CUPS Software Security Report

Easy Software Products
Copyright 1997-2003, All Rights Reserved

Table of Contents

1 Scope 2 References 3 Local Access Risks 4 Remote Access Risks A Glossary

1 Scope

1.1 Identification

This software security report provides an analysis of possible security concerns for the Common UNIX Printing System ("CUPS") Version 1.1.

1.2 System Overview

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:

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.

1.3 Document Overview

This software security report is organized into the following sections:

2 References

2.1 CUPS Documentation

The following CUPS documentation is referenced by this document:

2.2 Other Documents

The following non-CUPS documents are referenced by this document:

3 Local Access Risks

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.

3.1 Security Breaches

There is one known security vulnerability with local access:

  1. 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.

4 Remote Access Risks

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.

4.1 Denial of Service Attacks

Like all Internet services, the CUPS server is vulnerable to denial of service attacks, including:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

4.2 Security Breaches

The current CUPS server supports Basic, Digest, and local certificate authentication:

  1. 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.
  2. 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.
  3. 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 (

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.

A Glossary

A.1 Terms

A computer language.
Sending or receiving data more than 1 bit at a time.
A one-way communications channel between two programs.
Sending or receiving data 1 bit at a time.
A two-way network communications channel.

A.2 Acronyms

American Standard Code for Information Interchange
Common UNIX Printing System
EPSON Standard Code for Printers
File Transfer Protocol
Hewlett-Packard Graphics Language
Hewlett-Packard Page Control Language
Hewlett-Packard Printer Job Language
Internet Engineering Task Force
Internet Printing Protocol
International Standards Organization
Line Printer Daemon
Multimedia Internet Mail Exchange
PostScript Printer Description
Server Message Block
Trivial File Transfer Protocol