<PAGE>   1
                                                                  EXHIBIT 10.14


                                [ISS Letterhead]



July 10, 2000


Mr. Lawrence Costanza
444 Shelbourne Drive
Wyckoff, New Jersey  07481

Dear Larry:

I am pleased to make this offer for you to join our team at Internet Security
Systems. As Vice-President, America's Operation, your total compensation
package will be $480,000 annually at plan. The annual compensation will consist
of a base salary of $300,000 annually ($12,500 paid bi-monthly salary) plus
performances compensation paid quarterly at plan in the amount of $30,000 per
quarter. The actual performance compensation paid will be a function of
strategic business unit objectives, operational performance objectives and
revenue objectives as defined in the attached performance compensation plan. In
addition, you will be eligible for a $60,000 annual bonus based upon ISS
performance and ISS achievement of its 2000 sales and performance objectives.
You will be granted an option to purchase 200,000 shares of ISS Common Stock,
contingent upon the approval by the ISS Board of Directors, at an exercise
price equal to the closing price of ISS common stock at the date of the grant
by the Board of Directors at or after your date of employment. These options
will have our standard four-year annual vesting schedule. In addition, you will
be granted an additional 75,000 shares of ISS common stock at the same time
which vests as of October 1, 2000. These options shall be subject to all of the
terms and conditions set forth in ISS' Amended and Restated 1995 Stock Option
Plan. ISS also agrees to provide you with an additional bonus in the amount of
$40,000 per quarter gross for the first five quarters of your employment,
payable based on completion of employment for that quarter.

Additionally, if ISS terminates your employment without cause, ISS will
accelerate the vesting of your 200,000 share option grant to the same extent as
if you had remained employed by ISS for one (1) year following the date of the
termination. Vesting will not accelerate if your resign from your employment
with ISS or if your employment with ISS is terminated for Cause. As used
herein, "Cause" means a termination based on (i) your conviction of a felony or
any other offense evidencing fraud or moral turpitude, (ii) your failure or
refusal to follow the lawful and proper directives of the Company which is not
corrected within seven (7) days after your receipt written notice identifying
such failure or refusal, (iii) your willful malfeasance or gross misconduct
which discredits or damages the Company, (iv) any breach of your obligations to
maintain the confidentiality of ISS' confidential and proprietary information
and trade secrets, or (v) your chronic absence from work.


<PAGE>   2


In the event that the Company is party to a corporate transaction or a change
of control, as those terms are defined in the ISS' Restated 1995 Stock
Incentive Plan, in the first two years of your employment with ISS, then the
standard acceleration of vesting of one year under Article Two, Section III of
the stock option plan will be modified as follows for your 200,000 share option
grant. If the conditions under Article Two, Section III are met in the first 12
months after a corporate transaction or change in control such that you would
be eligible for acceleration of options, then your acceleration will be the
next two annual installments of option shares. All other conditions for this
acceleration to occur, including termination of your employment subsequent to
the acquisition, will remain a condition for such acceleration. A copy of the
Restated 1995 stock Incentive Plan is provided for your reference.

Your job responsibilities shall be as outlined in the attached job position
responsibilities. ISS agrees to provide for your health benefits in a
accordance with the ISS' benefits programs in effect from time to time. ISS
will provide for three weeks of paid vacation/sick leave during your first year
of employment as defined in ISS' benefits program. In addition, you will be
eligible to participate in the ISS 401 K program, the ISS Employee Stock
Purchase Program (ESPP), and the ISS Flexible Benefits Program (FLEX).

As I explained to you, this offer of employment is conditional upon the
following:

1)       Your execution of the standard form ISS Confidential Information. Work
         Product and Nonsolicitation Agreement which obligates you to protect
         ISS proprietary and confidential information and in addition,
         obligates you to refrain from using any confidential information from
         previous employers.

2)       Your providing proof of personal identity and employability. In
         connection with this contingency, please complete the top portion of
         the enclosed I-9 form. Bring this I-9 form, and substantiating
         documents as outlined on the form to provide personal identity and
         employment eligibility, to my office of your first day of work.

3)       ISS satisfactory review of personal and professional references and
         satisfactory results of the ISS background investigation that will
         require your completion of an application and authorization form
         allowing such investigation. This process is, by our policy, a
         condition of employment by ISS. This process can be coordinated
         through our Director of Human Resources, Terri Jones.

ISS is an at-will employer, which means that employment with ISS is for no
specific period of time and either your or ISS are free to terminate your
employment relationship at any time for any reason, with or without cause. This
is the full and complete agreement between us on the terms of your employment
and supercedes any prior representations or agreements, whether oral or
written. Although your job duties, title, compensation and benefits, as well as
ISS personnel policies and procedures, may change form time-to-time, the
"at-will" nature of your employment may only be changed in a document signed by
you and the President or the Vice President of Finance and Administration of
the Company.


<PAGE>   3


Larry, we would like for you to join us July 11, 2000. I look forward to having
you join the ISS team, to setting up shop in New Jersey and to building a
dynamic, successful enterprise with you.



                                             Sincerely,

                                             /s/ Thomas E. Noonan

                                             Thomas E. Noonan
                                             Chairman and CEO

                                             /s/ Ken Walters

                                             Ken Walters
                                             President & General Manager
                                             Enterprise Solutions Business

Accepted:


/s/ Larry Costanza
---------------------------
Larry Costanza

8/18/00
---------------------------
Date