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<HTML>
<!-- SECTION: Getting Started -->
<HEAD>
<TITLE>Server Security</TITLE>
<LINK REL="STYLESHEET" TYPE="text/css" HREF="../cups-printable.css">
</HEAD>
<BODY>
<H1 CLASS="title">Server Security</H1>
<P>In the default "standalone" configuration, there are few
potential security risks - the CUPS server does not accept remote
connections, and only accepts shared printer information from the
local subnet. When you share printers and/or enable remote
administration, you expose your system to potential unauthorized
access. This help page provides an analysis of possible CUPS
security concerns and describes how to better secure your
server.</P>
<H2 CLASS="title"><A NAME="AUTHENTICATION">Authentication Issues</A></H2>
<P>When you enable remote administration, the server will use
Basic authentication for administration tasks. The current CUPS
server supports Basic, Digest, Kerberos, and local certificate
authentication:</P>
<OL>
<LI>Basic authentication essentially places the clear
text of the username and password on the network.
<P>Since CUPS uses the system username and password
account information, the authentication information could
be used to gain access to possibly privileged accounts on
the server.</P>
<P><B>Recommendation:</B> Enable encryption to hide the
username and password information - this is the default on
MacOS X and systems with GNU TLS or OpenSSL installed.</P></LI>
<LI>Digest authentication uses an MD5 checksum of the
username, password, and domain ("CUPS"), so the original
username and password is not sent over the network.
<P>The current implementation does not authenticate the
entire message and uses the client's IP address for the
nonce value, making it possible to launch "man in the
middle" and replay attacks from the same client.</P>
<P><B>Recommendation:</B> Enable encryption to hide the
username and password information.</P></LI>
<LI>Local certificate authentication passes 128-bit
"certificates" that identify an authenticated user.
Certificates are created on-the-fly from random data and
stored in files under <VAR>/var/run/cups/certs</VAR>.
They have restricted read permissions: root +
system-group(s) for the root certificate, and lp + lp
for CGI certificates.
<P>Because certificates are only available on the local
system, the CUPS server does not accept local
authentication unless the client is connected to the
loopback interface (127.0.0.1 or ::1) or domain
socket.</P>
<P><B>Recommendation:</B> Ensure that unauthorized users
are not added to the system group(s).</P></LI>
</OL>
<H2 CLASS="title"><A NAME="DOS">Denial of Service Attacks</A></H2>
<P>When printer sharing or remote administration is enabled, the
CUPS server, like all Internet services, is vulnerable to a
variety of denial of service attacks:</P>
<OL>
<LI>Establishing multiple connections to the server until
the server will accept no more.
<P>This cannot be protected against by any known
software. The <CODE>MaxClientsPerHost</CODE> directive
can be used to configure CUPS to limit the number of
connections allowed from a single host, however that does
not prevent a distributed attack.</P>
<P><B>Recommendation:</B> Limit access to trusted systems
and networks.</P></LI>
<LI>Repeatedly opening and closing connections to the
server as fast as possible.
<P>There is no easy way of protecting against this in the
CUPS software. If the attack is coming from outside the
local network, it may be possible to filter such an
attack. However, once the connection request has been
received by the server it must at least accept the
connection to find out who is connecting.</P>
<P><B>Recommendation:</B> None.</P></LI>
<LI>Flooding the network with broadcast packets on port
631.
<P>It might be possible to disable browsing if this
condition is detected by the CUPS software, however if
there are large numbers of printers available on the
network such an algorithm might think that an attack was
occurring when instead a valid update was being
received.</P>
<P><B>Recommendation:</B> Block browse packets from
foreign or untrusted networks using a router or
firewall.</P></LI>
<LI>Sending partial IPP requests; specifically, sending
part of an attribute value and then stopping
transmission.
<P>The current code will wait up to 1 second before
timing out the partial value and closing the connection.
This will slow the server responses to valid requests and
may lead to dropped browsing packets, but will otherwise
not affect the operation of the server.</P>
<P><B>Recommendation:</B> Block IPP packets from foreign
or untrusted networks using a router or
firewall.</P></LI>
<LI>Sending large/long print jobs to printers, preventing
other users from printing.
<P>There are limited facilities for protecting against
large print jobs (the <CODE>MaxRequestSize</CODE>
attribute), however this will not protect printers from
malicious users and print files that generate hundreds or
thousands of pages.</P>
<P><B>Recommendation:</B> Restrict printer access to
known hosts or networks, and add user-level access
controls as needed for expensive printers.</P></LI>
</OL>
<H2 CLASS="title"><A NAME="ENCRYPTION">Encryption Issues</A></H2>
<P>CUPS supports 128-bit SSL 3.0 and TLS 1.0 encryption of
network connections via the OpenSSL, GNU TLS, and CDSA encryption
libraries. In additional to the potential security issues posed
by the SSL and TLS protocols, CUPS currently has the following
additional issue:</P>
<OL>
<LI>Certification validation/revocation; currently CUPS
does not validate or revoke server or client certificates
when establishing a secure connection. This can
potentially lead to "man in the middle" and
impersonation/spoofing attacks over unsecured networks.
Future versions of CUPS will support both validation and
revocation of server certificates.
<P><B>Recommendation:</B> Do not depend on encryption for
security when connecting to servers over the Internet or
untrusted WAN links.</P></LI>
</OL>
</BODY>
</HTML>
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