DRAFT - CUPS Software Security Report
CUPS-SSR-1.0
Easy Software Products
Copyright 1997-2000, All Rights Reserved
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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:
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:
This problem can be alleviated by making the request directory readable only by the user specified in the CUPS configuration file.
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:
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.
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.
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.
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.
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.