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CUPS Software Security Report
CUPS Software Security Report
CUPS-SSR-1.1
Easy Software Products
Copyright 1997-2005, All Rights Reserved
1 Scope
2 References
3 Local Access Risks
4 Remote Access Risks
A Glossary
This software security report provides an analysis of possible
security concerns for the Common UNIX Printing System ("CUPS") Version
1.1.
CUPS provides a portable printing layer for UNIX®-based operating
systems. It has been developed by Easy
Software Products to promote a standard printing solution for all
UNIX vendors and users. CUPS provides the System V and Berkeley
command-line interfaces.
CUPS uses the Internet Printing Protocol ("IPP") as the basis for
managing print jobs and queues. The Line Printer Daemon ("LPD") Server
Message Block ("SMB"), and AppSocket (a.k.a. JetDirect) protocols are
also supported with reduced functionality. CUPS adds network printer
browsing and PostScript Printer Description ("PPD") based printing
options to support real-world printing under UNIX.
CUPS includes an image file RIP that supports printing of image files
to non-PostScript printers. A customized version of GNU Ghostscript
7.05 for CUPS called ESP Ghostscript is available separately to support
printing of PostScript files within the CUPS driver framework. Sample
drivers for Dymo, EPSON, HP, and OKIDATA printers are included that use
these filters.
Drivers for thousands of printers are provided with our ESP Print Pro
software, available at:
http://www.easysw.com/printpro/
CUPS is licensed under the GNU General Public License and GNU Library
General Public License. Please contact Easy Software Products for
commercial support and "binary distribution" rights.
This software security report is organized into the following
sections:
- 1 - Scope
- 2 - References
- 3 - Local Access Risks
- 4 - Remote Access Risks
- A - Glossary
The following CUPS documentation is referenced by this document:
- CUPS-CMP-1.1: CUPS Configuration Management Plan
- CUPS-IDD-1.1: CUPS System Interface Design Description
- CUPS-IPP-1.1: CUPS Implementation of IPP
- CUPS-SAM-1.1.x: CUPS Software Administrators Manual
- CUPS-SDD-1.1: CUPS Software Design Description
- CUPS-SPM-1.1.x: CUPS Software Programming Manual
- CUPS-SSR-1.1: CUPS Software Security Report
- CUPS-STP-1.1: CUPS Software Test Plan
- CUPS-SUM-1.1.x: CUPS Software Users Manual
- CUPS-SVD-1.1: CUPS Software Version Description
The following non-CUPS documents are referenced by this document:
-
Adobe PostScript Printer Description File Format Specification, Version
4.3.
-
Adobe PostScript Language Reference, Third Edition.
- IPP/1.1: Implementers Guide
- RFC 1179, Line Printer
Daemon Protocol
- RFC 2396, Uniform
Resource Identifiers (URI): Generic Syntax
- RFC 2567, Design Goals
for an Internet Printing Protocol
- RFC 2568, Rationale
for the Structure of the Model and Protocol for the Internet Printing
Protocol
- RFC 2569, Mapping
between LPD and IPP Protocols
- RFC 2616, Hypertext
Transfer Protocol -- HTTP/1.1
- RFC 2617, HTTP
Authentication: Basic and Digest Access Authentication
- RFC 2910, IPP/1.1:
Encoding and Transport
- RFC 2911, IPP/1.1:
Model and Semantics
- RFC 3380, IPP: Job and
Printer Set Operations
Local access risks are those that can be exploited only with a local
user account. This section does not address issues related to
dissemination of the root password or other security issues associated
with the UNIX operating system.
There is one known security vulnerability with local access:
- Device URIs are passed to backend filters in argv[0] and in an
environment variable. Since device URIs can contain usernames and
passwords it may be possible for a local user to gain access to a
remote resource.
We recommend that any password-protected accounts used for remote
printing have limited access priviledges so that the possible damages
can be minimized.
The device URI is "sanitized" (the username and password are removed)
when sent to an IPP client so that a remote user cannot exploit this
vulnerability.
Remote access risks are those that can be exploited without a local
user account and/or from a remote system. This section does not address
issues related to network or firewall security.
Like all Internet services, the CUPS server is vulnerable to denial
of service attacks, including:
- Establishing multiple connections to the server until the server
will accept no more.
Starting with CUPS 1.1.18, the MaxClientsPerHost
provides limited protection against DoS attacks, however it is not
effective against large-scale distributed attacks.
- Repeatedly opening and closing connections to the server as fast as
possible.
There is no easy way of protecting against this in the CUPS software.
If the attack is coming from outside the local network it might 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.
- Flooding the network with broadcast packets on port 631.
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.
- Sending partial IPP requests; specifically, sending part of an
attribute value and then stopping transmission.
The current code is structured to read and write the IPP request data
on-the-fly, so there is no easy way to protect against this for large
attribute values.
- Sending large/long print jobs to printers, preventing other users
from printing.
There are limited facilities for protecting against large print jobs
(the MaxRequestSize attribute), however this will not
protect printers from malicious users and print files that generate
hundreds or thousands of pages. In general, we recommend restricting
printer access to known hosts or networks, and adding user-level access
control as needed for expensive printers.
The current CUPS server supports Basic, Digest, and local certificate
authentication:
- Basic authentication essentially places the clear text of the
username and password on the network. Since CUPS uses the UNIX username
and password account information, the authentication information could
be used to gain access to accounts (possibly priviledged accounts) on
the server.
- 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. However, 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. The next minor release of CUPS
will support Digest authentication of the entire message body,
effectively stopping these methods of attack.
- 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
/etc/cups/certs
. They have restricted read permissions: root + system for the root
certificate, and lp + system for CGI certificates. Because certificates
are only available on the local system, the CUPS server does not accept
local authentication unless the client is connected to the localhost
address (127.0.0.1.)
The default CUPS configuration disables remote administration. We do
not recommend that remote administration be enabled for all hosts.
However, if you have a trusted network or subnet, access can be
restricted accordingly. Also, we highly recommend using Digest
authentication when possible. Unfortunately, most web browsers do not
support Digest authentication at this time.
- C
- A computer language.
- parallel
- Sending or receiving data more than 1 bit at a time.
- pipe
- A one-way communications channel between two programs.
- serial
- Sending or receiving data 1 bit at a time.
- socket
- A two-way network communications channel.
- ASCII
- American Standard Code for Information Interchange
- CUPS
- Common UNIX Printing System
- ESC/P
- EPSON Standard Code for Printers
- FTP
- File Transfer Protocol
- HP-GL
- Hewlett-Packard Graphics Language
- HP-PCL
- Hewlett-Packard Page Control Language
- HP-PJL
- Hewlett-Packard Printer Job Language
- IETF
- Internet Engineering Task Force
- IPP
- Internet Printing Protocol
- ISO
- International Standards Organization
- LPD
- Line Printer Daemon
- MIME
- Multimedia Internet Mail Exchange
- PPD
- PostScript Printer Description
- SMB
- Server Message Block
- TFTP
- Trivial File Transfer Protocol
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