CGI How To</a>.</p>
<p><a href="config/filter.html">FailedRequestFilter</a>
can be configured and used to reject requests that had errors during
request parameter parsing. Without the filter the default behaviour is
to ignore invalid or excessive parameters.</p>
<p><a href="config/filter.html">HttpHeaderSecurityFilter</a> can be
used to add headers to responses to improve security. If clients access
Tomcat directly, then you probably want to enable this filter and all the
headers it sets unless your application is already setting them. If Tomcat
is accessed via a reverse proxy, then the configuration of this filter needs
to be co-ordinated with any headers that the reverse proxy sets.</p>
</section>
<section name="General">
<p>BASIC and FORM authentication pass user names and passwords in clear
text. Web applications using these authentication mechanisms with clients
connecting over untrusted networks should use SSL.</p>
<p>The session cookie for a session with an authenticated user are nearly
as useful as the user's password to an attacker and in nearly all
circumstances should be afforded the same level of protection as the
password itself. This usually means authenticating over SSL and continuing
to use SSL until the session ends.</p>
</section>
</body>
</document>
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