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                                                                   EXHIBIT 10.15

                                GUARANTY OF NOTE

         FOR VALUE RECEIVED and in consideration for and as an inducement to
CA-GATEWAY OFFICE LIMITED PARTNERSHIP, A DELAWARE LIMITED PARTNERSHIP ("Payee")
to enter in to the Third Amendment (as defined below) respecting certain real
property leased to BACKWEB TECHNOLOGIES INC., A DELAWARE CORPORATION, as tenant
("Maker"), which is leased by Maker pursuant to that certain office lease by and
between Payee (as "Landlord") and Maker (as "Tenant") and dated December 23,
1998 (the "Original Lease"), which Original Lease has been previously amended by
that certain 1st Amendment to Lease - Expansion dated as of May 12, 2000, and by
that certain 2nd Amendment to Lease - Expansion dated as of November 7, 2000,
and further amended by that certain Third Amendment of even date herewith (the
"Third Amendment", and, together with the 1st Amendment to Lease - Expansion and
the 2nd Amendment to Lease - Expansion, collectively, the "Lease"), the
undersigned, BACKWEB TECHNOLOGIES LTD, AN ISRAELI CORPORATION ("Guarantor") does
hereby unconditionally and irrevocably guarantee to Payee the punctual payment
payable by Maker under the that certain Promissory Note of even date herewith
(the "Note"), including, without limitation, the Additional Reduction Fee (as
defined in the Third Amendment and in the Note), throughout the term of the
Lease and the Note, as the case may be, for which the undersigned shall be
jointly and severally liable with Maker. If any default on the part of Maker
shall occur under the Note, the undersigned does hereby covenant and agree to
pay to Payee upon demand in each and every instance such sum or sums of money
and to perform each and every covenant, condition and agreement under the Note
as Maker is and shall become liable for or obligated to pay or perform under the
Note, together with the costs reasonably incurred by Payee in connection
therewith, including, without limitation, reasonable attorneys' fees. Such
payments of the Principal Amount and other sums shall be made monthly or at such
other intervals as the same shall or may become payable under the Note, all
without requiring any notice from Payee (other than any notice required by the
Note, if any) of such non-payment or non performance, all of which the
undersigned hereby expressly waives.

         The maintenance of any action or proceeding by Payee to recover any sum
or sums that may be or become due under the Note and to secure the performance
of any of the other terms, covenants and conditions of the Note shall not
preclude Payee from thereafter instituting and maintaining subsequent actions or
proceedings for any subsequent default or defaults of Maker under the Note. The
undersigned does hereby consent that without affecting the liability of the
undersigned under this Guaranty and without notice to the undersigned, time may
be given by Payee to Maker for payment of the Principal Amount and such other
sums and performance of said other terms, covenants and conditions, or any of
them, and such time extended and indulgence granted, from time to time, or Maker
may be dispossessed or Payee may avail itself of or exercise any or all of the
rights and remedies against Maker provided by law or by the Note, and may
proceed either against Maker alone or jointly against Maker and the undersigned
or against the undersigned alone without first prosecuting or exhausting any
remedy or claim against Maker. The undersigned does hereby further consent to
any subsequent change, modification or amendment of the Note in any of its
terms, covenants or conditions, or in the Principal Amount payable thereunder,
or in the term thereof, and to any renewals or extensions thereof, all of which
may be made without notice to or consent

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of the undersigned and without in any manner releasing or relieving the
undersigned from liability under this Guaranty.

         The undersigned does hereby agree that the bankruptcy of Maker shall
have no effect on the obligations of the undersigned hereunder. The undersigned
does hereby further agree that in respect of any payments made by the
undersigned hereunder, the undersigned shall not have any rights based on
suretyship, subrogation or otherwise to stand in the place of Payee so as to
compete with Payee as a creditor of Maker, unless and until all claims of Payee
under the Note shall have been fully paid and satisfied.

         The undersigned hereby waives and agrees not to assert: (a) any right
to require Payee to proceed against Maker, or any other guarantor or person or
to pursue any other security or remedy before proceeding against the
undersigned; (b) any defense based on the validity or enforceability of the
Note; (c) any right or defense that may arise by reason of the incapacity, lack
of authority, death or disability of Maker or any other person; and (d) any
right or defense arising by reason of the absence, impairment, modification,
limitation, destruction or cessation (in bankruptcy, by an election of remedies,
or otherwise) of the liability of Maker of the subrogation rights of the
undersigned or of the right of the undersigned to proceed against Maker for
reimbursement. The undersigned waives any right, statutory, or otherwise, for
itself to require or for Maker to require Payee to apply rents received toward
the obligations of the undersigned under this Guaranty, or to otherwise
prioritize the receipt of rents as against the obligations of the undersigned
under this Guaranty. The undersigned hereby waives and agrees not to assert (x)
any right or defense based on the absence of any or all presentments, demands
(including demands for performance), notices (including notices of any adverse
change in the financial status of Maker, notices of any other facts which
increase the risk to the undersigned, notices of non-performance and notices of
acceptance of this Guaranty) and protests of each and every kind; (y) the
defense of any statute of limitations in any action under or related to this
Guaranty or the Note; and (z) any right or defense based on a lack of diligence
or failure or delay by Payee in enforcing its rights under this Guaranty or the
Note. The undersigned hereby waives and agrees not to assert or take advantage
of any right to (i) exoneration if Payee's actions shall impair any security or
collateral of the undersigned; (ii) any security or collateral held by Payee;
(iii) require Payee to proceed against or exhaust any security or collateral
before proceeding against the undersigned; and (iv) require Payee to pursue any
right or remedy for the benefit of the undersigned. Without limiting the
generality of any of the covenants and agreements of the undersigned set forth
in this Guaranty, the undersigned hereby waives any and all benefits of the
provisions of Sections 2809, 2810, 2819, 2822, 2845, 2848, 2849 and 2850 of the
California Civil Code and any similar or analogous statutes of California or any
jurisdiction.

         Until all the Maker's obligations under the Lease are fully performed,
the undersigned (a) shall have no right of subrogation or reimbursement against
the Maker by reason of any payments or acts of performance by the undersigned
under this Guaranty, (b) subordinates any liability or indebtedness of the Maker
now or hereafter held by the undersigned to the obligations of the Maker under,
arising out of or related to the Note; and (c) acknowledges that the actions of
Payee may affect or eliminate any rights of subrogation or reimbursement of the
undersigned as against Maker without any liability or recourse against Payee.

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         Neither this Guaranty nor any of the provisions hereof can be modified,
waived or terminated, except by a written instrument signed by Payee. The
provisions of this Guaranty shall apply to, bind and inure to the benefit of the
undersigned and Payee and their respective heirs, legal representatives,
successors and assigns. As used herein, the term "Maker" means the Maker
specifically named in the Note and also any assignee or transferee of said Note
and any successor to the interests of said Maker, assignee or transferee of such
Note or any part thereof, whether by assignment or otherwise including, without
limitation, any trustee in bankruptcy and any bankruptcy estate of Maker,
Maker's assignee or transferee. The undersigned, if there be more than one,
shall be jointly and severally liable hereunder, and for purposes of such
several liability the word "undersigned" wherever used herein shall be construed
to refer to each of the undersigned parties separately, all in the same manner
and with the same effect as if each of them had signed separate instruments, and
this Guaranty shall not be revoked or impaired as to any of such parties by the
death of another party or by revocation or release of any obligations hereunder
of any other party.

         If Payee should retain counsel and/or institute any suit against
Guarantor to enforce this Guaranty or any covenants or obligations hereunder,
then Guarantor shall pay to Payee, upon demand, all reasonable attorneys' fees,
costs and expenses, including, without limitation, court costs, filing fees,
recording costs, and all other costs and expenses incurred in connection
therewith (all of which are referred to herein as "Enforcement Costs"), in
addition to all other amounts due hereunder. This Guaranty shall be governed by
and construed in accordance with the internal laws of the state of California.
For the purpose solely of litigating any dispute under this Guaranty, the
undersigned submits to the jurisdiction of the courts of the United States of
America and of said state. The undersigned hereby waives any right to trial by
jury and further waives and agrees not to assert or take advantage of any
defense based on the claim that any arbitration decision binding upon Payee or
Maker is not binding on the undersigned.

         Any notice or other communication to be given to Payee or the
undersigned hereunder shall be in writing and sent in accordance with the notice
provisions of the Note. Notices to Payee shall be delivered to Payee's address
set forth in the Note. Notices to the undersigned shall be addressed as follows:
2077 Gateway Place, Suite 500, San Jose, CA 95110, Attention: Finance
Department. In the event Guarantor's notice address as set forth above changes,
Guarantor agrees to provide written notice to Payee of such change in address.

         This Guaranty shall be construed and enforced in accordance with the
laws of the State of California. Any legal action or proceeding with respect to
this Guaranty and any action for enforcement of any judgment in respect thereof
may be brought in the courts of California or the United States of America for
the Northern District of California and by execution and delivery of this
Guaranty, Guarantor hereby accepts unconditionally the non-exclusive
jurisdiction of the aforementioned courts and their respective appellate courts.
Guarantor hereby irrevocably consents to the service of process out of any of
these aforementioned courts in any such action or proceeding by the mailing of
copies thereof by a reputable international courier to Guarantor at the address
provided herein. Guarantor irrevocably waives any objection which it may now or
hereafter have to the laying of venue in any of the courts referred to above
arising out of or in connection any action or proceeding on this Guaranty
brought in any of the courts referred to above and further irrevocably waives
and agrees not to plead or claim

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in any such court that any such action or proceeding has been brought in an
inconvenient forum.

         IN WITNESS WHEREOF, the undersigned has executed this Guaranty as of
the date of the Note.

                                   GUARANTOR:

                                   BACKWEB TECHNOLOGIES, LTD, AN ISRAELI
                                   CORPORATION

                                   By: /s/ MICHAEL A. MORGAN
                                       -----------------------

                                   Name: Michael A. Morgan

                                   Title: CFO

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                            LANDLORD ACKNOWLEDGMENTS

STATE OF ____________)
COUNTY OF ___________)  ss:

         I, the undersigned, a Notary Public, in and for the County and State
aforesaid, do hereby certify that ____________________________, personally known
to me to be the __________ President of Equity Office Properties Trust, a
Maryland real estate investment trust, and personally known to me to be the same
person whose name is subscribed to the foregoing instrument, appeared before me
this day in person and acknowledged that as such officer of said entity being
authorized so to do, (s)he executed the foregoing instrument on behalf of said
entity, by subscribing the name of such entity by himself/herself as such
officer, as a free and voluntary act, and as the free and voluntary act and deed
of said entity, for the uses and purposes therein set forth.

         GIVEN under my hand and official seal this ___ day of __________, 20__.

                                                      __________________________
                                                         Notary Public

My Commission Expires: __________

                             TENANT ACKNOWLEDGMENTS
                                   INDIVIDUAL

STATE OF ____________)
COUNTY OF ___________)  ss:

         I, the undersigned, as Notary Public in and for the County and State
aforesaid, do hereby certify that ____________________________, personally known
to me to be the same person whose name is subscribed to the foregoing
instrument, appeared before me this day in person and acknowledged that (s)he
signed the said instrument as a free and voluntary act for the uses and purposes
therein set forth.

         GIVEN under my hand and official seal this ___ day of __________, 20__.

                                                      __________________________
                                                         Notary Public

My Commission Expires: __________

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                                   CORPORATION

STATE OF California  )
COUNTY OF Santa Clara) ss:

         On this the 19th day of August, 2003, before me a Notary Public duly
authorized in and for the said County in the State aforesaid to take
acknowledgments personally appeared Michael A. Morgan known to me to be CFO
President of BackWeb Technologies, Inc., one of the parties described in the
foregoing instrument, and acknowledged that as such officer, being authorized so
to do, (s)he executed the foregoing instrument on behalf of said corporation by
subscribing the name of such corporation by himself/herself as such officer and
caused the corporate seal of said corporation to be affixed thereto, as a free
and voluntary act, and as the free and voluntary act of said corporation, for
the uses and purposes therein set forth.

         IN WITNESS WHEREOF, I hereunto set my hand and official seal.

                                                [SEAL] /s/ Electra M. McBurnie
                                                       ----------------------
                                                           Notary Public

My Commission Expires: 2/9/06

                                   PARTNERSHIP

STATE OF ____________)
COUNTY OF ___________)  ss:

         On this the ___ day of ________________, 20__, before me a Notary
Public duly authorized in and for the said County in the State aforesaid to take
acknowledgments personally appeared _____________ known to me to be a general
partner of a partnership known as _______________________________, one of the
parties described in the foregoing instrument, and acknowledged that being
authorized so to do, (s)he executed the foregoing instrument on behalf of said
partnership by subscribing the name of said partnership by himself/herself, as a
free and voluntary act, and as the free and voluntary act of said partnership,
for the uses and purposes therein set forth.

         IN WITNESS WHEREOF, I hereunto set my hand and official seal.

                                                      __________________________
                                                         Notary Public

My Commission Expires: __________