doc/help/security.html

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   9 <H1 CLASS="title">Server Security</H1>
  10
  11 <P>In the default "standalone" configuration, there are few
  12 potential security risks - the CUPS server does not accept remote
  13 connections, and only accepts shared printer information from the
  14 local subnet. When you share printers and/or enable remote
  15 administration, you expose your system to potential unauthorized
  16 access. This help page provides an analysis of possible CUPS
  17 security concerns and describes how to better secure your
  18 server.</P>
  19
  20 <H2 CLASS="title"><A NAME="AUTHENTICATION">Authentication Issues</A></H2>
  21
  22 <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>
  23
  24 <OL>
  25
  26         <LI>Basic authentication essentially places the clear
  27         text of the username and password on the network.
  28
  29         <P>Since CUPS uses the system username and password
  30         account information, the authentication information could
  31         be used to gain access to possibly privileged accounts on
  32         the server.</P>
  33
  34         <P><B>Recommendation:</B> Enable encryption to hide the
  35         username and password information - this is the default on
  36         MacmacOS and systems with GNU TLS or OpenSSL installed.</P></LI>
  37
  38         <LI>Local certificate authentication passes 128-bit
  39         "certificates" that identify an authenticated user.
  40         Certificates are created on-the-fly from random data and
  41         stored in files under <VAR>/var/run/cups/certs</VAR>.
  42         They have restricted read permissions: root +
  43         system-group(s) for the root certificate, and lp + lp
  44         for CGI certificates.
  45
  46         <P>Because certificates are only available on the local
  47         system, the CUPS server does not accept local
  48         authentication unless the client is connected to the
  49         loopback interface (127.0.0.1 or ::1) or domain
  50         socket.</P>
  51
  52         <P><B>Recommendation:</B> Ensure that unauthorized users
  53         are not added to the system group(s).</P></LI>
  54
  55 </OL>
  56
  57 <H2 CLASS="title"><A NAME="DOS">Denial of Service Attacks</A></H2>
  58
  59 <P>When printer sharing or remote administration is enabled, the
  60 CUPS server, like all Internet services, is vulnerable to a
  61 variety of denial of service attacks:</P>
  62
  63 <OL>
  64
  65         <LI>Establishing multiple connections to the server until
  66         the server will accept no more.
  67
  68         <P>This cannot be protected against by any known
  69         software. The <CODE>MaxClientsPerHost</CODE> directive
  70         can be used to configure CUPS to limit the number of
  71         connections allowed from a single host, however that does
  72         not prevent a distributed attack.</P>
  73
  74         <P><B>Recommendation:</B> Limit access to trusted systems
  75         and networks.</P></LI>
  76
  77         <LI>Repeatedly opening and closing connections to the
  78         server as fast as possible.
  79
  80         <P>There is no easy way of protecting against this in the
  81         CUPS software. If the attack is coming from outside the
  82         local network, it may be possible to filter such an
  83         attack. However, once the connection request has been
  84         received by the server it must at least accept the
  85         connection to find out who is connecting.</P>
  86
  87         <P><B>Recommendation:</B> None.</P></LI>
  88
  89         <LI>Sending partial IPP requests; specifically, sending
  90         part of an attribute value and then stopping
  91         transmission.
  92
  93         <P>The current code will wait up to 1 second before
  94         timing out the partial value and closing the connection.
  95         This will slow the server responses to valid requests and
  96         may lead to dropped browsing packets, but will otherwise
  97         not affect the operation of the server.</P>
  98
  99         <P><B>Recommendation:</B> Block IPP packets from foreign
 100         or untrusted networks using a router or
 101         firewall.</P></LI>
 102
 103         <LI>Sending large/long print jobs to printers, preventing
 104         other users from printing.
 105
 106         <P>There are limited facilities for protecting against
 107         large print jobs (the <CODE>MaxRequestSize</CODE>
 108         attribute), however this will not protect printers from
 109         malicious users and print files that generate hundreds or
 110         thousands of pages.</P>
 111
 112         <P><B>Recommendation:</B> Restrict printer access to
 113         known hosts or networks, and add user-level access
 114         controls as needed for expensive printers.</P></LI>
 115
 116 </OL>
 117
 118 <H2 CLASS="title"><A NAME="ENCRYPTION">Encryption Issues</A></H2>
 119
 120 <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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