Vulnerability Note VU#211736

Alcatel ADSL modems grant unauthenticated TFTP access via Bounce Attacks

Original Release date: 10 Apr 2001 | Last revised: 11 Apr 2001

Overview

The San Diego Supercomputer Center (SDSC) has recently discovered several vulnerabilities in the Alcatel Speed Touch line of Asymmetric Digital Subscriber Line (ADSL) modems. These vulnerabilities are the result of weak authentication and access control policies and result in one or more of the following impacts: unauthorized access, unauthorized monitoring, information leakage, denial of service, and permanent disability of affected devices.

The SDSC has published additional information regarding these vulnerabilities at http://security.sdsc.edu/self-help/alcatel/.

Description

Alcatel ADSL modems allow unauthenticated Trivial File Transfer Protocol (TFTP) access from the local area network (LAN) as a method to update firmware and to make configuration changes to the device. In conjunction with one of several common vulnerabilities, a remote attacker may be able to gain unauthenticated access as well.


For example, if a system on the LAN side of the ADSL modem has the UDP echo service enabled, a remote attacker may be able to spoof packets such that the ADSL modem will believe that this traffic originated from the local network. By sending a packet to the UDP echo service with a spoofed source port of 69 (TFTP) and a source address of 255.255.255.255, the system providing the echo service can be tricked into sending a TFTP packet to the ADSL modem. If a system offering this service is accessible from the Internet it may be possible to use the system to attack the ADSL modem.

Any mechanism for "bouncing" UDP packets off systems on the LAN side of the network may potentially allow a remote attacker to gain TFTP access to the device. Gaining TFTP access to the device allows the remote attacker to essentially gain complete control of the device.

Impact

A remote attacker may be able to gain access to the perform TFTP operations. These operations include:

    - inspection of configuration data
    - recovery and setting of passwords
    - inspection and updates to the firmware
    - destructive updates to the firmware
    - malicious custom updates to the firmware

Note that affected Alcatel ADSL modems do not provide any mechanism for determining the validity of firmware updates, so a remote attacker may be able to install custom firmware that operated as a DDoS client or a network sniffer. Similarly, an attacker could produce an invalid firmware revision that would disable the device completely, leaving victims no alternative but to return the disabled unit to the manufacturer.

Solution

Block malicious traffic at your network perimeter

If you have a home firewall product you may be able to prevent the TFTP UDP bounce attack by filtering one or more of the following types of traffic:

    - Packets with spoofed source addresses
    - Packets with a source address of 255.255.255.255
    - Packets with a destination port of echo (or other "simple" services)

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
AlcatelAffected10 Apr 200110 Apr 2001
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A

References

Credit

The CERT Coordination Center would like to thank Tom Perrine and Tsutomu Shimomura of the San Diego Supercomputer Center for notifying us about this problem and their help in constructing this advisory.

This document is based on research by the SDSC and was written by Cory Cohen and Jeffrey P. Lanza.

Other Information

  • CVE IDs: Unknown
  • CERT Advisory: CA-2001-08
  • Date Public: 10 Apr 2001
  • Date First Published: 10 Apr 2001
  • Date Last Updated: 11 Apr 2001
  • Severity Metric: 27.56
  • Document Revision: 15

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