124 lines
4.4 KiB
HTML
124 lines
4.4 KiB
HTML
<HTML>
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<!-- SECTION: Getting Started -->
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<HEAD>
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<TITLE>Server Security</TITLE>
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<LINK REL="STYLESHEET" TYPE="text/css" HREF="../cups-printable.css">
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</HEAD>
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<BODY>
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<H1 CLASS="title">Server Security</H1>
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<P>In the default "standalone" configuration, there are few
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potential security risks - the CUPS server does not accept remote
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connections, and only accepts shared printer information from the
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local subnet. When you share printers and/or enable remote
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administration, you expose your system to potential unauthorized
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access. This help page provides an analysis of possible CUPS
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security concerns and describes how to better secure your
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server.</P>
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<H2 CLASS="title"><A NAME="AUTHENTICATION">Authentication Issues</A></H2>
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<P>When you enable remote administration, the server will use Basic authentication for administration tasks. The current CUPS server supports Basic, Kerberos, and local certificate authentication:</P>
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<OL>
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<LI>Basic authentication essentially places the clear
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text of the username and password on the network.
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<P>Since CUPS uses the system username and password
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account information, the authentication information could
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be used to gain access to possibly privileged accounts on
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the server.</P>
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<P><B>Recommendation:</B> Enable encryption to hide the
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username and password information - this is the default on
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macOS and systems with GNU TLS installed.</P></LI>
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<LI>Local certificate authentication passes 128-bit
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"certificates" that identify an authenticated user.
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Certificates are created on-the-fly from random data and
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stored in files under <VAR>/var/run/cups/certs</VAR>.
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They have restricted read permissions: root +
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system-group(s) for the root certificate, and lp + lp
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for CGI certificates.
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<P>Because certificates are only available on the local
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system, the CUPS server does not accept local
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authentication unless the client is connected to the
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loopback interface (127.0.0.1 or ::1) or domain
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socket.</P>
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<P><B>Recommendation:</B> Ensure that unauthorized users
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are not added to the system group(s).</P></LI>
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</OL>
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<H2 CLASS="title"><A NAME="DOS">Denial of Service Attacks</A></H2>
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<P>When printer sharing or remote administration is enabled, the
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CUPS server, like all Internet services, is vulnerable to a
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variety of denial of service attacks:</P>
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<OL>
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<LI>Establishing multiple connections to the server until
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the server will accept no more.
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<P>This cannot be protected against by any known
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software. The <CODE>MaxClientsPerHost</CODE> directive
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can be used to configure CUPS to limit the number of
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connections allowed from a single host, however that does
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not prevent a distributed attack.</P>
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<P><B>Recommendation:</B> Limit access to trusted systems
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and networks.</P></LI>
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<LI>Repeatedly opening and closing connections to the
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server as fast as possible.
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<P>There is no easy way of protecting against this in the
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CUPS software. If the attack is coming from outside the
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local network, it may be possible to filter such an
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attack. However, once the connection request has been
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received by the server it must at least accept the
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connection to find out who is connecting.</P>
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<P><B>Recommendation:</B> None.</P></LI>
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<LI>Sending partial IPP requests; specifically, sending
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part of an attribute value and then stopping
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transmission.
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<P>The current code will wait up to 1 second before
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timing out the partial value and closing the connection.
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This will slow the server responses to valid requests and
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may lead to dropped browsing packets, but will otherwise
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not affect the operation of the server.</P>
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<P><B>Recommendation:</B> Block IPP packets from foreign
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or untrusted networks using a router or
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firewall.</P></LI>
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<LI>Sending large/long print jobs to printers, preventing
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other users from printing.
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<P>There are limited facilities for protecting against
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large print jobs (the <CODE>MaxRequestSize</CODE>
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attribute), however this will not protect printers from
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malicious users and print files that generate hundreds or
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thousands of pages.</P>
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<P><B>Recommendation:</B> Restrict printer access to
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known hosts or networks, and add user-level access
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controls as needed for expensive printers.</P></LI>
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</OL>
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<H2 CLASS="title"><A NAME="ENCRYPTION">Encryption Issues</A></H2>
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<P>CUPS supports 128-bit TLS encryption of network connections via the GNU TLS library, macOS Security framework, and Windows Schannel APIs. Secure deployment of TLS depends on proper certificate management and software maintenance.</P>
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</BODY>
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</HTML>
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