<PAGE>

                                                                     EXHIBIT 4.2

                                SECOND AMENDMENT
                                       TO
                                RIGHTS AGREEMENT


      This SECOND AMENDMENT TO RIGHTS AGREEMENT (the "Second Amendment") is
entered into effective as of October 25, 2001, between Cidco Incorporated, a
Delaware corporation (the "Company"), and The Bank of New York, as successor
Rights Agent to United States Trust Company of New York, (the "Rights Agent").
Capitalized terms not defined herein shall have the meanings given them in the
Rights Agreements (as defined below).


                                    RECITALS

      WHEREAS, pursuant to the Rights Agreement dated as of January 27, 1997
(the "Rights Agreement"), the Board of Directors of the Company on that date (i)
authorized the issuance and declared a dividend of one preferred share purchase
right (a "Right") for each Common Share of the Company outstanding as of the
close of business on February 14, 1997, each Right representing the right to
purchase one one-thousandth of a share of Series A Junior Participating
Preferred Stock of the Company upon the terms and subject to the conditions set
forth in the Rights Agreement, and (ii) further authorized the issuance of one
Right with respect to each Common Share that shall become outstanding between
February 14, 1997, and the earliest to occur of (x) the Distribution Date, (y)
the date on which the Rights are redeemed or exchanged, or (z) January 27, 2001.

      WHEREAS, pursuant to Section 19 of the Rights Agreement, The Bank of New
York succeeded United States Trust Company of New York as the Rights Agent under
the Rights Agreement.

      WHEREAS, pursuant to Section 27 of the Rights Agreement, prior to the
Distribution Date, the Company and the Rights Agent shall, if the Company so
directs, supplement or amend any provision of the Rights Agreement without the
approval of any holders of certificates representing Common Shares.

      WHEREAS, to the knowledge of the Board of Directors of the Company, the
Distribution Date has not occurred, and accordingly the Board of Directors may
amend the Rights Agreement pursuant to Section 27 thereof.

      WHEREAS, the Board of Directors has determined that it is in the best
interest of the Company and its stockholders to amend the Rights Agreement as
set forth herein immediately prior to and in connection with the execution of
that certain Agreement and Plan of Merger dated as of October 17, 2001, as the
same may be amended from time to time (the "Merger Agreement") among EarthLink,
Inc., a Delaware corporation, EarthLink Acquisition Sub, a Delaware corporation
("Sub") and the Company pursuant to which EarthLink will cause Merger Sub to
commence a cash tender offer (the "Offer") to purchase all of the outstanding
shares of common stock, par value $0.01 per share, of the Company and to
consummate the acquisition of


<PAGE>

the Company by means of a merger (the "Merger") of Sub with and into the Company
in accordance with the terms and subject to the conditions set forth in the
Merger Agreement.

      WHEREAS, in accordance with Section 27 of the Rights Agreement, the
parties entered into that certain First Amendment to Rights Agreement (the
"First Amendment") effective as of October 17, 2001, whereby the Company and the
Rights Agent agreed to amend the Rights Agreement pursuant to the terms thereof.

      WHEREAS, the Company has requested that the Rights Agreement be further
amended in accordance with Section 27 of the Rights Agreement, as set forth in
this Second Amendment , and the Rights Agent, in accordance with the terms
thereof, hereby agrees to amend the Rights Agreement as set forth herein.

                                    AGREEMENT

      NOW, THEREFORE, the parties, intending to be legally bound, hereby agree
as follows:

      1.    The parties acknowledge and agree that this Second Amendment amends
and restates the First Amendment.

      2.    Section 7(a) of the Rights Agreement is hereby amended to read in
its entirety as follows:

            "(a)  Subject to Section 7(e) and 11(a)(ii) hereof, the registered
holder of any Right Certificate may exercise the Rights evidenced thereby
(except as otherwise provided herein), in whole or in part at any time after the
Distribution Date upon surrender of the Right Certificate, with the form of
election to purchase on the reverse side thereof duly executed (with such
signature duly guaranteed), to the Rights Agent at the principal office of the
Rights Agent, together with payment of the aggregate Purchase Price for the
total number of one one-thousandths of a Preferred Share (or other securities,
as the case may be) as to which such surrendered Rights are exercised, at or
prior to the earliest to occur (i) the close of business on January 27, 2007
(the "Final Expiration Date"), (ii) the time at which the Rights are redeemed as
provided in Section 23 hereof (such date being herein referred to as the
"Redemption Date"), (iii) the time at which all such Rights are exchanged as
provided in Section 24 hereof, or (iv) immediately prior to the consummation of
the Offer (as defined in that certain Agreement and Plan of Merger dated as of
October 17, 2001, as the same may be amended from time to time (the "Merger
Agreement") among EarthLink, Inc., a Delaware corporation, EarthLink Acquisition
Sub, a Delaware corporation and a wholly owned subsidiary of EarthLink, and the
Company).

      3.    Section 35 of the Rights Agreement is hereby added as follows:

      "35.  EarthLink Transactions. Notwithstanding any provision of this Rights
Agreement to the contrary, no Distribution Date, Shares Acquisition Date or
Triggering Event shall be deemed to have occurred, neither EarthLink nor any
Affiliate or Associate of EarthLink shall be deemed to have become an Acquiring
Person and no holder of Rights shall be entitled to exercise such Rights under
or be entitled to any rights pursuant to Section 7(a), 11(a) or 13(a) of this



                                       2
<PAGE>

Rights Agreement solely by reason of (x) the approval, execution, delivery or
effectiveness of the Merger Agreement, the Voting Agreements (as defined in the
Merger Agreement), the Stock Option Agreement (as defined in the Merger
Agreement), or any certificates, instruments and documents, as may be necessary
or appropriate to consummate the transactions contemplated in the Merger
Agreement (collectively, the "Ancillary Documents"), as each may be amended from
time to time, or (y) the consummation of the transactions contemplated under the
Merger Agreement, the Voting Agreements, the Stock Option Agreement, or the
Ancillary Documents in accordance with the terms thereof (including, without
limitation, the Offer and the Merger), provided that if, after October 17, 2001,
EarthLink or any its Subsidiaries or any of their respective Affiliates or
Associates becomes the Beneficial Owner of any shares of Common Stock of the
Company (other than by reason of the approval, execution, delivery or
effectiveness of the Merger Agreement, the Voting Agreements, the Stock Option
Agreement or Ancillary Documents or the consummation of any of the transactions
contemplated thereby) the provisions of this Section 35 (other than this
proviso) shall not be applicable."

      4.    This Amendment shall be deemed effective as of October 25, 2001 as
if executed by both parties on such date. Except as amended hereby, the Rights
Agreement shall remain unchanged and shall remain in full force and effect.

      5.    This Amendment may be executed in any number of counterparts, each
of which shall be an original, but all of which together shall constitute one
instrument.

      6.    This Amendment shall be governed under Delaware law, without regard
to principles of conflict of laws.



                                       3
<PAGE>

      IN WITNESS WHEREOF, the parties have caused this Amendment to be executed
themselves or by their respective duly authorized representatives as of the date
first above written.


CIDCO INCORPORATED


By: /s/ Paul G. Locklin
   ________________________________________
      Paul G. Locklin
      President and Chief Executive Officer



THE BANK OF NEW YORK AS SUCCESSOR RIGHTS
AGENT TO UNITED STATES TRUST COMPANY OF NEW YORK


By: /s/ Patricia Gallagher
    ________________________________________
    Patricia Gallagher

Its: Authorized Signatory
    _______________________________________



                                       4