1<HTML> 2<!-- SECTION: Getting Started --> 3<HEAD> 4 <TITLE>Server Security</TITLE> 5 <LINK REL="STYLESHEET" TYPE="text/css" HREF="../cups-printable.css"> 6</HEAD> 7<BODY> 8 9<H1 CLASS="title">Server Security</H1> 10 11<P>In the default "standalone" configuration, there are few 12potential security risks - the CUPS server does not accept remote 13connections, and only accepts shared printer information from the 14local subnet. When you share printers and/or enable remote 15administration, you expose your system to potential unauthorized 16access. This help page provides an analysis of possible CUPS 17security concerns and describes how to better secure your 18server.</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 macOS and systems with GNU TLS 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 60CUPS server, like all Internet services, is vulnerable to a 61variety 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> 121 122</BODY> 123</HTML> 124