The Apache Tomcat Servlet/JSP Container

Apache Tomcat 7

Version 7.0.28, Apr 8 2014
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Security Considerations

Table of Contents
Introduction

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.

Note: 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.

Non-Tomcat settings

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.

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.

File permissions should also be suitable restricted. 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 priviliges, 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.

At the network level, consider using a firewall to limit both incoming and outgoing connections to only those connections you expect to be present.

Default web applications

Tomcat ships with a number of web applications 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.

Security manager

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.

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.

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.

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.

server.xml
General

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.

If a component type is not listed, then there are no settings for that type that directly impact security.

Server

Setting the port attribute to -1 disables the shutdown port.

If the shutdown port is not disabled, a strong password should be configured for shutdown.

Listeners

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.

The Security Listener should be enabled and configured as appropriate.

Connectors

By default, an HTTP and an AJP connector are configured. Connectors that will not be used should be removed from server.xml.

The address attribute may be used to control which IP address the connector listens on for connections. By default, the connector listens on all configured IP addresses.

The allowTrace 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.

The maxPostSize 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.

The maxSavePostSize attribute controls the saving of POST requests during FORM and CLIENT-CERT authentication. The parameters are 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.

The maxParameterCount 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 FailedRequestFilter.

The xpoweredBy 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/7.0.0), 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.

The server attribute controls the value of the Server HTTP header. The default value of this header for Tomcat 4.1.x, 5.0.x, 5.5.x, 6.0.x and 7.0.x is Apache-Coyote/1.1. This header can provide limited information to both legitimate clients and attackers.

The SSLEnabled, scheme and secure 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.

The ciphers attribute controls the ciphers used for SSL connections. By default, the default ciphers for the JVM will be used. This usually means that the weak export grade ciphers will be included in the list of available ciphers. Secure environments will normally want to configure a more limited set of ciphers.

The tomcatAuthentication attribute is used with the AJP connectors to determine if Tomcat should authenticate the user or if authentication can be delegated to the reverse proxy that will then pass the authenticated username to Tomcat as part of the AJP protocol.

The allowUnsafeLegacyRenegotiation attribute provides a workaround for CVE-2009-3555, a TLS man in the middle attack. This workaround applies to the BIO connector. It is only necessary if the underlying SSL implementation is vulnerable to CVE-2009-3555. For more information on the current state of this vulnerability and the work-arounds available see the Tomcat 7 security page.

The requiredSecret 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.

Host

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 autoDeploy and deployOnStartup attributes. If both are false, only Contexts defined in server.xml will be deployed and any changes will require a Tomcat restart.

In a hosted environment where web applications may not be trusted, set the deployXml attribute to false to ignore any context.xml packaged with the web application that may try to assign increased privileges to the web application.

Context

This applies to Context elements in all places where they can be defined: server.xml file, default context.xml file, per-host context.xml.default file, web application context file in per-host configuration directory or inside the web application.

The crossContext attribute controls if a context is allowed to access the resources of another context. It is false by default and should only be changed for trusted web applications.

The privileged attribute controls if a context is allowed to use container provided servlets like the Manager servlet. It is false by default and should only be changed for trusted web applications.

The allowLinking attribute 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. To avoid this behaviour, use the aliases attribute. Changing this setting from the default of false 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.

Valves

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.

Any administrative application should be protected by a RemoteAddrValve. (Note that this Valve is also available as a Filter.) The allow attribute should be used to limit access to a set of known trusted hosts.

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, the version number can be changed by creating the file CATALINA_BASE/lib/org/apache/catalina/util/ServerInfo.properties with content as follows:

server.info=Apache Tomcat/7.0.x
      

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 version number.

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.

Realms

The MemoryRealm is not intended for production use as any changes to tomcat-users.xml require a restart of Tomcat to take effect.

The JDBCRealm is not recommended for production use as it is single threaded for all authentication and authorization options. Use the DataSourceRealm instead.

The UserDatabaseRealm is not intended for large-scale installations. It is intended for small-scale, relatively static environments.

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.

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.

Manager

The manager component is used to generate session IDs.

The default entropy value has been shown to generate predictable values under certain conditions. For more secure session generation, this should be set to a long string. This is done automatically if the APR/native library is installed; a random value will be obtained from the APR/native library.

The class used to generate random session IDs may be changed with the randomClass attribute.

The length of the session ID may be changed with the sessionIdLength attribute.

System Properties

Setting org.apache.catalina.connector.RECYCLE_FACADES system property to true 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.

The org.apache.catalina.connector.CoyoteAdapter.ALLOW_BACKSLASH and org.apache.tomcat.util.buf.UDecoder.ALLOW_ENCODED_SLASH 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.

The org.apache.catalina.connector.Response.ENFORCE_ENCODING_IN_GET_WRITER 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.

web.xml

This applies to the default conf/web.xml file and WEB-INF/web.xml files in web applications if they define the components mentioned here.

The DefaultServlet is configured with readonly set to true. Changing this to false 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.

The DefaultServlet is configured with listings set to false. 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.

FailedRequestFilter 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.

General

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.

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.


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