<PAGE>


                                                                   EXHIBIT 10.28

                        SIXTH LOAN MODIFICATION AGREEMENT

         This Sixth Loan Modification Agreement (this "Loan Modification
Agreement") is entered into as of November 24, 2004, by and between SILICON
VALLEY BANK, a California-chartered bank, with its principal place of business
at 3003 Tasman Drive, Santa Clara, California 95054 and with a loan production
office located at One Newton Executive Park, Suite 200, 2221 Washington Street,
Newton, Massachusetts 02462, doing business under the name "Silicon Valley East"
("Bank") and ART TECHNOLOGY GROUP, INC., a Delaware corporation with its
principal place of business at 25 First Street, Cambridge, Massachusetts 02141
("Borrower").

1.       DESCRIPTION OF EXISTING INDEBTEDNESS AND OBLIGATIONS. Among other
         indebtedness and obligations which may be owing by Borrower to Bank,
         Borrower is indebted to Bank pursuant to a loan arrangement dated as of
         June 13, 2002, evidenced by, among other documents, a certain Amended
         and Restated Loan and Security Agreement dated as of June 13, 2002,
         between Borrower and Bank, as amended by a certain First Loan
         Modification Agreement dated as of September 27, 2002, as further
         amended by a certain Amendment dated as of October __, 2002, as further
         amended by a certain Second Loan Modification Agreement dated as of
         December 24, 2002, as further amended by a certain Third Loan
         Modification Agreement dated as of October 20, 2003, as further amended
         by a certain Fourth Loan Modification Agreement dated November 26,
         2003, as further amended by a certain Letter Agreement dated June 16,
         2004, as further amended by a certain Fifth Loan Modification Agreement
         dated June 30, 2004 (as amended, the "Loan Agreement"). Capitalized
         terms used but not otherwise defined herein shall have the same meaning
         as in the Loan Agreement.

2.       DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by
         the Collateral as described in the Loan Agreement (together with any
         other collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the "Existing
Loan Documents".

3.       DESCRIPTION OF CHANGE IN TERMS.

         A.       Modifications to Loan Agreement.

                  1.       The Loan Agreement shall be amended by deleting the
                           following definition appearing in Section 13 thereof,
                           in its entirety:

                                    ""REVOLVING MATURITY DATE" is November 25,
                                    2004."

                           and inserting in lieu thereof the following:

                                    ""REVOLVING MATURITY DATE" is December 25,
                                    2004."


<PAGE>


4.       FEES. Borrower shall pay to Bank a modification fee equal to Seven
         Thousand Eight-Three Dollars and 00/100 ($7,083.00), which fee shall be
         due on the date hereof and shall be deemed fully earned as of the date
         hereof. The Borrower shall also reimburse Bank for all reasonable legal
         fees and expenses incurred in connection with this amendment to the
         Existing Loan Documents.

5.       RATIFICATION OF INTELLECTUAL PROPERTY SECURITY AGREEMENT. Borrower
         hereby ratifies, confirms and reaffirms, all and singular, the terms
         and conditions of a certain Intellectual Property Security Agreement
         dated as of June 13, 2002 between Borrower and Bank, and acknowledges,
         confirms and agrees that said Intellectual Property Security Agreement
         contains an accurate and complete listing of all Intellectual Property
         Collateral as defined in said Intellectual Property Security Agreement
         (with the exception of the Intellectual Property Collateral set forth
         on Schedule 5 attached hereto) as of June 13, 2002 or any subsequent
         amendment thereto and shall remain in full force and effect.

6.       RATIFICATION OF PERFECTION CERTIFICATE. Borrower hereby ratifies,
         confirms and reaffirms, all and singular, the terms and disclosures
         contained in a certain Perfection Certificate dated as of June 13, 2002
         between Borrower and Bank, and acknowledges, confirms and agrees the
         disclosures and information Borrower provided to Bank in said
         Perfection Certificate has not changed, as of the date hereof.

7.       CONSISTENT CHANGES. The Existing Loan Documents are hereby amended
         wherever necessary to reflect the changes described above.

8.       RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies, confirms, and
         reaffirms all terms and conditions of all security or other collateral
         granted to the Bank, and confirms that the indebtedness secured thereby
         includes, without limitation, the Obligations.

9.       NO DEFENSES OF BORROWER. Borrower agrees that, as of this date, it has
         no defenses against the obligations to pay any amounts under the
         Obligations.

10.      CONTINUING VALIDITY. Borrower understands and agrees that in modifying
         the existing Obligations, Bank is relying upon Borrower's
         representations, warranties, and agreements, as set forth in the
         Existing Loan Documents. Except as expressly modified pursuant to this
         Loan Modification Agreement, the terms of the Existing Loan Documents
         remain unchanged and in full force and effect. Bank's agreement to
         modifications to the existing Obligations pursuant to this Loan
         Modification Agreement in no way shall obligate Bank to make any future
         modifications to the Obligations. Nothing in this Loan Modification
         Agreement shall constitute a satisfaction of the Obligations. It is the
         intention of Bank and Borrower to retain as liable parties all makers
         of Existing Loan Documents, unless the party is expressly released by
         Bank in writing. No maker will be released by virtue of this Loan
         Modification Agreement.

11.      COUNTERSIGNATURE. This Loan Modification Agreement shall become
         effective only when it shall have been executed by Borrower and Bank
         (provided, however, in no event shall this Loan Modification Agreement
         become effective until signed by an officer of Bank in California).

                  [Remainder of page intentionally left blank]


<PAGE>


         This Loan Modification Agreement is executed as a sealed instrument
under the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                                               BANK:

ART TECHNOLOGY GROUP, INC.            SILICON VALLEY BANK, doing business
                                      as SILICON VALLEY EAST


By:  /s/ Edward Terino                By:  /s/ Michael D. Sinclair
    ------------------------------        --------------------------------------

Name:  Edward Terino                  Name:  Michael D. Sinclair
      ----------------------------          ------------------------------------

Title:  CFO                           Title:  Vice President
       ---------------------------           -----------------------------------


                                      SILICON VALLEY BANK

                                      By:
                                          --------------------------------------

                                      Name:
                                            ------------------------------------

                                      Title:
                                             -----------------------------------
                                      (signed in Santa Clara County, California)