Cisco Security Advisory
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AV:R/AC:L/Au:NR/C:P/I:P/A:P/B:N/E:F/RL:O/RC:C
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The Cisco VPN 3000 series concentrators are affected by two vulnerabilities when file management via File Transfer Protocol (FTP) is enabled that could allow authenticated or unauthenticated attackers to execute certain FTP commands and delete files on the concentrator.
None of the vulnerabilities allows unauthorized users to transfer files from or to the concentrator.
Cisco has made free software available to address these vulnerabilities for affected customers. There are workarounds available to mitigate these vulnerabilities as well.
This advisory is posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20060823-vpn3k.
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Vulnerable Products
Cisco VPN 3000 series concentrators 3005, 3015, 3020, 3030, 3060, and the 3080 are affected by these vulnerabilities if they are running a vulnerable software version and if the concentrator is configured to use FTP as a management protocol. FTP as a management protocol is enabled by default.
Vulnerable software versions are:
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Any version prior to 4.1
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Any 4.1.x version prior to, and including, 4.1(7)L
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Any 4.7.x version prior to, and including, 4.7(2)F
There are two ways to determine whether the FTP protocol is enabled as a file management protocol: using the web Graphical User Interface (GUI), or using a Command-Line Interface (CLI) via a console, telnet, or Secure Shell (SSH) connection.
To use the GUI to determine whether the FTP protocol is enabled as a file management protocol, connect to the web administration interface of the concentrator via the URL:
https://<IP address of concentrator>/admin/
Then log in to the concentrator using the device administrator credentials and go to the screen "Configuration | System | Management Protocols | FTP". This screen will indicate whether the FTP server on the concentrator is enabled.
To accomplish the same thing using the CLI, log in to the concentrator using the chosen access method (console, Telnet or SSH) and go to the same screen ("Configuration -> System Management -> Management Protocols -> Configure FTP"). The prompt will indicate with a number whether the FTP server is enabled.
Products Confirmed Not Vulnerable
The Cisco VPN 3002 Hardware Client is not affected by these vulnerabilities.
Please note that Cisco VPN 3000 Series concentrators are not affected by these vulnerabilities if FTP is not configured as a management protocol.
No other Cisco products are currently known to be affected by these vulnerabilities.
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Any version prior to 4.1
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The Cisco VPN 3000 series concentrators are a family of purpose-built, remote access Virtual Private Network (VPN) platforms for data encryption and authentication.
The File Transfer Protocol is an application-layer protocol that allows transfer of files between TCP/IP hosts. It uses Transmission Control Protocol (TCP) as the transport protocol and supports user authentication.
The Cisco VPN 3000 series concentrators can be configured to use the FTP protocol to manage files stored on the concentrator, like configuration files and certificates. Files can be uploaded to, or downloaded from, the concentrator for backup and configuration purposes.
Two vulnerabilities affect the Cisco VPN 3000 series concentrators when FTP is enabled as a file management protocol. By exploiting these vulnerabilities, an attacker could execute the following FTP commands:
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CWD - Change working directory
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MKD - Create (make) a directory
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CDUP - Change directory to the directory one level up
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RNFR - Rename file
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SIZE - Get file size
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RMD - Remove directory
These vulnerabilities are documented in Cisco Bug IDs CSCse10733 ( registered customers only) and CSCse10753 ( registered customers only) .
None of these vulnerabilities allows unauthorized users to download or upload files from/to the concentrator.
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CWD - Change working directory
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This section provides workarounds for these vulnerabilities.
Disabling FTP
If FTP is not needed to manage the VPN 3000 concentrator, network security best practices recommend that it be disabled. This will completely eliminate the vulnerability. Secure alternatives to FTP include Secure Hyper-Text Transfer Protocol (HTTPS) and Secure Copy (SCP) via Secure Shell (SSH).
The use of FTP as a management protocol can be disabled by connecting to the web administration interface of the concentrator via the URL:
https://<IP address of concentrator>/admin/
and then logging in to the concentrator using the device administrator credentials, going to the screen "Configuration | System | Management Protocols | FTP", and unchecking the "Enable" checkbox.
Limiting FTP Access
It is possible to limit the exposure of the concentrator by permitting remote FTP management connections only from known, trusted IP addresses. This is also considered a network security best practice.
Limiting FTP access to specific IP addresses can be accomplished on the VPN 3000 Series concentrators by defining rules and filters. The traffic filters can be mapped to the interfaces, LAN-to-LAN tunnels, and VPN groups. Traffic rules and filters are configured via the GUI in the screen "Configuration | Policy Management | Traffic Management".
To allow FTP access only from specific IP addresses, at least two traffic rules must be defined. The first rule would permit FTP traffic from a specific, trusted IP address or IP subnet. The second rule would deny all other FTP traffic directed at the private interface.
The parameters of the first rule could be:
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Rule Name: permit_ftp_in
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Direction: Inbound
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Action: Forward
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Protocol: TCP
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Source Address/Wildcard-mask: <Allowed IP address
range>
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Destination Address/Wildcard-mask: <IP address of
private interface>/0.0.0.0
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TCP/UDP Source Port: Range, from 0 to 65535
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TCP/UDP Destination Port: FTP (21)
This rule will permit inbound TCP port 21 (FTP) traffic but only when the traffic originates from a specific IP address range.
The parameters of the second rule could be:
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Rule Name: deny_ftp_in
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Direction: Inbound
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Action: Drop and Log
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Protocol: TCP
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Source Address/Wildcard-mask: 0.0.0.0/255.255.255.255
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Destination Address/Wildcard-mask: <IP address of
private interface>/0.0.0.0
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TCP/UDP Source Port: Range, from 0 to 65535
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TCP/UDP Destination Port: FTP (21)
This rule will drop and log all inbound TCP port 21 (FTP) traffic that is directed at the IP address of the private interface.
Note: When new rules are added to a filter, the new rules get added at the bottom of the list of existing rules in the filter. The concentrator processes rules in a filter in sequential order, so if at the bottom of the list there is a rule that permits all traffic (which is the case with the predefined "Private" filter applied by default to the Private interface), then the new rules will not be executed. For this reason it is important that the new rules intended to allow FTP traffic only from trusted addresses be moved to the top of the list of rules.
When the traffic rules have been created, they need to be added to a filter by going to the GUI screen "Configuration | Policy Management | Traffic Management | Filters", selecting the desired filter from the list, clicking on the "Assign Rules to Filter" button, and then selecting the new filter rules and clicking on the "<< Add" button.
To completely protect the VPN 3000 concentrator from unauthorized FTP traffic, the filter with the new traffic rules needs to be applied to the following:
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Interfaces: a filter is applied to an interface by going to the GUI
screen "Configuration | Interfaces", clicking on an interface name, and then
selecting a filter in the "General" tab.
By default the predefined filter "Private" is applied to the Private interface and the predefined filter "Public" is applied to the "Public" interface. The "Private" filter is very permissive and it is recommended that the above rules to limit FTP management traffic be added to this filter. The "Public" filter does not permit FTP traffic by default, and if changes have been made to permit FTP traffic, it is recommended that rules are added to only permit FTP traffic to the concentrator from specific IP addresses, as described above.
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LAN-to-LAN tunnels: a filter is applied to a LAN-to-LAN tunnel by
going to the GUI screen "Configuration | Tunneling and Security | IPSec |
LAN-to-LAN", selecting the LAN-to-LAN connection and clicking the "Modify"
button, and then selecting the specific filter in the "Modify an IPSec
LAN-to-LAN connection" screen. By default a LAN-to-LAN connection does not have
any filter applied to it.
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Remove Access VPN tunnels: a filter is applied to a remote access
VPN group by going to the GUI screen "Configuration | User Management |
Groups", selecting the VPN group and clicking the "Modify Group" button, and
then selecting the specific filter in the "General" tab of the "Modify Group"
screen. By default a VPN group does not have any filter applied to it.
By adding these two traffic rules to the filter applied to the an interface, LAN-to-LAN tunnel, and VPN group, the FTP traffic directed at the concentrator will only be allowed from the specific, trusted IP addresses, regardless of where the IP addresses are located, i.e. internal network or across a VPN tunnel. FTP traffic directed at the concentrator from anywhere else will be dropped and logged. All other FTP traffic is not affected by these rules.
If you do not wish to define multiple filters for each interface and the VPN tunnels, you can add the two traffic rules described above to the default "Private" filter and then apply this filter to the Private interface as well as to any LAN-to-LAN connections and Remote Access VPN groups.
Please note that using the "Access Control List" feature ("Administration | Access Rights | Access Control List") to control FTP access will not protect against these vulnerabilities because the check for valid IP addresses takes place at authentication time, and the actual vulnerability described in this document takes place before authentication.
In addition to the access control mechanism that can be implemented on the VPN 3000 concentrator itself, FTP access to the VPN 3000 concentrator could be blocked as part of an Infrastructure Access Control List (ACL) on screening routers, switches and firewalls controlling all access to the trusted network. Infrastructure ACLs are considered a network security best practice and should be considered as a long-term addition to good network security, as well as a workaround for this specific vulnerability. The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques:
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801a1a55.shtml
Backing Up Configuration and Certificates
Always keep backup copies, outside of the concentrator's file system, of the concentrator's configuration and certificates.
Management of configuration files can be done from the GUI screen "Administration | File Management". Please refer to the "File Management" section of the VPN 3000 Series Concentrator Reference, Volume II, for additional information:
http://www.cisco.com/en/US/docs/security/vpn3000/vpn3000_47/administration/guide/fileman.html
Management of certificates can be done from the GUI screen "Administration | Certificate Management". Please refer to the "Certificate Management" section of the VPN 3000 Series Concentrator Reference, Volume II, for additional information:
http://www.cisco.com/en/US/docs/security/vpn3000/vpn3000_47/administration/guide/certman.html
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Rule Name: permit_ftp_in
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When considering software upgrades, also consult http://www.cisco.com/go/psirt and any subsequent advisories to determine exposure and a complete upgrade solution.
In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco Technical Assistance Center ("TAC") or your contracted maintenance provider for assistance.
Both vulnerabilities are fixed in versions 4.1(7)M and 4.7(2)G of the VPN 3000 Series Concentrators software.
Fixed versions of the Cisco VPN 3000 Series concentrators software can be downloaded from the following location:
http://www.cisco.com/pcgi-bin/tablebuild.pl/vpn3000-3des?psrtdcat20e2 ( registered customers only)
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The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.
This vulnerability was reported to Cisco by the NCC Group (http://www.nccgroup.com). Cisco would like to thank the NCC Group for reporting this vulnerability and working with us towards resolution of this problem.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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