Improving TCP's Robustness to Blind In-Window Attacks
RFC 5961

Internet Engineering Task Force (IETF)                        A. Ramaiah
Request for Comments: 5961                                         Cisco
Category: Standards Track                                     R. Stewart
ISSN: 2070-1721                                                   Huawei
                                                                M. Dalal
                                                                   Cisco
                                                             August 2010

         Improving TCP's Robustness to Blind In-Window Attacks

Abstract

   TCP has historically been considered to be protected against spoofed
   off-path packet injection attacks by relying on the fact that it is
   difficult to guess the 4-tuple (the source and destination IP
   addresses and the source and destination ports) in combination with
   the 32-bit sequence number(s).  A combination of increasing window
   sizes and applications using longer-term connections (e.g., H-323 or
   Border Gateway Protocol (BGP) [RFC4271]) have left modern TCP
   implementations more vulnerable to these types of spoofed packet
   injection attacks.

   Many of these long-term TCP applications tend to have predictable IP
   addresses and ports that makes it far easier for the 4-tuple (4-tuple
   is the same as the socket pair mentioned in RFC 793) to be guessed.
   Having guessed the 4-tuple correctly, an attacker can inject a TCP
   segment with the RST bit set, the SYN bit set or data into a TCP
   connection by systematically guessing the sequence number of the
   spoofed segment to be in the current receive window.  This can cause
   the connection to abort or cause data corruption.  This document
   specifies small modifications to the way TCP handles inbound segments
   that can reduce the chances of a successful attack.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5961.

Ramaiah, et al.              Standards Track                    [Page 1]
RFC 5961                      TCP Security                   August 2010

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Applicability Statement ....................................3
      1.2. Basic Attack Methodology ...................................4
      1.3. Attack probabilities .......................................5
   2. Terminology .....................................................7
   3. Blind Reset Attack Using the RST Bit ............................7
      3.1. Description of the Attack ..................................7
      3.2. Mitigation .................................................7
   4. Blind Reset Attack Using the SYN Bit ............................9
      4.1. Description of the Attack ..................................9
      4.2. Mitigation .................................................9
   5. Blind Data Injection Attack ....................................10
      5.1. Description of the Attack .................................10
      5.2. Mitigation ................................................11
   6. Suggested Mitigation Strengths .................................12
   7. ACK Throttling .................................................12
   8. Backward Compatibility and Other Considerations ................13
   9. Middlebox Considerations .......................................14
      9.1. Middlebox That Resend RSTs ................................14
      9.2. Middleboxes That Advance Sequence Numbers .................15
      9.3. Middleboxes That Drop the Challenge ACK ...................15
   10. Security Considerations .......................................16