|
Advisory
Multiple Vulnerabilities in Alcatel ADSL-Ethernet Bridge devices |
||||||||||
|
[ The advisory in its original text form is available here ]
|
||||||||||
I. SummaryResearchers associated with the San Diego Supercomputer Center at the University of California, San Diego have identified multiple implementation flaws in the Alcatel Speed Touch ADSL "modem" (actually an ADSL-Ethernet router/bridge). These flaws can allow an intruder to take complete control of the device, including changing its configuration, uploading new firmware, and disrupting the communications between the telephone central office providing ADSL service and the device. These flaws allow the following malicious actions:
One of the more interesting discoveries was a cryptographic challenge-response back door that completely bypasses any password that a user may have set on the device. All testing to date has been done in LLC/SNAP bridge mode. Routing mode was not tested. There may be other flaws that are easier to exploit in that mode. (Speed Touch is a trademark of Alcatel.)
II. The Alcatel Speed Touch family of devicesThis advisory addresses the Speed Touch family of devices, and similar devices apparently based on related code such as the older Alcatel 1000 ADSL Network Termination device (1000 ADSL). All testing was performed on the "Speed Touch Home", and limited testing was performed on the 1000 ADSL. It is strongly suspected that the "Speed Touch Pro" software is at least very similar to that in the Speed Touch Home, so it is probable that the Pro is vulnerable to similar attacks. Other members of the family running software derived from the same code base would also be expected to share these vulnerabilities. Note that Alcatel renamed their entire Speed Touch product line a few weeks ago at CeBIT, so the Home and Pro may have new designations. The described flaws were demonstrated in all known firmware versions of the Speed Touch Home, including:
The Alcatel 1000 ADSL does not have a user-settable password and therefore does not share the cryptographic back door with the Speed Touch Home. It has the additional vulnerability that access through its HTTP server can not be restricted, and shares the TFTP vulnerabilities described below with the Speed Touch Home. The version of software in the 1000 ADSL tested was:
By default, the device uses the IP address 10.0.0.138, although this can be changed via HTTP, TFTP, or command line (TELNET) interface. The device can have multiple IP addresses at the same time. III. The Implementation FlawsThere are several flaws, including user authentication issues; fully-accessible TFTP servers, and a lack of validation of downloaded firmware. III.A Various user authentication issuesThe device has several flaws and one interesting "feature" in the area of authentication. III.A.1 Open front door - No default passwordAs shipped, the device allows for configuration read/write access with no password. This can be accomplished via TELNET or HTTP. The file structure of the device's file systems can be examined with FTP. The first mention of this appears to be from November 2000: http://www.vnunet.fr/actu/article.htm?numero=6197&date=2000-11-06 In this article (in French), they suggest that you might want to set the password before someone else does it for you. III.A.2 Missing roof - password may be stolen/changedThe password, if set, can be extracted from the device using TFTP. Or, TFTP can be used to set or change the existing password. None of these operations require any authentication at all. See (III.B) below on the use of TFTP. III.A.3 Cryptographic back door - bypassing the password completelyIf for some reason it is inconvenient to obtain or change the password with TFTP, it can be directly bypassed by logging in as the user "EXPERT", which will invoke a cryptographic challenge-response sequence. The password will then be the result of a cryptographic function applied to the "challenge" string presented immediately before the request for the password. For example, the FTP and TELNET dialogs look something like: ftp> open 10.0.0.138 Connected to 10.0.0.138. 220 Inactivity timer = 120 seconds. Use 'site idle In both examples above, the "challenge" string is 'SpeedTouch (00-90-D0-00-00-00)' and the response (typically a ten-digit integer) in this case is 1552815226. Each device will have a unique response as it has a different Ethernet MAC address, and the rest of the "challenge" string has sometimes changed between firmware versions. Neither the encryption algorithm nor its cryptovariables have been observed to change across devices or software versions. The biggest risk of this challenge-response scheme is that anyone who knows the cryptographic algorithm and cryptovariables used to validate the response has permanent access to ANY similar Alcatel SpeedTouch device. There is NO WAY currently known to us for anyone to disable this back door, other than by downloading our custom firmware (see III.C below). It is worth noting that all of these potentially passworded TCP services are supposedly available ONLY from the LAN side. As the device is a MAC-layer bridge, it has no way of actually enforcing this restriction, and in many cases these services are trivially reachable from the WAN side due to the configuration of the device and other devices on the LAN. III.B Open TFTP servers - via LAN, WAN and DSLAMThe open TFTP server is trivially accessible from the "inside" LAN, and access from the "outside" net may be only marginally more difficult. It appears to be accessible to the ADSL provider's DSLAM, or anyone with access to the copper ADSL loop, with no additional authentication. As shipped, the device provides an open TFTP server that can be used to transfer files both to and from the device. This can be used to extract the configuration from the device, or to change the configuration of the device, as well as change, destroy or subvert the device's firmware. For example, an attacker could replace the device's firmware with malicious code, such as a packet sniffer or a denial of service "zombie" such as Trin00 or TFN2K. There is no known way for the user/owner to disable the TFTP server. There is, of course, no authentication required for any TFTP access. III.B.1 Access via the inside LANSpecifically, the TFTP server is available over normal UDP/IP on the "inside" Ethernet, using any TFTP client. Using TFTP, one can extract the password and other configuration data, as well as a copy of the firmware. More importantly, one can also upload new configuration information, including a new (or no) password, as well as new (perhaps malicious) firmware. III.B.2 Access via the outside WAN (IP)It is possible to attack from the "outside" WAN via IP protocols by using any of the well-known methods to "bounce" UDP packets through a host on the internal network. This "attack" can be mounted no matter what the IP address of the Speed Touch device, whether it is still set to a non-routed address, such as the default 10.0.138, or whether the Speed Touch device has been set to an address on the inside network. The device's address does not even need to be known, as the TFTP server in the device listens to the IP broadcast address (255.255.255.255) IN ADDITION to any addresses configured by the user/owner. This behavior makes it trivial to "bounce" attacks through (for example) the UDP ECHO port of a host computer that is attached to the "inside" Ethernet network, without concern for what addresses the Speed Touch device may be configured for or the concern that it may be on a different logical subnet than the other systems on the inside Ethernet. In this example, one can send packets to the TFTP server from the outside by sending TFTP UDP packets with a source address of 255.255.255.255 and a source port of TFTP to the UDP ECHO port of any system on the internal network with a functioning UDP ECHO server. When the "ECHO server" replies to the request, it will interpret the (now) destination address of 255.255.255.255 as local broadcast, and the packet will be broadcast on the Ethernet with the destination port set to UDP TFTP. Many networking devices (including the Speed Touch) provide a UDP ECHO service, and in many cases (again, including the Speed Touch) there is no way to disable the service. It should be noted that the Speed Touch Home specifically does not process source-routed packets by default. This decision appears to be deliberate, as this is an easily configurable option that the documentation explicitly recommends not be changed. This configuration is presumably to discourage the obvious attack. The 1000 ADSL appears to not process source-routed packets at all. However, this option provides some possibilities for the attacker. If the attacker has only TFTP access (via a "bounce" or some other mechanism), they could write a new configuration to the device which would permit source-routing and default routing, and gain full access either by also setting a new password or by using the cryptographic back door. III.B.3 Access via the outside WAN (DSLAM)The Speed Touch device appears to have TFTP and SNMP servers listening directly on the WAN side on AAL5-encapsulated VPI/VCIs 15/16 and 15/64. This feature presumably exists so that the ADSL provider has full access to the device, without any form of authentication. Therefore the ADSL providers have the ability to upgrade the device, should Alcatel provide new firmware to address these or other issues. A paragraph from the Alcatel Speed Touch Installation and User Guide, 3EC 16830 AAAA TCZZA Ed. 02, p.152:
17.1 Software Download from the Network
This feature is controlled by the ADSL Provider. At some
point in time he might decide to upgrade the software in your
Speed Touch. This download will happen almost unnoticed.
You will see a change in the software version though if you
surf to the Speed Touch's Upgrade page.
These capabilities are also available to anyone with the proper equipment and access to the copper loop, such as at the residential TELCO DEMARC outside a home, or a street-side "ped". Theoretically, anyone who can emulate a central office DSLAM (ATU-C) can "clip on" to the phone line and take full control of the device. Note that since some of the same DMT chip sets are sold for use in both remote devices (ATU-R), such as the Speed Touch, and in central office equipment, such as DSLAMs (ATU-C), it is probable that constructing an improvised single-line "portable DSLAM" is not be out of reach for a somewhat determined attacker. III.C Inadequate validation of firmwareThe Alcatel devices do not appear to do any sort of authenticity or integrity checking on firmware downloaded to them. This behavior makes it easier for an abuser to generate a firmware file that will be accepted as a valid firmware "load". This bogus firmware could contain malicious code, such as a network sniffer or denial of service tool. As a demonstration a modified version of the firmware, with "interesting" security properties was loaded into a SpeedTouch Home. The firmware was accepted, decompressed, and executed without question. IV. Vendor informationSearches of the www.alcatel.com and associated sites turned up no security information, other than how to recover or reset the passwords and other configuration information from the router via physical access. An interesting item showed up at: http://www.alcatel.com/consumer/dsl/supfaqusa.htm#usa3
There's no firewall in the A1000 or Home, does it make these modems
unsafe to use?
Absolutely not. When in standard settings, these modems do not allow
any connection from the outside world to your modem or computer,
except when requested by your machine. This means it only allows
replies on your request, for example the loading of a webpage after
clicking a link. When a computer, unknown to your modem, is trying to
connect to your modem or computer, it will be blocked.
Caveat emptor. V. Commentary and ObservationsIt is remarkable that for every method provided for accessing the box (HTTP,TELNET, FTP, and TFTP) it is possible to directly bypass any access controls the owner may try to put in place. It seems very poor form to let a user set a password that they believe will be enforced while deliberately leaving such a back door, especially given that there are other (well documented) mechanisms for clearing or resetting a password should it become lost. A malicious firmware load could be carried as a worm or virus payload to a host on the inside Ethernet, and could survive the eradication of the worm or virus on the host platform. One solution to the insufficient firmware validation problem would be for the firmware loader to verify that the offered firmware load was signed with a known digital signature key before being accepted for use. VI. Some notes on the "EXPERT" mode of operationThe Speed Touch Home has an EXPERT mode (distinct from the use of EXPERT to bypass the password mechanism) which can be used to discover interesting information about the ADSL line operational parameters, ATM cell statistics, etc. This mode can also be used to set a wide variety of device and interface parameters, as well as partitioning, formatting, and erasing the flash file system. It can provide extremely valuable information for debugging an ADSL connection. Entry to this mode is restricted by the same cryptographic challenge-response mechanism that is used as a back door to bypass the password. If the ADSL provider has not provided the password to the device, a tool is available to provide the password in the "Alcatel ADSL Modem Owner's Self-Help Guide", at: http://security.sdsc.edu/self-help/alcatel This page has some additional information related to this advisory, as well as some tools and hints for the Alcatel ADSL modem owner. VII. Workarounds and patchesNone known at this time. VIII. AuthorsTsutomu Shimomura is a Senior Fellow of the San Diego Supercomputer Center at the University of California, San Diego. He is a well-known technologist and security researcher and co-author of Takedown, on his pursuit and capture of computer outlaw Kevin Mitnick. Tom Perrine is the Manager of Security Technologies at the San Diego Supercomputer Center. He works in the areas of critical infrastructure protection, scalable security infrastructure, and computer intrusion analysis. The San Diego Supercomputer Center is a research unit of the University of California, San Diego, and the leading-edge site of the National Partnership for Advanced Computational Infrastructure. SDSC is sponsored by the National Science Foundation through NPACI and by other federal agencies, the State and the University of California, and private organizations. For additional information about SDSC, see http://www.sdsc.edu/ or contact David Hart at dhart@sdsc.edu or +1.858.534.8314. Correspondence regarding this notice should be sent to security@sdsc.edu or by telephone at +1.858.534.5050. ( $Id: alcatel-bugs.html,v 1.4 2002/05/19 04:51:40 tep Exp $ ) |
-->