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Network Working Group                                        S. BellovinRequest for Comments: 1948                                 AT&T ResearchCategory: Informational                                         May 1996Defending Against Sequence Number AttacksStatus of This Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   IP spoofing attacks based on sequence number spoofing have become a   serious threat on the Internet (CERT Advisory CA-95:01).  While   ubiquitous crypgraphic authentication is the right answer, we propose   a simple modification to TCP implementations that should be a very   substantial block to the current wave of attacks.Overview and Rational   In 1985, Morris [1] described a form of attack based on guessing what   sequence numbers TCP [2] will use for new connections.  Briefly, the   attacker gags a host trusted by the target, impersonates the IP   address of the trusted host when talking to the target, and completes   the 3-way handshake based on its guess at the next initial sequence   number to be used.  An ordinary connection to the target is used to   gather sequence number state information.  This entire sequence,   coupled with address-based authentication, allows the attacker to   execute commands on the target host.   Clearly, the proper solution is cryptographic authentication [3,4].   But it will quite a long time before that is deployed.  It has   therefore been necessary for many sites to restrict use of protocols   that rely on address-based authentication, such as rlogin and rsh.   Unfortunately, the prevalence of "sniffer attacks" -- network   eavesdropping (CERT Advisory CA-94:01) -- has rendered ordinary   TELNET [5] very dangerous as well.  The Internet is thus left without   a safe, secure mechanism for remote login.   We propose a simple change to TCP implementations that will block   most sequence number guessing attacks.  More precisely, such attacks   will remain possible if and only if the Bad Guy already has the   ability to launch even more devastating attacks.Bellovin                     Informational                      [Page 1]

RFC 1948                Sequence Number Attacks                 May 1996Details of the Attack   In order to understand the particular case of sequence number   guessing, one must look at the 3-way handshake used in the TCP open   sequence [2].  Suppose client machine A wants to talk to rsh server   B.  It sends the following message:           A->B: SYN, ISNa   That is, it sends a packet with the SYN ("synchronize sequence   number") bit set and an initial sequence number ISNa.   B replies with           B->A: SYN, ISNb, ACK(ISNa)   In addition to sending its own initial sequence number, it   acknowledges A's.  Note that the actual numeric value ISNa must   appear in the message.   A concludes the handshake by sending           A->B: ACK(ISNb)   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 a   constant every second, and by another constant 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 ISNb.  It then   impersonates A and sends        Ax->B: SYN, ISNx   where "Ax" denotes a packet sent by X pretending to be A.   B's response to X's original SYN (so to speak)        B->A: SYN, ISNb', ACK(ISNx)Bellovin                     Informational                      [Page 2]

RFC 1948                Sequence Number Attacks                 May 1996   goes to the legitimate A, about which more anon.  X never sees that   message but can still send        Ax->B: ACK(ISNb')   using the predicted value for ISNb'.  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).  In   actual practice, X can gag A by exploiting a very common   implementation bug; this is described below.The Fix   The choice of initial sequence numbers for a connection is not   random.  Rather, it must be chosen so as to minimize the probability   of old stale packets being accepted by new incarnations of the same   connection [6,Appendix A].  Furthermore, implementations of TCP   derived from 4.2BSD contain special code to deal with such   reincarnations when the server end of the original connection is   still in TIMEWAIT state [7, pp. 945].  Accordingly, simple   randomization, as suggested in [8], will not work well.   But duplicate packets, and hence the restrictions on the initial   sequence number for reincarnations, are peculiar to individual   connections.  That is, there is no connection, syntactic or semantic,   between the sequence numbers used for two different connections.  We   can prevent sequence number guessing attacks by giving each   connection -- that is, each 4-tuple of <localhost, localport,   remotehost, remoteport> -- a separate sequence number space.  Within   each space, the initial sequence number is incremented according to   [2]; however, there is no obvious relationship between the numbering   in different spaces.   The obvious way to do this is to maintain state for dead connections,   and the easiest way to do that is to change the TCP state transition   diagram so that both ends of all connections go to TIMEWAIT state.   That would work, but it's inelegant and consumes storage space.   Instead, we use the current 4 microsecond timer M and set           ISN = M + F(localhost, localport, remotehost, remoteport).Bellovin                     Informational                      [Page 3]

RFC 1948                Sequence Number Attacks                 May 1996   It is vital that F not be computable from the outside, or an attacker   could still guess at sequence numbers from the initial sequence   number used for some other connection.  We therefore suggest that F   be a cryptographic hash function of the connection-id and some secret   data.  MD5 [9] is a good choice, since the code is widely available.   The secret data can either be a true random number [10], or it can be   the combination of some per-host secret and the boot time of the   machine.  The boot time is included to ensure that the secret is   changed on occasion.  Other data, such as the host's IP address and   name, may be included in the hash as well; this eases administration   by permitting a network of workstations to share the same secret data   while still giving them separate sequence number spaces.  Our   recommendation, in fact, is to use all three of these items: as   random a number as the hardware can generate, an administratively-   installed pass phrase, and the machine's IP address.  This allows for   local choice on how secure the secret is.   Note that the secret cannot easily be changed on a live machine.   Doing so would change the initial sequence numbers used for   reincarnated connections; to maintain safety, either dead connection   state must be kept or a quiet time observed for two maximum segment   lifetimes after such a change.A Common TCP Bug   As mentioned earlier, attackers using sequence number guessing have   to "gag" the trusted machine first.  While a number of strategies are   possible, most of the attacks detected thus far rely on an   implementation bug.   When SYN packets are received for a connection, the receiving system   creates a new TCB in SYN-RCVD state.  To avoid overconsumption of   resources, 4.2BSD-derived systems permit only a limited number of   TCBs in this state per connection.  Once this limit is reached,   future SYN packets for new connections are discarded; it is assumed   that the client will retransmit them as needed.   When a packet is received, the first thing that must be done is a   search for the TCB for that connection.  If no TCB is found, the   kernel searches for a "wild card" TCB used by servers to accept   connections from all clients.  Unfortunately, in many kernels this   code is invoked for any incoming packets, not just for initial SYN   packets.  If the SYN-RCVD queue is full for the wildcard TCB, any new   packets specifying just that host and port number will be discarded,   even if they aren't SYN packets.Bellovin                     Informational                      [Page 4]

RFC 1948                Sequence Number Attacks                 May 1996   To gag a host, then, the attacker sends a few dozen SYN packets to   the rlogin port from different port numbers on some non-existent   machine.  This fills up the SYN-RCVD queue, while the SYN+ACK packets   go off to the bit bucket.  The attack on the target machine then   appears to come from the rlogin port on the trusted machine.  The   replies -- the SYN+ACKs from the target -- will be perceived as   packets belonging to a full queue, and will be dropped silently.   This could be avoided if the full queue code checked for the ACK bit,   which cannot legally be on for legitimate open requests.  If it is   on, RST should be sent in reply.Security Considerations   Good sequence numbers are not a replacement for cryptographic   authentication.  At best, they're a palliative measure.   An eavesdropper who can observe the initial messages for a connection   can determine its sequence number state, and may still be able to   launch sequence number guessing attacks by impersonating that   connection.  However, such an eavesdropper can also hijack existing   connections [11], so the incremental threat isn't that high.  Still,   since the offset between a fake connection and a given real   connection will be more or less constant for the lifetime of the   secret, it is important to ensure that attackers can never capture   such packets.  Typical attacks that could disclose them include both   eavesdropping and the variety of routing attacks discussed in [8].   If random numbers are used as the sole source of the secret, they   MUST be chosen in accordance with the recommendations given in [10].Acknowledgments   Matt Blaze and Jim Ellis contributed some crucial ideas to this RFC.   Frank Kastenholz contributed constructive comments to this memo.References   [1]  R.T. Morris, "A Weakness in the 4.2BSD UNIX TCP/IP Software",        CSTR 117, 1985, AT&T Bell Laboratories, Murray Hill, NJ.   [2]  Postel, J., "Transmission Control Protocol", STD 7,RFC 793,        September 1981.   [3]  Kohl, J., and C. Neuman, "The Kerberos Network Authentication        Service (V5)",RFC 1510, September 1993.   [4]  Atkinson, R., "Security Architecture for the Internet        Protocol",RFC 1825, August 1995.Bellovin                     Informational                      [Page 5]

RFC 1948                Sequence Number Attacks                 May 1996   [5]  Postel, J., and J. Reynolds, "Telnet Protocol Specification",        STD 8,RFC 854, May 1983.   [6]  Jacobson, V., Braden, R., and L. Zhang, "TCP Extension for        High-Speed Paths",RFC 1885, October 1990.   [7]  G.R. Wright, W. R. Stevens, "TCP/IP Illustrated, Volume 2",        1995.  Addison-Wesley.   [8]  S. Bellovin, "Security Problems in the TCP/IP Protocol Suite",        April 1989, Computer Communications Review, vol. 19, no. 2, pp.        32-48.   [9]  Rivest, R., "The MD5 Message-Digest Algorithm",RFC 1321,        April 1992.   [10] Eastlake, D., Crocker, S., and J. Schiller, "Randomness        Recommendations for Security",RFC 1750, December 1994.   [11] L. Joncheray, "A Simple Active Attack Against TCP, 1995, Proc.        Fifth Usenix UNIX Security Symposium.Author's Address   Steven M. Bellovin   AT&T Research   600 Mountain Avenue   Murray Hill, NJ  07974   Phone: (908) 582-5886   EMail: smb@research.att.comBellovin                     Informational                      [Page 6]