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AUSCERT External Security Bulletin Redistribution
ESB-98.114 -- CIAC Bulletin I-070
Distributed DoS Attack Against NIS/NIS+ Networks
17 July 1998
U.S. Department of Energy Computer Incident Advisory Capability (CIAC)
has released the following advisory concerning a vulnerability in the
NIS/NIS+ system. This vulnerability may allow remote users to
perform a denial of service attack upon a host and its associated
local network, including all hosts on that network.
The following security bulletin is provided as a service to AusCERT's
members. As AusCERT did not write this document, AusCERT has had
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The U.S. Department of Energy
Computer Incident Advisory Capability
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Distributed DoS Attack Against NIS/NIS+ Networks
July 15, 1998 20:00 GMT Number I-070
PROBLEM: The finger service is being used in attacks on NIS servers.
PLATFORM: NIS servers
DAMAGE: If exploited, the attacker may cause a denial of service.
SOLUTION: Disable finger or apply workaround listed below under
VULNERABILITY Avoid using finger if it is allowed to do ambiguous lookups. If
ASSESSMENT: 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.
[ Start ISS Advisory ]
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.
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.
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
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
exec /usr/bin/finger.exe -m $*
# chmod +x /usr/bin/finger
These recommendations would permit the continued safe use of finger
(or as safe as finger ever gets).
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 firstname.lastname@example.org 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.
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
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.
We would like to extend our appreciation to Sun Microsystems, Inc.
for their assistance and consultation with regard to the vulnerability.
Michael H. Warfield
Internet Security Systems, Inc.
Copyright (c) 1998 by Internet Security Systems, Inc.
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[ End ISS Advisory ]
CIAC wishes to acknowledge the contributions of Internet Security Systems,
Inc. for the information contained in this bulletin.
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