TCP Maintenance and Minor Extensions                       L. Velvindron
Intended status: Informational                           August 11, 2016
Expires: February 12, 2017

                     Handling of TCP ACK throttling


   The functionality provided by the TCP ACK throttling mechanism can be
   exploited as a side channel vulnerablity to terminate connections
   between two arbitrary hosts and inject data in the communication

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on February 12, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Deprecation of ACK throttling mechanism . . . . . . . . . . .   3
   3.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   [RFC5961] defines the challenge ACK response mechanism as a technique
   to mitigate against blind in-window attacks.  Specifically, an ACK
   packet is sent in response to an incoming segment with a SYN bit to
   confirm that the preceding connection was lost.  Another case is
   sending an ACK packet if the RST packet is received but the sequence
   number does not match the next expected sequence number.  Lastly, to
   prevent data injection, the range of valid ACK value is reduced for
   better strictness, so the likelihood of old ACK values and very new
   ACK values are discarded.  In all of those cases, the ACK packet is
   referrered to as a "Challenge ACK" through the rest of this document.

   [RFC5961] also introduces an ACK throttling mechanism to reduce
   possible wastage of CPU and bandwidth resources by limiting the
   number of challenge ACK that can be sent in a given interval.

   An attacker can leverage the Challenge ACK and the ACK throttling
   mechanism to abuse on the global ACK throttling rate-limit on a
   target host.  Through a series of step, the attacker can send spoofed
   packets to the target host, affect the the global challenge ACK rate-
   limiter, count the number of challenge ACK received, and finally
   compare that number with the target system limit.

   The attacker can then gather clues about: the existence of a 4-tuple
   connection, the next expected sequence number, and the expected ACK

   Based on the gathered information, the attacker can mount connection
   reset attacks and data injection attacks.  Those attacks have been
   demonstrated to work in real-world constraints according to [CBR01].

   Due to the seriousness of the threat, it is sufficient to deprecate
   the ACK throttling mechanism, as defined in [RFC5961].

   This document updates [RFC5961].

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1.1.  Terminology

   Challenge ACK in this document denotes the ACK packet sent in
   response to an segment whose RST bit is set and the sequence number
   does not fully match the next expected sequence value, but is within
   the current receive window as defined in [RFC5961].

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

2.  Deprecation of ACK throttling mechanism

   An implementation is not required to implement an ACK throttling
   mechanism which is conservative as defined in section 7 of [RFC5961].
   However, if there is a concern about CPU or bandwidth usage, an
   implementation may have a per-socket ACK throttling mechanism which
   is not shared across the system.  This makes it more difficult to
   abuse compared to having a single (global) ACK throttling mechanism.
   Additionally, an implementation may also introduce a randomized value
   to the interval defined in Section 7 of [RFC5961].  This makes the
   attacks defined in section 1 much more difficult.

3.  Operations

   It will take time to update all of the TCP implementations that fully
   implement the ACK throttling mechanism as described in [RFC5961].

   An operator can increase the value of the ACK throttling limit to the
   highest value possible to mitigate the risk of the vulnerabilities
   defined in section 1.

4.  IANA Considerations

   None of the proposed measures have an impact on IANA.

5.  Security Considerations

   The purpose of this document is to deprecate a feature of TCP that
   has been shown to lead to security vulnerabilities.  Specific
   examples of those vulnerabilities can be found in [CBR01].  In
   particular, the ACK throttling mechanism leads to a side-channel
   vulnerability that can be leveraged for connection reset and data
   injection attacks.  A description of this functionality can be found
   in section 1.

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6.  Normative References

   [CBR01]    Cao, Y., Wang, Z., Dao, T., Krishnamurthy, S., and L.
              Marvel, "Off-Path TCP Exploits: Global Rate Limit
              Considered Dangerous", University of California , 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5961]  Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
              Robustness to Blind In-Window Attacks", RFC 5961,
              DOI 10.17487/RFC5961, August 2010,

Author's Address

   Loganaden Velvindron
   88 Avenue De Plevitz Roches Brunes
   Rose Hill  71259

   Phone: +230 59762817

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