[Solovay Edlin & Eiseman, P.C. Letterhead]




                                                                March 21, 2000


Network-1 Security Solutions, Inc.
1601 Trapelo Road, Reservoir Place
Waltham, Massachusetts  02451

                     Re: Registration Statement on Form S-3

Ladies and Gentlemen:

         We have acted as counsel for Network-1 Security Solutions, Inc., a
Delaware corporation (the "Company"), in connection with the registration under
the Securities Act of 1933, as amended (the "Act"), pursuant to the
above-referenced registration statement (the "Registration Statement"), of the
offer and sale from time to time, by the selling stockholders named therein, of
3,278,296 shares of the Company's common stock, par value $.01 per share (the
"Common Stock"), consisting of (i) 997,810 shares of Common Stock issuable upon
the exercise of warrants granted by the Company to certain selling stockholders
named therein (the "Warrant Shares"), (ii) 294,879 shares of Common Stock
issuable upon the exercise of options granted by the Company to a certain
selling stockholder named therein (the "Option Shares"), (iii) 352,820 shares of
Common Stock previously issued by the Company to certain selling stockholders
named therein (the "Issued Shares"), (iv) 491,803 shares of Common Stock
issuable upon the conversion of shares of preferred stock granted by the Company
to certain selling stockholders named therein (the "Conversion Shares"), and (v)
1,140,984 shares of Common Stock issuable upon the conversion of promissory
notes and its underlying securities granted by the Company to certain selling
stockholders named therein (the "Note Shares").

         In connection with this opinion, we have examined and relied upon a
copy of the Registration Statement. We have also examined originals, or copies
of originals certified to our satisfaction, of such agreements, documents,
certificates and other instruments as we have deemed relevant and necessary to
enable us to express an opinion on the matters covered hereby.

         In all such examinations, we have assumed the completeness and
authenticity of all records and documents submitted to us as originals and the
conformity to original records and documents of all copies submitted to us as
reproduced or conformed copies.



Network-1 Security Solutions, Inc.
March 21, 2000
Page 2 of 3 pages

         Based upon the foregoing, and subject to the limitations and
assumptions heretofore and hereinafter set forth, it is our opinion that:

         1. The Warrant Shares and the Option Shares, when issued in accordance
with the terms of the relative warrants and options and when certificates
representing such shares have been duly executed, countersigned and delivered to
the persons entitled thereto against payment to the Company for the exercise
price provided for in such warrants and options, will be validly issued, fully
paid and non-assessable;

         2. The Issued Shares are validly issued, fully paid and non-assessable;

         3. The Conversion Shares, when issued in accordance with the terms of
the relative convertible securities and when certificates representing such
shares have been duly executed, countersigned and delivered to the persons
entitled thereto, will be validly issued, fully paid and non-assessable;

         4. The Note Shares, when issued in accordance with the terms of the
relative promissory notes and the underlying securities, and when certificates
representing such shares have been duly executed, countersigned and delivered to
the persons entitled thereto, will be validly issued, fully paid and
non-assessable.

         We hereby consent to the filing of this opinion letter as an exhibit to
the Registration Statement and to the reference to our firm under the caption
"Legal Matters" in the Registration Statement. In giving such consent, we do not
hereby admit that we come within the category of persons whose consent is
required under Section 7 of the Act or the rules and regulations thereunder.

         The foregoing opinion is based on and limited to the Delaware General
Corporation Law and the relevant federal laws of the United States. We express
no opinion with respect to the laws of any other jurisdiction.




Network-1 Security Solutions, Inc.
March 21, 2000
Page 3 of 3 pages

         The opinions expressed herein are based upon the laws in effect on the
date hereof, and we assume no obligation to revise or supplement this opinion
should any such law be changed by legislative action, judicial decision or
otherwise.

                                                   Very truly yours,

                                                   SOLOVAY EDLIN & EISEMAN, P.C.


                                                   By: /s/ Sam Schwartz
                                                       -----------------------
                                                           Sam Schwartz