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<sect1 id="security">
<title>HTTP Client Security</title>
<para>&neon; is intended to be secure against a specific threat
model: use of a malicious HTTP server. Under this threat model, a
range of attacks are possible against a client when the user (or
application) can be tricked into accessing an HTTP server which is
controlled by an attacker. This section documents various types of
possible attack and describes what mitigation is used in
&neon;.</para>
<sect2>
<title>CPU or memory consumption attacks</title>
<para>&neon; uses fixed resource limits to prevent the following
attacks:</para>
<itemizedlist>
<listitem>
<para>memory/CPU consumption attack using an unbounded number
of response header fields</para>
</listitem>
<listitem>
<para>memory consumption attack using an unbounded length of
individual response header lines (or continuation
headers)</para>
</listitem>
<listitem>
<para>memory consumption attack against the PROPFIND code
using an unbounded number of properties (propstat elements)
per resource</para>
</listitem>
<listitem>
<para>memory consumption attack against the PROPFIND code
using an unbounded CDATA element in a "flat property"
value</para>
</listitem>
</itemizedlist>
<para>Memory consumption attacks against applications based on
&neon; by use of unbounded response length are also possible, but
must be mitigated at application level. Memory consumption in
&neon; itself is fixed and is not proportional to the response
size.</para>
<para>Test cases for all the above attacks are present in the
&neon; test suite.</para>
</sect2>
<sect2>
<title>SSL/TLS connection security</title>
<para>When using a connection secured by SSL/TLS, it is necessary
for clients to verify that the X.509 certificate presented by the
server matches the server's expected identity. The algorithm
required for this purpose is described in RFC 2818 and RFC 3280,
and is implemented by &neon; in the following manner:</para>
<itemizedlist>
<listitem>
<para>the hostname argument passed to <xref
linkend="ne_session_create"/> is the expected identity of the
server</para>
</listitem>
<listitem>
<para>the subjectAltName extension of the certificate is used
for comparision against the expected identity, in preference
to the Subject name's commonName attribute.</para>
</listitem>
<listitem>
<para>the dNSName, iPAddress, and uniformResourceIdentifier
classes of GeneralName are supported in subjectAltName
comparison.</para>
</listitem>
<listitem>
<para>if no subjectAltName is present in the certificate, the
most specific commonName attribute in the Subject name is used
for comparison instead.</para>
</listitem>
</itemizedlist>
<para>In the case where a server certificate is presented that
does not match the expected identity (or is otherwise not
trusted), &neon; will fail the request by default. This behaviour
can be overridden by the use of a callback installed using <xref
linkend="ne_ssl_set_verify"/>, which allows the application to
present the certificate details to a user for manual/off-line
verification, if possible.</para>
<para>Test cases for the correctness of the implementation of the
identity verification algorithm are present in the &neon; test
suite.</para>
</sect2>
<sect2>
<title>Control character insertion in error messages</title>
<para>Where error messages (as returned by
(<xref linkend="ne_get_error"/>) contain data supplied by the
server, the untrusted data is sanitised to remove both control
characters and non-ASCII characters. This prevents any attacks
where such error messages are exposed to the user and can
potentially distort the presentation of the interface (for
example, through the use of a carriage return character in a text
user interface).</para>
</sect2>
<sect2>
<title>Attacks against authentication credentials</title>
<para>Authentication credentials can be compromised by a
"downgrade attack" by an active attacker; for example, where a
MITM presents a Basic authentication challenge in place of the
server's Digest challenge. &neon; mitigates these attacks by
allowing the application (and hence, user) to specify that only a
specific set of authentication protocols is permitted.</para>
<para>&neon; supports the Digest and Negotiate authentication
schemes, which both allow authentication of users without passing
credentials in cleartext over the wire. The "domain" parameter is
supported in Digest, allowing the server to restrict an
authentication session to a particular set of URIs.</para>
</sect2>
</sect1>
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