Forwarded From: x-forcet_private -----BEGIN PGP SIGNED MESSAGE----- ISS Security Alert Advisory June 29, 1998 Distributed DoS attack against NIS/NIS+ based networks. For purposes of this report, NIS refers to both NIS and NIS+ as this problem has been observed and reproduced on both services. Synopsis: It is possible, through a well orchestrated attack using the finger service against multiple NIS clients, to disrupt an entire NIS based network and/or starve the NIS servers for resources. The problem is in the finger service but the attack causes long duration, network-wide, congestion and resource exhaustion on NIS servers. Recommended action: Disable finger service on any systems connected to any NIS based network. Disable access to internal finger services at all perimeter defenses (firewalls, gateways, filtering routers, etc.). If the finger service is required for some specific purpose, limit it to the minimum number of restricted hosts or to hosts which are NOT participating in NIS. For those who wish to continue to run the finger service on their systems, there are other possible actions that could be taken, like using a public domain fingerd that doesn't do ambiguous lookups. The finger service can also be protected by even something as simple as wrapping finger to not do ambiguous lookups like as follows: # mv /usr/bin/finger /usr/bin/finger.exe # cat > /usr/bin/finger #!/bin/sh exec /usr/bin/finger.exe -m $* ^D # chmod +x /usr/bin/finger These recommendations would permit the continued safe use of finger (or as safe as finger ever gets). Description: A finger request results in multiple NIS requests as the responding daemon attempts to locate all account records matching the finger request. A request for finger foot_private will result in one finger daemon searching incrementally through all of the entries in the passwd map to locate any accounts with foo in the name. As a consequence, a single finger request can result in a significantly larger amount of traffic between the NIS client and the NIS server than the originating traffic to and from the finger service. The amount of NIS traffic is dependent on the size of the NIS passwd map. With a passwd map of 10,000 entries, a single finger request would result in roughly 10,000 NIS requests and 10,000 NIS responses. This does NOT count retries from packet loss or other failures (a highly significant factor in this attack). By sending a large number of overlapping finger requests to a single host, it is possible to load that host down with a very significant amount of traffic just processing the NIS requests. If this is done to multiple hosts, the network traffic rises dramatically. Eventually, a condition arises in which congestion and/or resource exhaustion on the NIS server begin to cause a significant rise in lost packets and failed requests. This results in retry attempts from the NIS clients, adding to the already overloaded network traffic. The failure / retry / failure cycle becomes an NIS traffic "storm" in which the retry traffic dominates and little other traffic can squeeze through. Network congestion combined with NIS server resource exhaustion work together to not only deny service to the requesting clients but also to rapidly clog the network bandwidth and render the network unusable by anything on the network. Analysis and details: In analyzing this attack, a perl script was used to generate finger traffic attacking a dozen hosts with four finger requests for each of approximately 100 names (~400 finger requests per host). A demonstration NIS map of approximately 1000 accounts was used. At an issue rate of approximately 4 finger requests every two seconds against a given host, 10's to 100's of lingering finger requests would build up even as some finger requests would be fufilled. These lingering finger processes would be attempting to paw their way through the entire NIS password map. A typical test run attack lasted approximately 30-50 seconds in duration. During analysis of this attack, network traffic from even a short ~30 seconds blast from the perl test script resulted in traffic levels that caused network disruptions extending for as much as 45 minutes to an hour after cessation of the attack. During this time, some systems were impacted to the extent that screen savers froze and systems were unresponsive to the keyboards. Many systems were left with seemingly hung finger processes. These stayed on the system for a half an hour or more while the network congestion cleared. Some systems ran out of swap space because of the resource demands of the finger processes. On a few of the test runs the network traffic was observed to have risen to a level which caused a switched ethernet hub to disable ports due to excesive collisions. Finger requests to perform this action have to be distributed and timed properly. Too many requests, too quickly, seem to result in inetd disabling the finger service. Too slowly, and the network traffic rises too slowly and fails to reach the catastrophic level where packet loss and retries become the dominant traffic input to the network. Because the finger requests are TCP based and not dependent on preauthentication, finger requests can still be delivered by the attacker to the systems under attack even in the face of increasing network congestion. By the time the attacking connections are significantly impacted by the network congestion, the network has been rendered unusable by systems requiring NIS or other services. Timed correctly, an attack of only a few seconds, targeting as few as a dozen NIS clients on a network with a moderate NIS passwd map can render even a small network unusable for as long as a half an hour to an hour or more. Increasing the size of the NIS passwd map, the number of attacked clients, or the number of requests sent to any given client causes the recovery time to extend out dramatically and disproportionately to any particular increases in any particular factor. If the NIS server is also one of the attacked systems, it can rapidly run out of system resources, causing NIS request failures and accelerating the resulting NIS traffic "storm". When the NIS server was one of the systems attacked by finger requests, it was not unusual to see warnings about unable to grow stack, exhausted virtual memory, or other resource related errors. MOST client systems seem to clean themselves up EVENTUALLY. This can take anywhere from a few moments for some Linux boxes, to a significant fraction of an hour for some SUN boxes. It was observed that some IRIX boxes and AIX boxes would become unreachable from the network and unresponsive to the keyboard, requiring a power cycle to recover. These last systems may have recovered on their own eventually, but that time frame appears to be geological. Recovery time seems to also be dependent on the recovery time of the NIS server for those clients which were observed to recover. Resetting the targeted systems permits the network to recover. All tested systems were affected to some extent. Because the resulting traffic and congestion is proportional to the size of the NIS passwd map times the number of attacked hosts times the number of requests in the attack, large networks are disproportionately vulnerable to this attack. Even small networks of a few dozen systems can be disabled by a determined attacker if they have a sufficiently large NIS passwd map. Conclusion: The finger service permits a condition where a limited number of requests can result in a vastly larger number of internal requests against a central naming service such as NIS. This permits an attacker to mount a distributed attack by launching smaller attacks against numerous hosts. These combine to form a disasterous level of congestion on the internal systems, disrupting an internal network for an extended period of time. Afterword: It is unknown, at this time, if any other services exhibit similar characteristics with regard to NIS traffic as does finger. Disabling finger prevents it from being exploited against a network. It obviously does not guarantee that some other service might be similarly exploitable. Credits: We would like to extend our appreciation to Sun Microsystems, Inc. for their assistance and consultation with regard to the vulnerability. Michael H. Warfield Senior Researcher ISS X-Force Internet Security Systems, Inc. ________ Copyright (c) 1998 by Internet Security Systems, Inc. Permission is hereby granted for the redistribution of this alert electronically. It is not to be edited in any way without express consent of X-Force. If you wish to reprint the whole or any part of this alert in any other medium excluding electronic medium, please email xforcet_private for permission. Disclaimer The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties with regard to this information. In no event shall the author be liable for any damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk. X-Force PGP Key available at: http://www.iss.net/xforce/sensitive.html as well as on MIT's PGP key server and PGP.com's key server. X-Force Vulnerability and Threat Database: http://www.iss.net/xforce Please send suggestions, updates, and comments to: X-Force <xforcet_private> of Internet Security Systems, Inc. -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBNZfaJjRfJiV99eG9AQEFDAP/Y5k+8q5lmasrZotzcVw5z43LBUy9AL6l By8onp07PmIdkQD7vCyiSIHqEvWiGigvhl76gdnkZxUowSYjdI+PfZBbB9S1HoZB eMRNjd0m5X7aEimIH208TwdegxwLu+A7/FKnl3EImd182Nzma1PhAqXpclPEmJ38 mUbk7Lm6Yls= =HXJi -----END PGP SIGNATURE----- -o- Subscribe: mail majordomot_private with "subscribe isn". Today's ISN Sponsor: Repent Security Incorporated [www.repsec.com]
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