[LOGO]
NETWORK-1
SECURITY SOLUTIONS, INC.

                                                             Monday, Feb 7, 2000

Mr. Robert Russo
33-20 28th St.
Long  Island City, NY  11106

Dear Bob:

         In connection with the proposed sale of Network-1's Professional
Services Group ("PSG") to a strategic acquirer (Sinai), set forth below is a
summary of our respective obligations to each other:

         Prior to or on the day of closing of the transaction with Sinai,
Network-1 will:


         1.  Receive from you your final expense report by fax or e-mail and
             with the FedEx of the originals overnight and reimburse you for any
             out of out-of-pocket expenses that were paid by you, as indicated
             on your expense report by FedEx overnight. Please include in this
             package the Company AMEX card, keys, phone cards, cell phones and
             pagers.

         2.  Receive from you your final billable days / hours report by fax or
             e-mail and with the signed originals sent FedEx overnight which
             will include the time spent with Sinai this week.

         3.  Provide you with your final payroll check through the day of
             closing by FedEx overnight.

         4.  Provide you with your accrued vacation, if any through the day of
             closing by FedEx overnight and any remaining balance on unpaid
             health insurance bills against the bills being provided to us.

         5.  Provide you with a grant of 25,000 Non Qualified Options, subject
             to the Sinai transaction closing, at an exercise price of $2.91,
             with 10,000 of the options vesting immediately and 15,000 options
             vesting on the day after the one year anniversary of the PSG Group
             Sale; provided, that you are still employed by Sinai on such date.

         I am also happy to inform you that the Board of Directors' compensation
committee has approved accelerated vesting of your 5,500 options at an exercise
price of $6.00 per share, that were due to vest on 10/22/00 to a new vesting
date that will be the day after closing of the Sinai transaction.

         The Board of Directors' compensation committee has also approved
accelerated vesting of your 1,125 options at an exercise price of $3.75 per
share that were due to vest on 6/22/00 to a new vesting date that will be the
day after closing of the Sinai transaction.

         Network-1 also agrees to extend the time period of your vested
Incentive Stock Options following the closing from the current exercise period
of thirty (30) days to ninety (90) days. A schedule, with your ISO and Non
Qualified Options distribution, quantities and exercise prices is described
below.

         In addition, Network-1 agrees to pay you a special bonus, payable
within ten (10) days following the one year anniversary of the closing of the
Sinai transaction, conditioned upon your continuing to be employed by Sinai at
such time, in an amount equal to 3% of the net funds received by Network-1 from
Sinai by the first anniversary of the closing which are in excess of $3M (the
payment to Network-1 from Sinai, with respect to funds held in escrow will be up
to $1.3 million and is subject to reduction upon certain events, including the
failure of Sinai to retain the 7 PSG team members ,any breach of the
representations and warranties set forth in
<PAGE>

[LOGO]
NETWORK-1
SECURITY SOLUTIONS, INC.


Section 4 of the Asset Purchase Agreement between Network-1 and Sinai, and PO's
of less than $300K in the first 90 days after closing).

         In addition we have secured the agreement of the Network-1 investors to
release your founder shares from lockup as follows:

         1) 25,000 shares will be released on 3/31/00

         2) 25,000 shares will be released on 6/30/00

         3) 25,000 shares will be released on 9/30/00

         4) The balance of your shares will be released on 11/12/00.

         A waiver and extension to your employment will be executed on or before
Sinai closing to indicate:

         1)  That the termination is by mutual agreement and that no severance
             is due to you

         2)  That a "non-compete" clause will remain in effect, pertaining to
             your participation in a company that develops competing products to
             those of Network-1, other than Sinai.


         You will also execute an appropriate certificate, as one of the two
NSSI officers involved in the consulting practice, confirming to the best of
your knowledge that the representations and warranties set forth in Section 4 of
the Asset Purchase Agreement are true and correct.

         This shall also serve as Network-1 consent for you to practice network
security consulting within less than two years after termination as a
modification of clause #r 13 in your employment agreement, dated June 30, 1998,
under the following circumstances only:

         1)  Work performed as an employee of Sinai

         2)  Work performed after termination at Sinai, if termination was not
             for cause

         You understand that this letter (and the benefits set forth herein)
shall be effective upon the Closing of the Sinai Transaction. If the foregoing
confirms our understanding, kindly execute below at the appropriate place
provided. Mutual signatures are required to make this letter effective.


                                            Very truly yours,

                                            Network-1 Security Solutions, Inc.
                                            -----------------------------------
                                            By:  Avi A. Fogel
                                                 -----------------
                                                 President and CEO
<PAGE>

[LOGO]
NETWORK-1
SECURITY SOLUTIONS, INC.


AGREED TO AND ACCEPTED BY:
/s/ Robert Russo
------------------------------------

By:   Mr. Robert Russo

Date: 2/8/00
---------------------------






                                 Option Summary


<TABLE>
<CAPTION>
                                                                                      2000 &
                              Ex                     1998            1999            Special         2001         Total Vested
Type           Grant Date    Price     Total       Vesting         Vesting           Vesting       Vesting        Post Closing
----           ----------    -----     -----       -------         -------           -------       -------        ------------
<S>             <C>          <C>        <C>          <C>              <C>              <C>         <C>                  <C>
ISO             10/22/98     $6.00      25,000       8,500            5,500             5500                            19,500
ISO              6/22/99     $3.75      20,000           0            6,800             1125                             7,925
ISO             10/25/99     $1.50      11,250           0             3825                0                             3,825
                                       ---------------------------------------------------------------------------------------
Totals                                  56,250       8,500           16,125            6,625              0             31,250

NQO              11/8/99     $2.91      25,000           0           10,000                0         15,000             25,000
                                              --------------------------------------------------------------------------------
Grand Total                                          8,500           26,125            6,625         15,000             56,250
                                              ================================================================================


</TABLE>