<PAGE>
                                                                     EXHIBIT 4.6

THIS WARRANT AND THE SECURITIES ISSUABLE UPON EXERCISE HEREOF HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER ANY STATE
SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED
IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER
SAID ACT OR AN OPINION OF COUNSEL SATISFACTORY TO THE CORPORATION THAT SUCH
REGISTRATION IS NOT REQUIRED.

                               CRITICAL PATH, INC.

             WARRANT TO PURCHASE UP TO 25,000 SHARES OF COMMON STOCK

Issue date: December 29, 1999

        1. General. THIS CERTIFIES THAT, Ecker-Folsom Properties, LLC (the
"HOLDER") is entitled to subscribe for and purchase up to twenty-five thousand
(25,000) fully paid and nonassessable shares of common stock of Critical Path,
Inc., a California corporation (the "COMPANY"), at a price of $90 per share (the
"EXERCISE PRICE") subject to the provisions and upon the terms and conditions
hereinafter set forth. This warrant (this "WARRANT") is being issued to the
Holder in connection with the lease of certain office space located at 530
Folsom Street and 33 Clementina in San Francisco, California between the Company
and the Holder of even date herewith.

        2. Exercise Period; Vesting This Warrant may be exercised by the Holder
at any time and from time to time (but no more than four (4) times) from the
issue date above until December 20, 2006. The number of shares that shall be
issuable upon exercise of this Warrant shall initially be zero (0), and shall
increase by 521 shares each calendar month (the "VESTING RATE") on the first day
of each month, beginning January 1, 2000, until this Warrant is exercisable for
the entire amount hereunder or until it terminates. In the event that the lease
for 530 Folsom Street terminates for any reason other than due to the default of
the Company thereunder, the Vesting Rate shall be reduced by 347 shares per
month; in the event that the lease for 33 Clementina terminates for any reason
other than due to the default of the Company thereunder, the Vesting Rate shall
be reduced by 174 shares per month.

        3. Method of Exercise; Payment

               a. Cash Exercise. The Holder may exercise this Warrant in whole
or in part, by surrendering this Warrant (with the notice of exercise form
attached hereto as Exhibit A duly executed) at the principal office of the
Company and by the payment to the Company, by certified, cashier's or other
check acceptable to the Company, of an amount equal to the aggregate purchase
price of the shares of common stock being purchased.


<PAGE>

               b. Net Issue Exercise. In lieu of exercising this Warrant with
cash, the Holder may elect to receive shares equal to the value of this Warrant
(or the portion thereof being canceled) by surrendering this Warrant at the
principal office of the Company together with notice of such election, in which
event the Company shall issue to the Holder a number of shares of common stock
computed using the following formula:

             X = Y (A-B)
                 -------
                    A

        where:

              X    =     the number of shares of common stock to be issued to
                         the Holder;

              Y    =     the number of shares of common stock purchasable under
                         this Warrant or, if only a portion of this Warrant is
                         being exercised, the portion of this Warrant being
                         canceled (at the date of such calculation);

              A    =     the Fair Market Value (as defined below) of a share of
                         common stock (at the date of such calculation); and

              B    =     the Exercise Price (on the date of such calculation).

               c. Fair Market Value. The fair market value of a share of common
stock of the Company shall equal the market price as quoted on the NASDAQ
National Market (the "FAIR MARKET VALUE").

               d. Stock Certificates. In the event the Holder exercises any of
the rights represented by this Warrant to purchase shares of common stock, the
Company shall deliver to the Holder certificates representing such shares within
a reasonable time and, unless the Holder has fully exercised this Warrant or the
Warrant has expired, a new warrant representing the remaining shares underlying
this Warrant.

        4. Reservation of Shares. The Company covenants and agrees that all
shares of common stock which may be issued upon the exercise of this Warrant
will, upon issuance, be duly authorized, validly issued, fully paid and
nonassessable, free from all preemptive rights of any stockholder and free from
all taxes, liens and charges created by the Company with respect to the issue
thereof. During the period within which the Holder may exercise this Warrant,
the Company will at all times have authorized, and reserved for the purpose of
issuance upon exercise of this Warrant, a sufficient number of shares of common
stock to provide for the exercise of the rights represented by this Warrant.

        5. Adjustment of Exercise Price and Number of Shares. The number and
kind of securities purchasable upon the exercise of this Warrant and the
Exercise Price shall be subject to


<PAGE>

adjustment from time to time upon the occurrence of certain events as follows:

               a. Reclassification or Merger. In case of any reclassification,
change or conversion of securities of the class issuable upon exercise of this
Warrant (other than a change in par value or as a result of a subdivision or
combination), or in case of any merger of the Company with or into another
company (other than (i) a merger effected solely for the purpose of changing the
Company's jurisdiction of incorporation or (ii) a merger with another company in
which the Company is the acquiring and surviving corporation and which does not
result in any reclassification or change of outstanding securities issuable upon
exercise of this Warrant), or in case of any sale of all or substantially all of
the assets of the Company, the Company, or such successor or purchasing company,
as the case may be, shall duly execute and deliver to the Holder a new warrant,
so that the Holder shall have the right to receive, at a total purchase price
not to exceed that payable upon the exercise of the unexercised portion of this
Warrant, and in lieu of the shares of common stock theretofore issuable upon
exercise of this Warrant, the kind and amount of shares of stock, other
securities, money and property receivable upon such reclassification, change or
merger by a holder of the number of shares of common stock under this Warrant.
Such new warrant shall provide for adjustments as nearly equivalent as may be
practicable to the adjustments provided for in this section. The provisions of
this subparagraph (a) shall similarly apply to successive reclassifications,
changes, mergers, consolidations and transfers.

               b. Combination or Subdivision of Shares. If the Company at any
time while this Warrant remains outstanding and unexpired shall combine its
outstanding shares of common stock, the number of shares purchasable shall be
proportionally decreased and the Exercise Price proportionally increased
effective concurrently with such combination. In the case of a subdivision, the
number of shares purchasable shall be proportionally increased and the Exercise
Price proportionally decreased effective concurrently with such subdivision.

               c. Stock Dividends. If the Company at any time while this Warrant
is outstanding and unexpired shall pay a dividend with respect to shares of
common stock in shares of common stock, then the Exercise Price shall be
adjusted, from and after the date of determination of stockholders entitled to
receive such dividend or distribution, to that price determined by multiplying
the Exercise Price in effect immediately prior to such date of determination by
a fraction (i) the numerator of which shall be the total number of shares of
common stock outstanding immediately prior to such dividend or distribution and
(ii) the denominator of which shall be the total number of shares of common
stock outstanding immediately after such dividend or distribution.

        6. Fractional Shares. No fractional shares will be issued in connection
with any exercise hereunder, but in lieu of such fractional shares the Company
shall make a cash payment therefor upon the basis of the Exercise Price then in
effect.

        7. Compliance with Securities Law. The Holder, by acceptance hereof,
agrees that the Holder is acquiring this Warrant, and the shares of common stock
to be issued upon exercise hereof, for investment and will not offer, sell or
otherwise dispose of this Warrant, or any shares of common stock to be issued
upon exercise hereof, except under circumstances which will not result in a


<PAGE>

violation of the Securities Act of 1933 (the "SECURITIES ACT"). Upon exercise of
this Warrant, unless the shares being acquired are registered under the
Securities Act or an exemption from such registration is available, the Holder
hereof shall confirm in writing, by executing the form attached as Schedule 1 to
Exhibit A hereto, that the shares of common stock so acquired are being acquired
for investment and not with a view towards distribution or resale. All shares of
common stock issued upon exercise of this Warrant shall be stamped or imprinted
with a legend in substantially the following form:

"THE SECURITIES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED, OR UNDER ANY STATE SECURITIES LAWS. THEY MAY NOT BE
SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE
REGISTRATION STATEMENT AS TO THE SECURITIES UNDER SAID ACT OR AN OPINION OF
COUNSEL SATISFACTORY TO THE CORPORATION THAT SUCH REGISTRATION IS NOT REQUIRED.

        8. Transferability. This Warrant and all rights hereunder are not
transferable without the prior written consent of the Company. In the event that
the Company agrees to any such transfer, such transfer shall be effected,
without charge to the Holder hereof (except for transfer taxes), upon surrender
of this Warrant properly endorsed.

        9. Rights as Stockholder. No Holder, solely as such, shall be entitled
to vote or receive dividends or be deemed a stockholder of the Company.

        10. Modification and Waiver. This Warrant and any provision hereof may
be changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

        11. Notices of Change.

               a. Promptly upon any adjustment in the number or class of shares
subject to this Warrant and of the Exercise Price, the Company shall give
written notice thereof to the Holder, setting forth in reasonable detail and
certifying the calculation of such adjustment.

               b. The Company shall give written notice to the Holder at least
ten (10) business days prior to the date on which the Company closes its books
or takes a record for determining rights to receive any dividends or
distributions.

        12. Transfer Books. The Company will at no time close its transfer books
against the transfer of this Warrant or of any shares of common stock issued or
issuable upon the exercise of this Warrant in any manner which interferes with
the timely exercise of this Warrant.

        13. Loss, Theft, Destruction, or Mutilation. The Company represents and
warrants to the Holder that, upon receipt of evidence reasonably satisfactory to
the Company of the loss, theft, destruction, or mutilation of this Warrant and,
in the case of any such loss, theft or destruction, upon


<PAGE>

receipt of an indemnity reasonably satisfactory to the Company, or in the case
of any such mutilation upon surrender and cancellation of this Warrant, the
Company, at the Holder's expense, will make and deliver a new warrant of like
tenor in lieu of the lost, stolen, destroyed or mutilated Warrant.

        14. Notices. Any notice, request, communication or other document
required or permitted to be given or delivered to the Holder or the Company
shall be delivered via overnight courier by certified or registered mail,
postage prepaid, to the Holder's address as shown on the books of the Company or
to the Company at the address indicated on the signature page of this Warrant.

        15. Binding Effect on Successors. Except as otherwise set forth herein,
this Warrant shall be binding upon any company succeeding the Company by merger,
consolidation or acquisition of all or substantially all of the Company's
assets.

        16. Descriptive Headings. The descriptive headings of the several
paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant.

        17. Governing Law. This Warrant shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the State of California.

        18. Acceptance. Receipt of this Warrant by the holder hereof shall
constitute acceptance of and agreement to the foregoing terms and conditions.



                                        CRITICAL PATH, INC.
                                        320 First Street
                                        San Francisco, California 94105


                                        By:     /s/ DAVID THATCHER
                                                -------------------------------

                                        Name:   DAVID THATCHER
                                                -------------------------------

                                        Title:  EVP/CFO
                                                -------------------------------



<PAGE>

                                    EXHIBIT A

                               NOTICE OF EXERCISE

TO:    Critical Path, Inc.

        1. The undersigned hereby elects to purchase ___________ shares of
common stock of Critical Path, Inc. pursuant to the terms of the attached
Warrant.

        2. Method of Exercise (Please initial the applicable blank.):

        ______ The undersigned elects to exercise the attached Warrant by means
of a cash payment, and tenders herewith payment in full for the purchase price
of the shares being purchased, together with all applicable transfer taxes, if
any.

        ______ The undersigned elects to exercise the attached Warrant by means
of the net exercise provisions of the Warrant.

        3. Please issue a certificate or certificates representing said shares
of common stock in the name of the undersigned or in such other name as is
specified below:


               ____________________________________
               (Name)

               ____________________________________

               ____________________________________

               ____________________________________
               (Address)


        4. The undersigned hereby represents and warrants that the aforesaid
shares of common stock are being acquired for the account of the undersigned for
investment and not with a view to or for resale in connection with the
distribution thereof, and that the undersigned has no present intention of
distributing or reselling such shares. The undersigned hereby delivers an
Investment Representation Statement in the form attached to the Warrant as
Schedule I to Exhibit A.

Date: _____________________                  By:    ___________________________

                                             Name:  ___________________________

                                             Title: ___________________________
                                                    (if applicable)



<PAGE>

                                   Schedule I

                      INVESTMENT REPRESENTATION STATEMENT

Purchaser:     _________________________________

Security:      Shares of Common Stock

Amount:        _________________________________

Date:          _________________________________

        In connection with the purchase of the above-listed shares of common
stock (the "SECURITIES"), the undersigned (the "PURCHASER") represents to
Critical Path, Inc. (the "COMPANY") as follows:

        (a) The Purchaser is aware of the Company's business affairs and
financial condition, and has acquired sufficient information about the Company
to reach an informed and knowledgeable decision to acquire the Securities. The
Purchaser is purchasing the Securities for its own account for investment
purposes only and not with a view to, or for the resale in connection with, any
"distribution" thereof for purposes of the Securities Act of 1933, as amended
(the "SECURITIES ACT").

        (b) The Purchaser understands that the Securities have not been
registered under the Securities Act in reliance upon a specific exemption
therefrom, which exemption depends upon, among other things, the bona fide
nature of the Purchaser's investment intent as expressed herein. In this
connection, the Purchaser understands that, in the view of the Securities and
Exchange Commission (the "SEC"), the statutory basis for such exemption may be
unavailable if the Purchaser's representation was predicated solely upon a
present intention to hold these Securities for the minimum capital gains period
specified under tax statutes, for a deferred sale, for or until an increase or
decrease in the market price of the Securities, or for a period of one year or
any other fixed period in the future.

        (c) The Purchaser further understands that the Securities must be held
indefinitely unless subsequently registered under the Securities Act or unless
an exemption from registration is otherwise available. Moreover, the Purchaser
understands that the Company is under no obligation to register the Securities.
In addition, the Purchaser understands that the certificate evidencing the
Securities will be imprinted with the legend referred to in the Warrant under
which the Securities are being purchased.

        (d) The Purchaser is aware of the provisions of Rule 144, promulgated
under the Securities Act, which, in substance, permit limited public resale of
"restricted securities" acquired, directly or indirectly, from the issuer
thereof (or from an affiliate of such issuer), in a non-public offering subject
to the satisfaction of certain conditions, if applicable, including, among other
things:


<PAGE>

The availability of certain public information about the Company, the resale
occurring not less than one year after the party has purchased and paid for the
securities to be sold; the sale being made through a broker in an unsolicited
"broker's transaction" or in transactions directly with a market maker (as said
term is defined under the Securities Exchange Act of 1934, as amended) and the
amount of securities being sold during any three-month period not exceeding the
specified limitations stated therein.

        (c) The Purchaser further understands that at the time it wishes to sell
the Securities there may be no public market upon which to make such a sale, and
that, even if such a public market then exists, the Company may not be
satisfying the current public information requirements of Rule 144 and that, in
such event, the Purchaser may be precluded from selling the Securities under
Rule 144 even if the one-year minimum holding period had been satisfied.

        (f) The Purchaser further understands that in the event all of the
requirements of Rule 144 are not satisfied, registration under the Securities
Act, compliance with Regulation A, or some other registration exemption will be
required, and that, notwithstanding the fact that Rule 144 is not exclusive, the
SEC has expressed its opinion that persons proposing to sell private placement
securities other than in a registered offering and otherwise than pursuant to
Rule 144 will have a substantial burden of proof in establishing that an
exemption from registration is available for such offers or sales, and that such
persons and their respective brokers who participate in such transactions do so
at their own risk.


                                             PURCHASER


                                             By:    ___________________________

                                             Name:  ___________________________

                                             Title: ___________________________
                                             (if applicable)