[[modules-scripting-security]]
== Scripting and security

While Elasticsearch contributors make every effort to prevent scripts from
running amok, security is something best done in
https://en.wikipedia.org/wiki/Defense_in_depth_(computing)[layers] because
all software has bugs and it is important to minimize the risk of failure in
any security layer. Find below rules of thumb for how to keep Elasticsearch
from being a vulnerability.

[discrete]
=== Do not run as root
First and foremost, never run Elasticsearch as the `root` user as this would
allow any successful effort to circumvent the other security layers to do
*anything* on your server. Elasticsearch will refuse to start if it detects
that it is running as `root` but this is so important that it is worth double
and triple checking.

[discrete]
=== Do not expose Elasticsearch directly to users
Do not expose Elasticsearch directly to users, instead have an application
make requests on behalf of users. If this is not possible, have an application
to sanitize requests from users. If *that* is not possible then have some
mechanism to track which users did what. Understand that it is quite possible
to write a <<search, `_search`>> that overwhelms Elasticsearch and brings down
the cluster. All such searches should be considered bugs and the Elasticsearch
contributors make an effort to prevent this but they are still possible.

[discrete]
=== Do not expose Elasticsearch directly to the Internet
Do not expose Elasticsearch to the Internet, instead have an application
make requests on behalf of the Internet. Do not entertain the thought of having
an application "sanitize" requests to Elasticsearch. Understand that it is
possible for a sufficiently determined malicious user to write searches that
overwhelm the Elasticsearch cluster and bring it down. For example:

Good:

* Users type text into a search box and the text is sent directly to a
<<query-dsl-match-query>>, <<query-dsl-match-query-phrase>>,
<<query-dsl-simple-query-string-query>>, or any of the <<search-suggesters>>.
* Running a script with any of the above queries that was written as part of
the application development process.
* Running a script with `params` provided by users.
* User actions makes documents with a fixed structure.

Bad:

* Users can write arbitrary scripts, queries, `_search` requests.
* User actions make documents with structure defined by users.

[discrete]
[[modules-scripting-other-layers]]
=== Other security layers
In addition to user privileges and script sandboxing Elasticsearch uses the
http://www.oracle.com/technetwork/java/seccodeguide-139067.html[Java Security Manager]
and native security tools as additional layers of security.

As part of its startup sequence Elasticsearch enables the Java Security Manager
which limits the actions that can be taken by portions of the code. Painless
uses this to limit the actions that generated Painless scripts can take,
preventing them from being able to do things like write files and listen to
sockets.

Elasticsearch uses
https://en.wikipedia.org/wiki/Seccomp[seccomp] in Linux,
https://www.chromium.org/developers/design-documents/sandbox/osx-sandboxing-design[Seatbelt]
in macOS, and
https://msdn.microsoft.com/en-us/library/windows/desktop/ms684147[ActiveProcessLimit]
on Windows to prevent Elasticsearch from forking or executing other processes.

Below this we describe the security settings for scripts and how you can
change from the defaults described above. You should be very, very careful
when allowing more than the defaults. Any extra permissions weakens the total
security of the Elasticsearch deployment.

[[allowed-script-types-setting]]
[discrete]
=== Allowed script types setting

By default all script types are allowed to be executed.  This can be modified using the
setting `script.allowed_types`.  Only the types specified as part of the setting will be
allowed to be executed.  To specify no types are allowed, set `script.allowed_types` to
be `none`.

[source,yaml]
----
script.allowed_types: inline <1>
----
<1> This will allow only inline scripts to be executed but not stored scripts
(or any other types).

[[allowed-script-contexts-setting]]
[discrete]
=== Allowed script contexts setting

By default all script contexts are allowed to be executed.  This can be modified using the
setting `script.allowed_contexts`.  Only the contexts specified as part of the setting will
be allowed to be executed.  To specify no contexts are allowed, set `script.allowed_contexts`
to be `none`.

[source,yaml]
----
script.allowed_contexts: score, update <1>
----
<1> This will allow only scoring and update scripts to be executed but not
aggs or plugin scripts (or any other contexts).
