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Defending Against Sequence Number Attacks (draft-gont-tcpm-rfc1948bis-00.txt)

Defending Against Sequence Number Attacks (draft-gont-tcpm-rfc1948bis-00.txt). Fernando Gont (UTN/FRH) Steven Bellovin (Columbia U.) 80th IETF meeting, Prague, Czech Republic March 27-April 1, 2011. Introduction.

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Defending Against Sequence Number Attacks (draft-gont-tcpm-rfc1948bis-00.txt)

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  1. Defending Against Sequence Number Attacks(draft-gont-tcpm-rfc1948bis-00.txt) Fernando Gont (UTN/FRH) Steven Bellovin (Columbia U.) 80th IETF meeting, Prague, Czech Republic March 27-April 1, 2011

  2. Introduction • The current standard algorithm for genearting Initial Sequence Numbers (ISNs) produces sequences that are trivially predicable by off-path attackers • The security implications of predictable TCP sequence numbers have been known for a long time (e.g., Morris paper in 1985) • RFC 1948 [Bellovin, 1996] proposed an algorithm for selecting ISNs such that they are not easily predictable by off-path attackers

  3. RFC 1948 • Proposed to generate ISNs with: ISN = M + F(localhost, localport, remotehost, remoteport) • Where M is a timer, and F is suggested to be a cryptographic hash function such as MD5 • This expression leads to monotonically-increasing ISNs that are unpredictable by off-path attackers • RFC 1948 was published as an Informational RFC • It has been widely implemented and deployed

  4. draft-gont-tcpm-rfc1948bis • New document aims at Standards Track (rather than Informational): TCP SHOULD generate its Initial Sequence Numbers with the expression: ISN = M + F(localip, localport, remoteip, remoteport) • The discussion of address-based trust relationship attacks IN rfc 1948 was updated to reflect current attack scenarios, and moved to an Appendix. • Documentation of an old BSD bug was also moved to an Appendix • In version -00 of the document, the recommended hash algorithm had been changed to SHA-256 [FIPS-SHS] • This had been motivated by non-technical reasons • Based on later discussions on the mailing-list, we will switch back to MD5 in the next revision

  5. Moving Forward • This is TCP maintenance work, that is within the charter of the TCPM WG • So far, the document has received some support on the mailing-list (e.g., William Simpson and Richard Scheffenegger) • Should TCPM adopt this as a WG item?

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