CUPS Software Security Report
Author: Easy Software Products

# Scope

## Identification

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

## Document Overview

This software security report is organized into the following sections:

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

# 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.

## 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.

# 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.

## 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.

## 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 (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.