Apache Tomcat 9 (9.0.75) - Security Considerations

## Security Considerations

### Table of Contents

- Introduction
- Non-Tomcat settings
  1. JMX
- Default web applications
  1. General
  2. ROOT
  3. Documentation
  4. Examples
  5. Manager
  6. Host Manager
  7. Securing Management Applications
- Security manager
- server.xml
  1. General
  2. Server
  3. Listeners
  4. Connectors
  5. Host
  6. Context
  7. Valves
  8. Realms
  9. Manager
  10. Cluster
- System Properties
- web.xml
- General

### 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 suitably restricted. In the `.tar.gz` 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 `0027` to maintain these permissions for files created while Tomcat is running (e.g. log files, expanded WARs, etc.).

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.

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

#### JMX

The security of the JMX connection is dependent on the implementation provided by the JRE and therefore falls outside the control of Tomcat.

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.

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.

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:

- configuring a strong password for all JMX users;
- binding the JMX listener only to an internal network;
- limiting network access to the JMX port to trusted clients; and
- providing an application specific health page for use by external monitoring systems.

### Default web applications

#### General

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.

#### ROOT

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.

#### Documentation

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.

#### Examples

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.

#### Manager

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 Securing Management Applications section should be followed.

#### Host Manager

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 Securing Management Applications section should be followed.

#### Securing Management Applications

When deploying a web application that provides management functions for the Tomcat instance, the following guidelines should be followed:

- Ensure that any users permitted to access the management application have strong passwords.
- Do not remove the use of the LockOutRealm which prevents brute force attacks against user passwords.
- Configure the RemoteAddrValve in the context.xml 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.

### 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.

Enabling the security manager changes the defaults for the following settings:

- The default value for the deployXML attribute of the Host element is changed to `false`.

### 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 JNI Library Loading Listener may be used to load native code. It should only be used to load trusted libraries.

The Security Lifecycle Listener should be enabled and configured as appropriate.

#### Connectors

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.

AJP Connectors should only be used on trusted networks or be appropriately secured with a suitable `secret` attribute.

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 `allowedRequestAttributesPattern` attribute.

The address 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.

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 discardFacades attribute set 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 encodedSolidusHandling 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.

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 the request body during FORM and CLIENT-CERT authentication and HTTP/1.1 upgrade. For FORM authentication, the request body is cached in the HTTP session for the duration of the authentication so the cached request body is limited to 4KB by default to reduce exposure to a DOS attack. To further reduce exposure to a DoS attack by limiting the permitted duration of the FORM authentication, a reduced session timeout is used if the session is created by the FORM authentication. This reduced timeout is controlled by the `authenticationSessionTimeout` attribute of the FORM authenticator.

The maxParameterCount attribute controls the maximum total number of request parameters (including uploaded files) obtained from the query string and, for POST requests, the request body if the content type is `application/x-www-form-urlencoded` or `multipart/form-data`. 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/9.0), 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 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.

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 tomcatAuthentication and tomcatAuthorization 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.

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. Note that if the security manager is enabled that the deployXML attribute will default to `false`.

#### 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 of a nested Resources 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 `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.

The sessionCookiePathUsesTrailingSlash 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 `/*`. 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.

#### 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 (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, you can explicitly configure an ErrorReportValve and set its showServerInfo 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:

    server.info=Apache Tomcat/9.0.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 correct 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. Alternatively, you can explicitly configure an ErrorReportValve and set its showReport attribute to `false`.

The RewriteValve uses regular expressions and poorly formed regex patterns may be vulnerable to "catastrophic backtracking" or "ReDoS". See Rewrite docs for more details.

#### 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 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.

The persistAuthentication 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.

When using the JDBCStore, the session store should be secured (dedicated credentials, appropriate permissions) such that only the JDBCStore is able to access the persisted session data. In particular, the JDBCStore should not be accessible via any credentials available to a web application.

#### Cluster

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.

If you require confidentiality and/or integrity protection then you can use the EncryptInterceptor to encrypt traffic between nodes. This interceptor does not protect against all the risks of running on an untrusted network, particularly DoS attacks.

### System Properties

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, the `/WEB-INF/tomcat-web.xml` and the `/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.

The DefaultServlet is configured with showServerInfo set to `true`. 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 showServerInfo 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:

    server.info=Apache Tomcat/9.0.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 correct version number.

The CGI Servlet is disabled by default. If enabled, the debug initialisation parameter should not be set to `10` or higher on a production system because the debug page is not secure.

When using the CGI Servlet on Windows with `enableCmdLineArguments` enabled, review the setting of `cmdLineArgumentsDecoded` 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 CGI How To.

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.

HttpHeaderSecurityFilter 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.

### 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 is nearly as useful as the user's password to an attacker and 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.