ServerCrash.qhelp

<!DOCTYPE qhelp PUBLIC
"-//Semmle//qhelp//EN"
"qhelp.dtd">
<qhelp>

<overview>

	<p>

		Servers handle requests from clients until terminated
		deliberately by a server administrator. A client request that results
		in an uncaught server-side exception causes the current server
		response generation to fail, and should not have an effect on
		subsequent client requests.

	</p>

	<p>

		Under some circumstances, uncaught exceptions can however
		cause the entire server to terminate abruptly. Such a behavior is
		highly undesirable, especially if it gives malicious users the ability
		to turn off the server at will, which is an efficient
		denial-of-service attack.

	</p>

</overview>

<recommendation>

	<p>

		Ensure that the processing of client requests can not cause
		uncaught exceptions to terminate the entire server abruptly.

	</p>

</recommendation>

<example>

	<p>

		The following server implementation checks if a client-provided
		file path is valid and throws an exception if the check fails. It can
		be seen that the exception is uncaught, and it is therefore reasonable to
		expect the server to respond with an error response to client requests
		that cause the check to fail.

		But since the exception is uncaught in the context of an
		asynchronous callback invocation (<code>fs.access(...)</code>), the
		entire server will terminate instead.

	</p>

	<sample src="examples/server-crash.BAD.js"/>

	<p>
		To remedy this, the server can catch the exception explicitly with
		a <code>try/catch</code> block, and generate an appropriate error
		response instead:

	</p>

	<sample src="examples/server-crash.GOOD-A.js"/>

	<p>

		An alternative is to use an <code>async</code> and
		<code>await</code> for the asynchronous behavior, since the server
		will then print warning messages about uncaught exceptions instead of
		terminating, unless the server was started with the commandline option
		<code>--unhandled-rejections=strict</code>:

	</p>

	<sample src="examples/server-crash.GOOD-B.js"/>

</example>

<references>

</references>

</qhelp>