From: Adam Baldwin (
Date: 06/02/04

  • Next message: Jose Nazario: "RE: Suggestions"
    Date: Tue, 1 Jun 2004 17:13:01 -0700 (PDT)

    Comments inline...

    > What does it mean if the packet that trigger this
    > alert is the TCP "ACK"
    > packet.
    To understand some of the why aspects of why a
    signature is being triggered you need to understand
    the underlying protocol, I suggest TCP/IP Illustrated
    by W. Richard Stevens (ISBN: 0201633469)

    After the 3-way TCP handshake (SYN, SYN+ACK, ACK) all
    of your packets containing any data are going to have
    the ACK flag set. (In a good little, abide by the
    rules TCP session that is :)

    This data portion of the packet is going to be the
    "interesting" part that the signature is going to look
    at. In the case of the BARE BYTE UNICODE ENCODING
    signature we are checking for that particular encoding

    Below is some good info from the snort
    ( README.http_inspect doc

    * bare_byte [yes/no] *
    Bare byte encoding is an IIS trick that uses non-ASCII
    chars as valid values in decoding UTF-8 values. This
    is NOT in the HTTP standard, as all non-ASCII values
    have to be encoded with a %. Bare byte encoding
    allows the user to emulate an IIS server and interpret
    non-standard encodings correctly.

    The alert on this decoding should be enabled, because
    there are no legitimate clients that encoded UTF-8
    this way, since it is non-standard.

    > When I traced back, I couldn't find the
    > "SYN" packet. Is this
    > always the case that any packet that cause "BARE
    > the ACK packet?
    It is very possible that the packet that triggered
    that alert didn't have a SYN packet associated with
    it. If it is a single packet or there is a series of
    these packets, with no SYN packet in the same stream,
    they may have been created by hand or with a tool.

    Adam Baldwin

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  • Next message: Jose Nazario: "RE: Suggestions"

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