DRAFT - CUPS Software Security Report
CUPS-SSR-1.0
Easy Software Products
Copyright 1997-1999, All Rights Reserved
1 Scope
2 References
3 Local Access Risks
4 Remote Access Risks
A Glossary
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This software security report provides an analysis of possible
security concerns for the Common UNIX Printing System ("CUPS") Version
1.0.
The Common UNIX Printing System provides a portable printing layer for
UNIX® 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 (IETF-IPP) as the basis for
managing print jobs and queues. The Line Printer Daemon (LPD, RFC1179),
Server Message Block (SMB), and AppSocket protocols are also supported
with reduced functionality.
CUPS adds network printer browsing and PostScript Printer
Description ("PPD")-based printing options to support real world
applications under UNIX.
CUPS also includes a customized version of GNU GhostScript
(currently based off GNU GhostScript 4.03) and an image file RIP that
can be used to support non-PostScript printers.
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-1.0: CUPS Configuration Management Plan
- CUPS-IDD-1.0: CUPS System Interface Design Description
- CUPS-SAM-1.0.x: CUPS Software Administrators Manual
- CUPS-SDD-1.0: CUPS Software Design Description
- CUPS-SPM-1.0: CUPS Software Programming Manual
- CUPS-SSR-1.0: CUPS Software Security Report
- CUPS-STP-1.0: CUPS Software Test Plan
- CUPS-SUM-1.0.x: CUPS Software Users Manual
- CUPS-SVD-1.0.x: CUPS Software Version Description
The following non-CUPS documents are referenced by this document:
- IEEE 1387.4, System Administration: Printing (draft)
- IPP/1.0: Additional Optional Operations - Set 1
- RFC 1179, Line Printer Daemon Protocol
- RFC 2565, IPP/1.0: Encoding and Transport
- RFC 2566, IPP/1.0: Model and Semantics
- RFC 2639, IPP/1.0: Implementers Guide
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 are two known security vulnerabilities with local access:
- Since the default installation creates a world-readable request
directory, it is possible for local users to read the contents of
print files before they are printed.
This problem can be alleviated by making the request directory
readable only by the user specified in the CUPS configuration file.
- 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.
This cannot be protected against by the current software. It is
possible that future versions of the CUPS software could be configured
to limit the number of connections allowed from a single host, however
that still would not prevent a determined attack.
- 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 only supports Basic authentication with
usernames and passwords. This 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.
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.
The next minor release of CUPS will support Digest authentication of
the entire message body using separate MD5-based username and password
files. This will protect password information and prevent unauthorized
access due to compromised account passwords.
- 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 Printer 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
- PCL
- Page Control Language
- PPD
- PostScript Printer Description
- SMB
- Server Message Block
- TFTP
- Trivial File Transfer Protocol