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RFC 1858 - Security Considerations for IP Fragment Filtering

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Network Working Group                                         G. Ziemba
Request for Comments: 1858                                      Alantec
Category: Informational                                         D. Reed
                                                              P. Traina
                                                          cisco Systems
                                                           October 1995

           Security Considerations for IP Fragment Filtering

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.


   IP fragmentation can be used to disguise TCP packets from IP filters
   used in routers and hosts. This document describes two methods of
   attack as well as remedies to prevent them.

1. Background

   System administrators rely on manufacturers of networking equipment
   to provide them with packet filters; these filters are used for
   keeping attackers from accessing private systems and information,
   while permitting friendly agents to transfer data between private
   nets and the Internet.  For this reason, it is important for network
   equipment vendors to anticipate possible attacks against their
   equipment and to implement robust mechanisms to deflect such attacks.

   The growth of the global Internet has brought with it an increase in
   "undesirable elements" manifested in antisocial behavior.  Recent
   months have seen the use of novel attacks on Internet hosts, which
   have in some cases led to the compromise of sensitive data.

   Increasingly sophisticated attackers have begun to exploit the more
   subtle aspects of the Internet Protocol; fragmentation of IP packets,
   an important feature in heterogeneous internetworks, poses several
   potential problems which we explore here.

2. Filtering IP Fragments

   IP packet filters on routers are designed with a user interface that
   hides packet fragmentation from the administrator; conceptually, an
   IP filter is applied to each IP packet as a complete entity.

   One approach to fragment filtering, described by Mogul [1], involves
   keeping track of the results of applying filter rules to the first
   fragment (FO==0) and applying them to subsequent fragments of the
   same packet.  The filtering module would maintain a list of packets
   indexed by the source address, destination address, protocol, and IP
   ID.  When the initial (FO==0) fragment is seen, if the MF bit is set,
   a list item would be allocated to hold the result of filter access
   checks.  When packets with a non-zero FO come in, look up the list
   element with a matching SA/DA/PROT/ID and apply the stored result
   (pass or block).  When a fragment with a zero MF bit is seen, free
   the list element.

   Although this method (or some refinement of it) might successfully
   remove any trace of the offending whole packet, it has some
   difficulties.  Fragments that arrive out of order, possibly because
   they traveled over different paths, violate one of the design
   assumptions, and undesired fragments can leak through as a result.
   Furthermore, if the filtering router lies on one of several parallel
   paths, the filtering module will not see every fragment and cannot
   guarantee complete fragment filtering in the case of packets that
   should be dropped.

   Fortunately, we do not need to remove all fragments of an offending
   packet.  Since "interesting" packet information is contained in the
   headers at the beginning, filters are generally applied only to the
   first fragment.  Non-first fragments are passed without filtering,
   because it will be impossible for the destination host to complete
   reassembly of the packet if the first fragment is missing, and
   therefore the entire packet will be discarded.

   The Internet Protocol allows fragmentation of packets into pieces so
   small as to be impractical because of data and computational
   overhead.  Attackers can sometimes exploit typical filter behavior
   and the ability to create peculiar fragment sequences in order to
   sneak otherwise disallowed packets past the filter.  In normal
   practice, such pathalogical fragmentation is never used, so it is
   safe to drop these fragments without danger of preventing normal

3. Tiny Fragment Attack

   With many IP implementations it is possible to impose an unusually
   small fragment size on outgoing packets.  If the fragment size is
   made small enough to force some of a TCP packet's TCP header fields
   into the second fragment, filter rules that specify patterns for
   those fields will not match.  If the filtering implementation does
   not enforce a minimum fragment size, a disallowed packet might be
   passed because it didn't hit a match in the filter.

   STD 5, RFC 791 states:

      Every internet module must be able to forward a datagram of 68
      octets without further fragmentation.  This is because an internet
      header may be up to 60 octets, and the minimum fragment is 8

   Note that, for the purpose of security, it is not sufficient to
   merely guarantee that a fragment contains at least 8 octets of data
   beyond the IP header because important transport header information
   (e.g., the CODE field of the TCP header) might be beyond the 8th data

   3.1 Example of the Tiny Fragment Attack

      In this example, the first fragment contains only eight octets of
      data (the minimum fragment size).  In the case of TCP, this is
      sufficient to contain the source and destination port numbers, but
      it will force the TCP flags field into the second fragment.

      Filters that attempt to drop connection requests (TCP datagrams
      having SYN=1 and ACK=0) will be unable to test these flags in the
      first octet, and will typically ignore them in subsequent

      FRAGMENT 1

      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+
      |     | ... | Fragment Offset = 0 | ... |     |
      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+

      |        Source Port            |       Destination Port        |
      |                       Sequence Number                         |

      FRAGMENT 2

      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+
      |     | ... | Fragment Offset = 1 | ... |     |
      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+

      |                    Acknowledgment Number                      |
      |  Data |           |U|A|P|R|S|F|                               |
      | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
      |       |           |G|K|H|T|N|N|                               |

   3.2 Prevention of the Tiny Fragment Attack

      In a router, one can prevent this sort of attack by enforcing
      certain limits on fragments passing through, namely, that the
      first fragment be large enough to contain all the necessary header

      There are two ways to guarantee that the first fragment of a
      "passed" packet includes all the required fields, one direct, the
      other indirect.

      3.2.1 Direct Method

         There is some number TMIN which is the minimum length of a
         transport header required to contain "interesting" fields
         (i.e., fields whose values are significant to packet filters).
         This length is measured from the beginning of the transport
         header in the original unfragmented IP packet.

         Note that TMIN is a function of the transport protocol involved
         and also of the particular filters currently configured.

         The direct method involves computing the length of the
         transport header in each zero-offset fragment and comparing it
         against TMIN.  If the transport header length is less than
         TMIN, the fragment is discarded.  Non-zero-offset fragments
         need not be checked because if the zero-offset fragment is
         discarded, the destination host will be unable to complete
         reassembly.  So far we have:

            if FO=0 and TRANSPORTLEN < tmin then
                    DROP PACKET

         However, the "interesting" fields of the common transport
         protocols, except TCP, lie in the first eight octets of the
         transport header, so it isn't possible to push them into a
         non-zero-offset fragment. Therefore, as of this writing, only
         TCP packets are vulnerable to tiny-fragment attacks and the
         test need not be applied to IP packets carrying other transport
         protocols.  A better version of the tiny fragment test might
         therefore be:

            if FO=0 and PROTOCOL=TCP and TRANSPORTLEN < tmin then
                    DROP PACKET

         As discussed in the section on overlapping fragments below,
         however, this test does not block all fragmentation attacks,
         and is in fact unnecessary when a more general technique is

      3.2.2 Indirect Method

         The indirect method relies on the observation that when a TCP
         packet is fragmented so as to force "interesting" header fields
         out of the zero-offset fragment, there must exist a fragment
         with FO equal to 1.

         If a packet with FO==1 is seen, conversely, it could indicate
         the presence, in the fragment set, of a zero-offset fragment
         with a transport header length of eight octets Discarding this
         one-offset fragment will block reassembly at the receiving host
         and be as effective as the direct method described above.

4. Overlapping Fragment Attack

   RFC 791, the current IP protocol specification, describes a
   reassembly algorithm that results in new fragments overwriting any
   overlapped portions of previously-received fragments.

   Given such a reassembly implementation, an attacker could construct a
   series of packets in which the lowest (zero-offset) fragment would
   contain innocuous data (and thereby be passed by administrative
   packet filters), and in which some subsequent packet having a non-
   zero offset would overlap TCP header information (destination port,
   for instance) and cause it to be modified.  The second packet would
   be passed through most filter implementations because it does not
   have a zero fragment offset.

   RFC 815 outlines an improved datagram reassembly algorithm, but it
   concerns itself primarily with filling gaps during the reassembly
   process.  This RFC remains mute on the issue of overlapping

   Thus, fully-compliant IP implementations are not guaranteed to be
   immune to overlapping-fragment attacks.  The 4.3 BSD reassembly
   implementation takes care to avoid these attacks by forcing data from
   lower-offset fragments to take precedence over data from higher-
   offset fragments.  However, not all IP implementations are based on
   the original BSD code, and it is likely that some of them are

   4.1 Example of the Overlapping Fragment Attack

      In this example, fragments are large enough to satisfy the minimum
      size requirements described in the previous section.  The filter
      is configured to drop TCP connection request packets.

      The first fragment contains values, e.g., SYN=0, ACK=1, that
      enable it to pass through the filter unharmed.

      The second fragment, with a fragment offset of eight octets,
      contains TCP Flags that differ from those given in the first
      fragment, e.g., SYN=1, ACK=0.  Since this second fragment is not a
      0-offset fragment, it will not be checked, and it, too will pass
      through the filter.

      The receiving host, if it conforms fully to the algorithms given
      in RFC 791, will reconstitute the packet as a connection request
      because the "bad" data arrived later.

      FRAGMENT 1

      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+
      |     | ... | Fragment Offset = 0 | ... |     |
      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+

      |        Source Port            |       Destination Port        |
      |                       Sequence Number                         |
      |                    Acknowledgment Number                      |
      |  Data |           |U|A|P|R|S|F|                               |
      | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
      |       |           |G|K|H|T|N|N|                               |
      |                        (Other data)                           |

      FRAGMENT 2

      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+
      |     | ... | Fragment Offset = 1 | ... |     |
      +-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+

      |                    Acknowledgment Number                      |
      |  Data |           |U|A|P|R|S|F|                               |
      | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
      |       |           |G|K|H|T|N|N|                               |
      |                        (Other data)                           |

      If the receiving host has a reassembly algorithm that prevents new
      data from overwriting data received previously, we can send
      Fragment 2 first, followed by Fragment 1, and accomplish the same
      successful attack.

   4.2 Prevention of the Overlapping Fragment Attack

      Since no standard requires that an overlap-safe reassembly
      algorithm be used, the potential vulnerability of hosts to this
      attack is quite large.

      By adopting a better strategy in a router's IP filtering code, one
      can be assured of blocking this "attack".  If the router's
      filtering module enforces a minimum fragment offset for fragments
      that have non-zero offsets, it can prevent overlaps in filter
      parameter regions of the transport headers.

      In the case of TCP, this minimum is sixteen octets, to ensure that
      the TCP flags field is never contained in a non-zero-offset
      fragment.  If a TCP fragment has FO==1, it should be discarded
      because it starts only eight octets into the transport header.
      Conveniently, dropping FO==1 fragments also protects against the
      tiny fragment attack, as discussed earlier.

      RFC 791 demands that an IP stack must be capable of passing an 8
      byte IP data payload without further fragmentation (fragments sit
      on 8 byte boundaries).  Since an IP header can be up to 60 bytes
      long (including options), this means that the minimum MTU on a
      link should be 68 bytes.

      A typical IP header is only 20 bytes long and can therefore carry
      48 bytes of data.  No one in the real world should EVER be
      generating a TCP packet with FO=1, as it would require both that a
      previous system fragmenting IP data down to the 8 byte minimum and
      a 60 byte IP header.

      A general algorithm, then, for ensuring that filters work in the
      face of both the tiny fragment attack and the overlapping fragment
      attack is:

         IF FO=1 and PROTOCOL=TCP then
                 DROP PACKET

      If filtering based on fields in other transport protocol headers
      is provided in a router, the minimum could be greater, depending
      on the position of those fields in the header.  In particular, if
      filtering is permitted on data beyond the sixteenth octet of the
      transport header, either because of a flexible user interface or

      the implementation of filters for some new transport protocol,
      dropping packets with FO==1 might not be sufficient.

5. Security Considerations

   This memo is concerned entirely with the security implications of
   filtering fragmented IP packets.

6. Acknowledgements

   The attack scenarios described above grew from discussions that took
   place on the firewalls mailing list during May of 1995.  Participants
   included: Darren Reed <avalon@coombs.anu.edu.au>, Tom Fitzgerald
   <fitz@wang.com>, and Paul Traina <pst@cisco.com>.

7. References

   [1] Mogul, J., "Simple and Flexible Datagram Access Controls for
       Unix-based Gateways", Digital Equipment Corporation, March 1989.

   [2] Postel, J., Editor, "Internet Protocol - DARPA Internet Program
       Protocol Specification", STD 5, RFC 791, USC/Information Sciences
       Institute, September 1981.

   [3] Postel, J., Editor, "Transmission Control Protocol - DARPA
       Internet Program Protocol Specification", STD 7, RFC 793,
       USC/Information Sciences Institute, September 1981.

   [4] Clark, D., "IP Datagram Reassembly Algorithms", RFC 815, MIT
       Laboratory for Computer Science/Computer Systems and
       Communications Group, July 1982.

Authors' Addresses

   G. Paul Ziemba
   2115 O'Nel Drive
   San Jose, CA 95131

   EMail: paul@alantec.com

   Darren Reed
   1275A Malvern Rd
   Melbourne, Vic 3144

   EMail: darrenr@cyber.com.au

   Paul Traina
   cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA 95028

   EMail: pst@cisco.com


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