Merge #15223: Doc: add information about security to the JSON-RPC doc

5a5ea93e87c766dac1251f0369b2737d8e9dadde Doc: add information about security to the JSON-RPC doc (David A. Harding)

Pull request description:

  This documents some information about using the RPC interface securely, as suggested in https://github.com/bitcoin-core/bitcoincore.org/pull/637 by @luke-jr and @TheBlueMatt.  I think it should fit in well with #14458, but is not dependent on it (and shouldn't have any significant merge conflicts with it).

Tree-SHA512: e09d82c3029ed17a8bcf50722ea25a8c6c514731f3bce01908cbb6fe48bc96a3068a025beabebc602d18e1bc0dc3f2602848abc05dca1d3efe2a988ee50068c0

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diff --git a/doc/JSON-RPC-interface.md b/doc/JSON-RPC-interface.md
index 59df541567..982afd5d56 100644
--- a/doc/JSON-RPC-interface.md
+++ b/doc/JSON-RPC-interface.md
@@ -5,6 +5,85 @@ The headless daemon `bitcoind` has the JSON-RPC API enabled by default, the GUI
 option. In the GUI it is possible to execute RPC methods in the Debug Console
 Dialog.
 
+## Security
+
+The RPC interface allows other programs to control Bitcoin Core,
+including the ability to spend funds from your wallets, affect consensus
+verification, read private data, and otherwise perform operations that
+can cause loss of money, data, or privacy.  This section suggests how
+you should use and configure Bitcoin Core to reduce the risk that its
+RPC interface will be abused.
+
+- **Securing the executable:** Anyone with physical or remote access to
+  the computer, container, or virtual machine running Bitcoin Core can
+  compromise either the whole program or just the RPC interface.  This
+  includes being able to record any passphrases you enter for unlocking
+  your encrypted wallets or changing settings so that your Bitcoin Core
+  program tells you that certain transactions have multiple
+  confirmations even when they aren't part of the best block chain.  For
+  this reason, you should not use Bitcoin Core for security sensitive
+  operations on systems you do not exclusively control, such as shared
+  computers or virtual private servers.
+
+- **Securing local network access:** By default, the RPC interface can
+  only be accessed by a client running on the same computer and only
+  after the client provides a valid authentication credential (username
+  and passphrase).  Any program on your computer with access to the file
+  system and local network can obtain this level of access.
+  Additionally, other programs on your computer can attempt to provide
+  an RPC interface on the same port as used by Bitcoin Core in order to
+  trick you into revealing your authentication credentials.  For this
+  reason, it is important to only use Bitcoin Core for
+  security-sensitive operations on a computer whose other programs you
+  trust.
+
+- **Securing remote network access:** You may optionally allow other
+  computers to remotely control Bitcoin Core by setting the `rpcallowip`
+  and `rpcbind` configuration parameters.  These settings are only meant
+  for enabling connections over secure private networks or connections
+  that have been otherwise secured (e.g. using a VPN or port forwarding
+  with SSH or stunnel).  **Do not enable RPC connections over the public
+  Internet.**  Although Bitcoin Core's RPC interface does use
+  authentication, it does not use encryption, so your login credentials
+  are sent as clear text that can be read by anyone on your network
+  path.  Additionally, the RPC interface has not been hardened to
+  withstand arbitrary Internet traffic, so changing the above settings
+  to expose it to the Internet (even using something like a Tor hidden
+  service) could expose you to unconsidered vulnerabilities.  See
+  `bitcoind -help` for more information about these settings and other
+  settings described in this document.
+
+    Related, if you use Bitcoin Core inside a Docker container, you may
+    need to expose the RPC port to the host system.  The default way to
+    do this in Docker also exposes the port to the public Internet.
+    Instead, expose it only on the host system's localhost, for example:
+    `-p 127.0.0.1:8332:8332`
+
+- **Secure authentication:** By default, Bitcoin Core generates unique
+  login credentials each time it restarts and puts them into a file
+  readable only by the user that started Bitcoin Core, allowing any of
+  that user's RPC clients with read access to the file to login
+  automatically.  The file is `.cookie` in the Bitcoin Core
+  configuration directory, and using these credentials is the preferred
+  RPC authentication method.  If you need to generate static login
+  credentials for your programs, you can use the script in the
+  `share/rpcauth` directory in the Bitcoin Core source tree.  As a final
+  fallback, you can directly use manually-chosen `rpcuser` and
+  `rpcpassword` configuration parameters---but you must ensure that you
+  choose a strong and unique passphrase (and still don't use insecure
+  networks, as mentioned above).
+
+- **Secure string handling:** The RPC interface does not guarantee any
+  escaping of data beyond what's necessary to encode it as JSON,
+  although it does usually provide serialized data using a hex
+  representation of the bytes.  If you use RPC data in your programs or
+  provide its data to other programs, you must ensure any problem
+  strings are properly escaped.  For example, multiple websites have
+  been manipulated because they displayed decoded hex strings that
+  included HTML `<script>` tags.  For this reason, and other
+  non-security reasons, it is recommended to display all serialized data
+  in hex form only.
+
 ## RPC consistency guarantees
 
 State that can be queried via RPCs is guaranteed to be at least up-to-date with