CA-95:01 CERT Advisory
January 23, 1995
IP Spoofing Attacks and Hijacked Terminal Connections
The CERT Coordination Center has received reports of attacks in which
intruders create packets with spoofed source IP addresses. These attacks
exploit applications that use authentication based on IP addresses. This
exploitation leads to user and possibly root access on the targeted system.
Note that this attack does not involve source routing. Recommended solutions
are described in Section III below.
In the current attack pattern, intruders may dynamically modify the kernel of
a Sun 4.1.X system once root access is attained. In this attack, which is
separate from the IP spoofing attack, intruders use a tool to take control of
any open terminal or login session from users on the system. Note that
although the tool is currently being used primarily on SunOS 4.1.x systems,
the system features that make this attack possible are not unique to SunOS.
As we receive additional information relating to this advisory, we will place
it, along with any clarifications, in a CA-95:01.README file. CERT advisories
and their associated README files are available by anonymous FTP from
info.cert.org. We encourage you to check the README files regularly for
updates on advisories that relate to your site.
This description summarizes both the IP spoofing technique that can
lead to root access on a system and the tool that intruders are using to
take over open terminal and login connections after they get root access.
We are currently seeing attacks in which intruders combine IP spoofing
with use of the tool. However, these are two separate actions. Intruders
can use IP spoofing to gain root access for any purpose; similarly, they
can highjack terminal connections regardless of their method of gaining
To gain access, intruders create packets with spoofed source IP
addresses. This exploits applications that use authentication based on
IP addresses and leads to unauthorized user and possibly root access
on the targeted system. It is possible to route packets through
filtering-router firewalls if they are not configured to filter
incoming packets whose source address is in the local domain. It
is important to note that the described attack is possible even if
no reply packets can reach the attacker.
Examples of configurations that are potentially vulnerable include
– routers to external networks that support multiple internal
– routers with two interfaces that support subnetting on the
– proxy firewalls where the proxy applications use the source
IP address for authentication
The IP spoofing attacks we are currently seeing are similar to those
described in two papers: 1) “Security Problems in the TCP/IP Protocol
Suite” by Steve Bellovin, published in _Computer Communication Review_
vol. 19, no. 2 (April 1989) pages 32-48; 2) “A Weakness in the 4.2BSD
Unix TCP/IP Software” by Robert T. Morris. Both papers are available
by anonymous FTP from
Bellovin paper: ipext.ps.Z
Morris paper: 117.ps.Z
Services that are vulnerable to the IP spoofing attack include
SunRPC & NFS
BSD UNIX “r” commands
anything wrapped by the tcp daemon wrappers – site dependent; check
other applications that use source IP addresses for authentication
Once the intruders have root access on a system, they can use a tool
to dynamically modify the UNIX kernel. This modification allows them
to hijack existing terminal and login connections from any user on the
In taking over the existing connections, intruders can bypass one-time
passwords and other strong authentication schemes by tapping the
connection after the authentication is complete. For example, a
legitimate user connects to a remote site through a login or terminal
session; the intruder hijacks the connection after the user has
completed the authentication to the remote location; the remote site
is now compromised. (See Section I for examples of vulnerable
Currently, the tool is used primarily on SunOS 4.1.x systems. However,
the system features that make this attack possible are not unique to
Current intruder activity in spoofing source IP addresses can lead to
unauthorized remote root access to systems behind a filtering-router
After gaining root access and taking over existing terminal and login
connections, intruders can gain access to remote hosts.
If you monitor packets using network-monitoring software such as
netlog, look for a packet on your external interface that has
both its source and destination IP addresses in your local domain.
If you find one, you are currently under attack. Netlog is
available by anonymous FTP from
MD5 checksum: 1dd62e7e96192456e8c75047c38e994b
Another way to detect IP spoofing is to compare the process
accounting logs between systems on your internal network. If
the IP spoofing attack has succeeded on one of your systems,
you may get a log entry on the victim machine showing a remote
access; on the apparent source machine, there will be no
corresponding entry for initiating that remote access.
When the intruder attaches to an existing terminal or login
connection, users may detect unusual activity, such as commands
appearing on their terminal that they did not type or a blank window
that will no longer respond to their commands. Encourage your users
to inform you of any such activity. In addition, pay particular
attention to connections that have been idle for a long time.
Once the attack is completed, it is difficult to detect. However,
the intruders may leave remnants of their tools. For example, you
may find a kernel streams module designed to tap into existing TCP
The best method of preventing the IP spoofing problem is to install
a filtering router that restricts the input to your external
interface (known as an input filter) by not allowing a packet
through if it has a source address from your internal network. In
addition, you should filter outgoing packets that have a source
address different from your internal network in order to prevent
a source IP spoofing attack originating from your site.
The following vendors have reported support for this feature:
Bay Networks/Wellfleet routers, version 5 and later
Cabletron – LAN Secure
Cisco – RIS software all releases of version 9.21 and later
Livingston – all versions
If you need more information about your router or about firewalls,
please contact your vendor directly.
If your vendor’s router does not support filtering on the inbound
side of the interface or if there will be a delay in incorporating
the feature into your system, you may filter the spoofed IP packets
by using a second router between your external interface and your
outside connection. Configure this router to block, on the outgoing
interface connected to your original router, all packets that have a
source address in your internal network. For this purpose, you can
use a filtering router or a UNIX system with two interfaces that
supports packet filtering.
NOTE: Disabling source routing at the router does not protect you
from this attack, but it is still good security practice to
There is no specific way to prevent use of the tool other than
preventing intruders from gaining root access in the first place.
If you have experienced a root compromise, see Section C for general
instructions on how to recover.
C. Recovery from a UNIX root compromise
1. Disconnect from the network or operate the system in
single-user mode during the recovery. This will keep users
and intruders from accessing the system.
2. Verify system binaries and configuration files against the
vendor’s media (do not rely on timestamp information to
provide an indication of modification). Do not trust any
verification tool such as cmp(1) located on the compromised
system as it, too, may have been modified by the intruder.
In addition, do not trust the results of the standard UNIX
sum(1) program as we have seen intruders modify system
files in such a way that the checksums remain the same.
Replace any modified files from the vendor’s media, not
— or —
Reload your system from the vendor’s media.
3. Search the system for new or modified setuid root files.
find / -user root -perm -4000 -print
If you are using NFS or AFS file systems, use ncheck to
search the local file systems.
ncheck -s /dev/sd0a
4. Change the password on all accounts.
5. Don’t trust your backups for reloading any file used by
root. You do not want to re-introduce files altered by an
The CERT Coordination Center thanks Eric Allman, Steve Bellovin, Keith Bostic,
Bill Cheswick, Mike Karels, and Tsutomu Shimomura for contributing to our
understanding of these problems and their solutions.
If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in Forum of Incident
Response and Security Teams (FIRST).
If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the e-mail be
encrypted. The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on info.cert.org), or PEM (contact
CERT staff for details).
Internet E-mail: firstname.lastname@example.org
Telephone: +1 412-268-7090 (24-hour hotline)
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Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890
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