Defending Against Sequence Number Attacks
RFC 1948
Document | Type |
RFC - Informational
(May 1996; Errata)
Obsoleted by RFC 6528
Was draft-rfced-info-bellovin (individual)
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Author | Steven Bellovin | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 1948 (Informational) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group S. Bellovin Request for Comments: 1948 AT&T Research Category: Informational May 1996 Defending Against Sequence Number Attacks Status 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 1996 Details 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 commonShow full document text