The RISKS Digest
Volume 16 Issue 76

Tuesday, 24th January 1995

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

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CERT Advisory CA-95:01.IP.spoofing.attacks.and.hijacked.terminal.connections
More on the new CERT advisory
Steve Bellovin
Phil Agre
Another post office stamp machine story (they *almost* got it right!)
Jonathan I. Kamens

CERT Advisory CA-95:01.IP.spoofing.attacks.and.hijacked.terminal.connections

CERT Advisory <>
Mon, 23 Jan 1995 13:49:47 -0500

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 We encourage you to check the README files regularly for
updates on advisories that relate to your site.

I.   Description

     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
     root access.

     IP spoofing
        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
          internal network
        - 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:
           Morris paper:

        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
               your configuration
           X windows
           other applications that use source IP addresses for authentication

     Hijacking tool
        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

II. Impact

     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.

III. Solutions

     A. Detection

        IP spoofing
       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.

        Hijacking tool
           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

     B. Prevention

        IP spoofing
           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
                 do so.

        Hijacking tool
           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
           from backups.
                                — 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, or PEM (contact CERT staff
for details).

Internet E-mail:
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
           and are on call for emergencies during other hours.
Fax: +1 412-268-6989

CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

Past advisories, CERT bulletins, information about FIRST representatives,
and other information related to computer security are available for anonymous
FTP from

CERT is a service mark of Carnegie Mellon University.

More on the new CERT advisory

Steve Bellovin <>
Tue, 24 Jan 1995 07:09:12 -0500
   [The CERT advisory is the previous item in this issue.
   Also, see John Markoff's front-page story in yesterday's
   editions of The New York Times, 23 Jan 1995.  PGN]

There's a great deal of confusion about what kind of attack the recent CERT
advisory is referring to.  Let me try to clear things up.

The specific attack is a sequence number guessing attack, originally
described by R.T. Morris in Bell Labs Computer Science Technical Report
#117, February 25, 1985.  I generalized (and publicized) the attack in my
1989 paper ``Security Problems in the TCP/IP Protocol Suite'', Computer
Communications Review 19:2, April 1989, pp. 32-48 (URLs below).  Both his
attack and my generalizations are special cases of a more general attack, IP
source address spoofing, in which the attacker illegitimately uses a trusted
machine's IP address in conjunction with some protocol (such as rsh) that
does address-based authentication.

In order to understand the particular case of sequence number guessing, you
have to look at the 3-way handshake used in the TCP open sequence.  Suppose
client machine A wants to talk to rsh server B.  It sends the following

    A->B: SYN, ISSa

That is, it sends a packet with the SYN (``synchronize sequence
number'') bit set and an initial sequence number ISSa.

B replies with

    B->A: SYN, ISSb, ACK(ISSa)

In addition to sending its own initial sequence number, it acknowledges A's.
Note that the actual numeric value ISSa must appear in the message.

A concludes the handshake by sending

    A->B: ACK(ISSb)

The initial sequence numbers are intended to be more or less random.  More
precisely, RFC 793 specifies that the 32-bit counter be incremented by 1 in
the low-order position about every 4 microseconds.  Instead,
Berkeley-derived kernels increment it by 128 every second, and 64 for each
new connection.  Thus, if you open a connection to a machine, you know to a
very high degree of confidence what sequence number it will use for its next
connection.  And therein lies the attack.

The attacker X first opens a real connection to its target B — say, to the
mail port or the TCP echo port.  This gives ISSb.  It then impersonates A
and sends

    A->B: SYN, ISSx

B's response to X's original SYN (so to speak)

    B->A: SYN, ISSb', ACK(ISSx)

goes to the legitimate A, about which more anon.  X never sees that message
but can still send

    A->B: ACK(ISSb')

using the predicted value for ISSb'.  If the guess is right — and usually
it will be — B's rsh server thinks it has a legitimate connection with A,
when in fact X is sending the packets.  X can't see the output from this
session, but it can execute commands as more or less any user — and in that
case, the game is over and X has won.

There is a minor difficulty here.  If A sees B's message, it will realize
that B is acknowledging something it never sent, and will send a RST packet
in response to tear down the connection.  There are a variety of ways to
prevent this; the easiest is to wait until the real A is down (possibly as a
result of enemy action, of course).

There are several possible defenses.  Most obvious is to take advantage of
topological knowledge: don't let packets purporting to be from a local
machine arrive on an outside interface.  That works very well if you only
trust local machines.  If trust is granted to outside machines (say, via
.rhosts files) and if the attacker can identify the patterns of trust (which
isn't that difficult), the topological solution won't work.  In that case,
you have to block all protocols that use TCP and address-based
authentication.  (UDP is a separate can of worms.)

Best of all, don't use address-based authentication; it's a disaster
waiting to happen.  The only real solution is cryptographic

Firewalls based on tcpd have a special problem: address-based authentication
is their business.  If you have a set of rules that grants special
permission to inside addresses, and you don't use a screening router as
well, you may be vulnerable.  The question is this: can an attacker do
damage by just sending commands and not seeing any output? If the answer is
yes, you are vulnerable.

        --Steve Bellovin

For further information, see the two papers cited above:


Phil Agre <>
Mon, 16 Jan 1995 19:05:36 -0800
     [Excerpted with permission from Phil's The Network Observer]

  Bouncemail top ten.

  In running a large mailing list for the past year or so, I
  have become acquainted with a depressing variety of dysfunctional
  mail handling software.  I've gathered here my top ten least
  favorite phenomena in hopes that a Universal Union of Large List
  Maintainers might spring up to force them to get fixed:

    10.  Mailers that give intermittent "user unknown" messages
         for users who perfectly well exist, perhaps because they
         cannot detect transient local network problems well enough
         to postpone delivering mail.

     9.  The confusion over the Errors-To: field, which sure
         seems like a good idea to me but which apparently is not
         part of the standard.  It is supported by most but not
         all mailers.  If it didn't exist then I'd have to run my
         mailing list from a separate account.

     8.  Mailers that generate messages lacking well-formed
         headers, most commonly addresses like "someone@local"
         without proper domain information.

     7.  Mailers that tell me "Press F1 for help with VNM error
         codes" even though my function keys are unlikely to be
         programmed the same way as they are for users at the
         site that generates the bouncemail message.  In general,
         mailers designed on the assumption that all senders and
         recipients of messages would use that same mailer --
         particularly when the mailer in question does not think in
         terms of standard IP domain formats.

     6.  Mailers that complain that a certain message could not be
         delivered but do not specify who in particular the message
         could not be delivered to.  Also, mailers that complain
         that a forwarded message could not be delivered without
         providing any indication of what address(es) the message
         was forwarded from.

     5.  Vacation programs that respond to bulk or mailing-list
         mail or that do not keep track of who they've replied to,
         with the result that I get batches of spurious vacation
         messages (sometimes in German) as each holiday approaches.

     4.  Mailers that generate mail that cannot be replied to.
         Sometimes a message says "From:",
         even though the user's account is actually called "fqs".
         Sometimes I have no idea why I cannot reply to a message,
         and the mailer offers me no help in figuring it out.
         This is particularly annoying when the sender in question
         starts sending further messages to the effect of, "you
         should reply to my messages, you rude person!"  It is
         even more annoying when the machine that generated the
         bogus message does not have a "postmaster" alias defined.

     3.  Mailers that take a month before giving up on the delivery
         of messages to a missing user, whereupon they initiate
         a monthlong stream of error messages, individually, for
         every one of the messages I've sent in the last month.

     2.  Mail-reading programs that automatically generate a little
         message to the effect of "so-and-so read your message
         about "routine administrative notes" on December 3rd at
         08:41" — even when the message was sent to a mailing list
         and not directly to the person reading it.  The people
         whose mail readers generate these messages are usually not
         aware of them, and their site maintainers usually do not
         know how to shut them off.

     1.  The astoundingly baroque and uninformative error messages
         that I have gotten from the Novell mailer.

  Of course errors happen.  The basic point here is that the error
  messages are so incomprehensible, so incomplete, so inconsistent,
  and so difficult to adjust or control.  The right way to do this,
  in my view, would be for mailers to be talking to one another and
  maintaining updated status tables for the process of delivering
  (or not delivering) each message.  A reasonable amount of useful
  information could travel over these lines of communication, and
  my mailer could consequently provide me with some significantly
  more useful functionalities.  Imagine a GUI interface with a
  window showing the messages that got bounced, deferred, and so
  forth.  And imagine that I could just click on each one to say,
  in one nice clean operation, "okay, let's just take this person
  off the mailing list, send them a nice explanatory note in case
  they're magically back on-line, and expunge from the system all
  remaining traces of existing messages from me to them or error
  messages from these mailers about them".

Another post office stamp machine story (they *almost* got it right!)

"Jonathan I. Kamens" <>
Thu, 19 Jan 1995 09:46:39 -0500
Some readers of RISKS will recall numerous previous articles about
suboptimal behavior of the electronic postal stamp vending machines which
have been appearing in United States Post Offices over the past few years.
I am one of the many people who at one point or another has lost money in
one of the machines.  However, this message isn't about that; it's about a
case I witnessed recently in which the vending machine *almost* got it
right, but still has a few kinks that need to be worked out.

On the first business day after the recent US postal rate increase, I went
to the Post Office to buy some new stamps.  Of course, it was a madhouse,
because everyone else had the same idea I did.  However, although the line
in front of the service windows was many people long, there were only one
person using the vending machine, with no line behind him.  I therefore
decided to get in line behind him, since I had enough crisp bills (or so I
thought) to buy stamps from the machine.

The man was standing in front of the machine, whose LED display claimed that
he had a $5.00 credit (presumably because of a five-dollar bill he'd
inserted), trying to get the machine to take a one-dollar bill (he wanted to
buy a book of twenty 32-cent stamps for $6.40).  The bill slot in the
machine wasn't pulling in his bill part way and then rejecting it, as you
might expect it to do if it could not verify the bill.  The slot wasn't
engaging at all, i.e., the man was pushing the bill into the slot over and
over, and nothing was happening.

He did this five or ten times, then stopped, turned around to look at the
service windows as if trying to attract someone's attention, wavered
momentarily as if contemplating actually going over to the windows to ask
for assistance, and then turned around and started shoving the bill at the
bill slot again.  He repeated this cycle three or four times, with me
standing there thinking, "Look, idiot, if the slot wouldn't take the bill
the first time, it isn't going take it the hundredth time either," and
wondering why he didn't just buy a booklet of ten stamps and come back for
more later.  Finally, he got up the courage to go over to the windows and
ask for help.

A few seconds after he walked away from the machine, it started beeping,
with the LED display asking if he needed more time, and saying that if he
didn't press the "YES" button he'd lose his money.  Since I'm a nice guy, I
pressed the "YES" button for him.

Meanwhile, the woman he was talking to at a service window was telling him
something to the effect of, "Well, I don't know anything about the vending
machine.  You'll have to talk to <so-and-so>.  She's in charge of it.  She's
in back right now.  Hold on, I'll send her out.  Go back to the machine and
wait for her."

So, the man went back to the machine and stood there waiting.  I said to
him, "Why don't you just buy a booklet of ten stamps and come back later for
more?"  He said, gesturing at the sign on the machine which says that change
over a dollar is given in dollars, "Because then I'll lose money."  I'm not
sure what gave him that impression, but I explained to him that what the
sign meant was that the machine issues Susan B. Anthony dollars when it has
to issue change of more than a dollar.

Once he understood that, he went ahead and pushed the button sequence for
the booklet of ten stamps.  The machine then spit his five-dollar bill out
of the bill slot and informed him on the LED display that it was currently
unable to issue change of over a dollar.  At this point, the woman "in
charge of" the vending machine showed up and asked what the story was.  The
man told her what had happened, and she basically repeated everything he'd
done (putting in his five-dollar bill, trying repeatedly to get the machine
to take the additional one-dollar bill, giving up and trying to buy ten
stamps, getting back the five-dollar bill), at which point she said, "Well,
I don't know what's wrong.  I guess you'll just have to get in line with
everybody else."

At this point, I decided that trying to use the vending machine myself would
not be a productive endeavor, so I picked up a copy of the USPS
stamps-by-mail pamphlet and walked out.  It was only when I'd made it out of
the Post Office a few steps that I realized what astute RISKS readers have
realized already — the vending machine's behavior made perfect sense, and
was in reality protecting the user from losing money, although its user
interface needs some improvement.  As I see it, the explanation for the
machine's behavior is as follows....

Most bill readers, in vending machines, change machines, etc., can only
return to the user the last bill inserted into the machine.  I.e., if I
insert a five-dollar bill into a reader and then a one-dollar bill, the
reader can return the one-dollar bill to me if necessary, but the
five-dollar bill is lost in the innards of the machine.

The stamp machine in this case was programmed to know that it couldn't give
back more than a dollar in change (presumably because its supply of dollar
coins was exhausted), and also to know that if it allowed the user to insert
more than one bill, it might not be able to give back adequate change after
the user completed his purchase.  For example, if the user inserted two
five-dollar bills and then purchased $6.40 worth of stamps, the machine
would have to issue $3.60 in change, which it couldn't do.  It therefore
refused to accept the second bill.

So, what are the problems?  I see several:

1) The machine should have told him on its display that it couldn't
   accept any more bills, after he inserted the first bill.

2) Instructions should have been displayed on the machine or on a
   placard within sight of the machine, explaining the machine's
   behavior when it runs out of dollar coins.

3) Better yet, there should be a light on the machine, with the words,
   "When lit, only one bill per purchase will be accepted, and money
   deposited into machine must be less than one dollar above purchase
   price of desired item," and it should be lit at the right time.

4) The woman "in charge of" the machine should have a clue about how
   it works.  I suspect that the USPS just wheels the machines into
   Post Offices, sets them down, says, "Here you go, a new vending
   machine!" and then periodically comes back to empty the money and
   restock them.  Perhaps they should invest a little in training at
   least one person at each Post Office to know how the machines work.

5) The USA really needs a dollar coin in common circulation :-).

Jonathan Kamens  |  OpenVision Technologies, Inc.  |

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