<PAGE>

                                                                   EXHIBIT 10.6

                               SECURITY AGREEMENT


         This Security Agreement (as may be amended, restated, and supplemented
from time to time, this "Agreement") is entered into as of December 26, 2000,
between Bentley Systems, Incorporated, a Delaware corporation (the "Debtor"),
whose principal place of business, chief executive office and address for
purposes of notice is 685 Stockton Drive, Exton, Pennsylvania 19341, and
Intergraph Corporation, a Delaware corporation ("Secured Party"), whose address
for purposes of notice is 1 Madison Industrial Park, Huntsville, Alabama
35894-0001. In consideration of the following premises and covenants and other
good and valuable consideration, the Debtor and Secured Party agree as follows:

         1. Definitions. All accounting terms not specifically defined in this
Agreement (which also includes the Schedules and Exhibits hereto and the
attachments to such Schedules and Exhibits) shall be construed in accordance
with generally accepted accounting principles, consistently applied ("GAAP").
"Asset Purchase Agreement" means the Asset Purchase Agreement among Debtor,
Secured Party, et al. dated on or about the date hereof. "Bank" means PNC Bank,
N.A., as agent for itself and the lenders under Debtor's credit facility, and
its successors. "Business Day" means any day other than a day on which banking
institutions in the State of Alabama are authorized or required by law to close.
All other terms used in this Agreement to describe the Collateral or Secured
Party's security interests and related rights which are not specifically defined
in this Agreement shall have the definitions given the same in the Uniform
Commercial Code, as adopted in the State of Alabama (the "UCC"). All other
capitalized terms shall have the meanings stated in this Agreement (which also
includes the Schedules and Exhibits hereto and the attachments to such Schedules
and Exhibits).

         2. Collateral.

                  (a) As security for the Obligations (as such term is hereafter
defined and used), the Debtor assigns and grants to Secured Party a security
interest and lien in the following property (collectively, the "Collateral"):

                           (i) Any and all of the Debtor's rights, title and
         interests (whether now or hereafter existing, arising or acquired) in,
         to and under the property, rights and interests described on SCHEDULE 1
         hereto, whether now or hereafter existing, arising or acquired (the
         "Maintenance Agreements and Receivables"); and

                           (ii) Any and all cash and non-cash proceeds
         (including, without limitation, insurance proceeds), products of and
         additions to and substitutions and replacements for, the foregoing,
         whether now or hereafter existing, arising or acquired.
<PAGE>
                  (b) "Obligations" means all of Debtor's debts, obligations and
liabilities to Secured Party evidenced by or referenced in (i) that certain
Secured Note executed by Debtor in favor of Secured Party on or about the date
hereof (as may be amended, restated or replaced, the "Promissory Note") and (ii)
this Agreement, whether now or hereafter arising or existing, and whether direct
or indirect, absolute or contingent, joint or several, due or not due,
contractual or tortious, liquidated or unliquidated and all extensions,
renewals, modifications and refinancings thereof.

         3. Collection of Accounts. At any time after an Event of Default, the
Debtor shall notify Secured Party of any collections received on the Maintenance
Agreements and Receivables and shall hold the same in trust for Secured Party
without commingling the same with other funds of the Debtor and, if Secured
Party shall request, shall turn the same over to Secured Party immediately upon
receipt in the identical form received. Proceeds transmitted to Secured Party
may be handled and administered in and through a remittance or special account;
the maintenance of any such account shall be solely for the convenience of
Secured Party, and the Debtor shall not have any right, title, or interest in or
to any such account or in the amounts at any time appearing to the credit
thereof. Secured Party may apply and credit Proceeds transmitted to or otherwise
received by Secured Party against any Obligations; however, Secured Party shall
not be required to credit any Obligations with the amount of any check or other
instrument constituting provisional payment until Secured Party has received
final payment thereof at its office in cash or solvent credits accepted by
Secured Party. At any time after an Event of Default, (a) the Debtor shall, at
the request of Secured Party, notify the obligors on the Maintenance Agreements
and Receivables of the security interest of Secured Party therein and shall
instruct the obligors on the Maintenance Agreements and Receivables to remit
payments directly to Secured Party, and (b) Secured Party may itself so notify
and instruct such obligors.

         4. Representations and Warranties. The Debtor represents and warrants
to Secured Party as follows:

                  (a) The Debtor is (i) a corporation duly organized, validly
existing and in good standing under the laws of the State of Delaware; (ii) duly
qualified and licensed to do business and is in good standing in every
jurisdiction where qualification is necessary except where the failure to
qualify or to obtain such license would not have a material adverse effect on
the Debtor or the Collateral; and (iii) duly authorized and empowered to create,
issue, execute, deliver and perform this Agreement and any related instruments
and agreements;

                  (b) The Debtor has timely filed and paid all federal, state,
and local tax reports, taxes and returns required by any law or regulation to be
filed and paid;

                  (c) The Debtor has and at all times will have good and
marketable title to the Collateral, free and clear of all liens, security
interests, or adverse claims, except for the Permitted Liens described in
Exhibit A hereto (the "Permitted Liens") and liens and security interests
granted to Secured Party;
<PAGE>
                  (d) This Agreement and the Promissory Note are the legal,
valid and binding obligation of the Debtor and any necessary consents of any
person or entity related to the execution, delivery and performance of this
Agreement and the Promissory Note have been obtained;

                  (e) No Event of Default has occurred and is continuing;

                  (f) Neither the Debtor's name nor the location of any of
Debtor's offices or the Collateral has changed in the previous five years;

                  (g) The Debtor is and shall be the lawful owner of all
Collateral and does and shall have good right to subject the same to a security
interest in favor of Secured Party. No Collateral shall be sold, assigned, or
transferred to any person other than Secured Party or in any way encumbered
except for the Permitted Liens and except to Secured Party, and the Debtor shall
defend the same against the claims and demands of all persons other than Secured
Party and the holders of the Permitted Liens, provided that so long as no Event
of Default has occurred and so long as the value of the Collateral is not
materially impaired or reduced, Debtor may settle and compromise in the ordinary
course of business, consistent with prudent business practices, the claims of
any obligor on any Maintenance Agreements or Receivables;

                  (h) There are no judgments, actions, suits, claims,
proceedings or investigations existing, outstanding, pending, or to the best of
the Debtor's knowledge after due inquiry, threatened or in prospect, before any
court, agency or tribunal, or governmental authority against or involving the
Debtor which do or could materially affect the business, properties, prospects,
financial condition, earnings, results of operations or earnings capacity of the
Debtor or which question the validity of the Promissory Note, this Agreement, or
any action or instrument contemplated hereby;

                  (i) Debtor has not executed any prior assignments of the
Maintenance Agreements and Receivables, and will not in the future execute any
assignments of the same, provided that so long as no Event of Default has
occurred and so long as the value of the Collateral is not materially impaired
or reduced, Debtor may settle and compromise in the ordinary course of business,
consistent with prudent business practices, the claims of any obligor on any
Maintenance Agreements or Receivables; and

                  (j) Debtor has not performed any acts or executed any other
documents, and will not take any action or execute any other documents, that
might prevent, limit or restrict Secured Party from enforcing any of the terms
or conditions of this Agreement or exercising any of its rights or remedies
hereunder.

         5. Affirmative Covenants. Until the Obligations are fully paid and
satisfied, the Debtor agrees and covenants to do the following:

                  (a) Maintain its existence, rights, and franchises and
maintain, preserve, and protect the Collateral provided that this Section 5(a)
shall not be deemed to prohibit Debtor from merging or consolidating with
another entity, changing its corporate structure or amending its
<PAGE>
certificate of incorporation or bylaws in any respect so long as any such action
does not impair the Collateral or adversely affect Secured Party's security
interest or lien thereon;

                  (b) (i) Comply with all material laws, statutes and government
regulations (including, without limitation, all applicable federal and state
environmental laws, rules, regulations, decrees, and court orders); (ii) pay all
of its indebtedness when due except for trade payables which Debtor is disputing
in good faith consistent with prudent business practices and for which Debtor
has established sufficient reserves; (iii) pay when due all taxes, assessments
and governmental charges or levies imposed on it, the Collateral or any part of
the Collateral, its income and profits, or any of its property (including,
without limitation, all environmental clean up costs); and (iv) pay all lawful
claims for labor, materials, supplies or other claims;

                  (c) At any time after an Event of Default, permit Secured
Party to visit its locations and to examine and make and take away copies or
reproductions of those books and records of Debtor which relate to the
Collateral (which records Debtor agrees to keep in a manner satisfactory to
Secured Party), to discuss the Debtor with its officers and accountants at all
reasonable times, and to inspect and audit the Collateral, and at any time after
an Event of Default permit Secured Party to verify the Debtor's receivables
constituting Collateral and other Collateral directly with account debtors and
by other methods selected by Secured Party and perform a Collateral audit all as
Secured Party desires at Debtor's expense;

                  (d) Promptly inform Secured Party, in writing, of (i) any and
all material adverse changes in the Debtor's financial condition; and (ii) any
Event of Default;

                  (e) Execute and deliver and file, or cause to be executed and
delivered and filed, any and all other agreements, instruments, or documents
which Secured Party may reasonably request in order to give effect to the
transactions contemplated by this Agreement and to perfect and protect Secured
Party's rights and interests. Debtor hereby appoints and empowers Secured Party,
or any employee of Secured Party which Secured Party may designate for the
purpose, as Debtor's attorney-in-fact (which appointment is coupled with an
interest and is irrevocable), to execute and/or endorse (and file, as
appropriate) at any time after an Event of Default for and in the name of Debtor
any documents, instruments, agreements, papers, checks, financing statements and
other documents which, in Secured Party's reasonable judgment, are necessary to
be executed and/or filed in order to (i) perfect or preserve the perfection and
priority of Secured Party's security interests and (ii) collect or realize upon
the Collateral or otherwise exercise its rights and remedies under this
Agreement, the Promissory Note or any related instrument or agreement or
applicable law; and

                  (f) Comply with all of the terms of all schedules, attachments
and exhibits hereto which are hereby incorporated herein by reference.

         6. Negative Covenants. Until the Obligations are fully paid and
satisfied, the Debtor will not:
<PAGE>
                  (a) Sell or otherwise dispose of any Collateral or create or
permit or suffer to occur the existence of any lien or encumbrance on any
Collateral except for Permitted Liens and security interests and liens in favor
of Secured Party; nor

                  (b) Except with thirty (30) days prior written notice to
Secured Party, change the location of the Debtor's principal place of business
or the Debtor's name or structure or consummate any merger or consolidation or
purchase or sale of substantially all of the Debtor's assets; provided that
notice of a merger or consolidation shall not be required if such event does not
adversely affect the Collateral or Secured Party's security interest therein or
the perfection thereof.

         7. Events of Default. The occurrence of any of the following events or
conditions shall constitute an "Event of Default" under this Agreement:

                  (a) The failure or refusal or neglect of the Debtor within
five (5) days after written notice from Secured Party to pay all or any part of
the Obligations or any related charges, fees and expenses;

                  (b) The failure of the Debtor within five (5) days after
written notice from Secured Party to timely and properly observe, keep, or
perform any covenant, agreement or warranty required by or otherwise set forth
in this Agreement, the Promissory Note or the Asset Purchase Agreement; provided
however, that no such notice shall be required with respect to any failure to
observe, keep or perform any covenant, agreement or warranty pursuant to which
Debtor is to inform, advise or otherwise provide notice or information to
Secured Party;

                  (c) If any representation, warranty, or statement contained in
this Agreement, in the Promissory Note or in any related instrument or agreement
delivered by the Debtor to Secured Party proves to have been false or misleading
in any material respect as of the date such representation, warranty, or
statement was made;

                  (d) If a default by the Debtor (as principal or guarantor or
other surety) in the payment of any indebtedness for borrowed money in excess of
$500,000 owing to any person or entity (other than Secured Party) occurs, and
such default shall remain unremedied in excess of the period of grace or cure,
if any, for payment of such indebtedness and repayment of such indebtedness is
accelerated or demanded;

                  (e) If (i) a receiver, trustee or liquidator is appointed for
the Debtor or all or a substantial part of its assets, provided Debtor shall
have forty (40) days to have removed any receiver, trustee or liquidator which
is appointed without Debtor's consent or request; or (ii) a petition commencing
a bankruptcy or other insolvency proceeding shall be filed (a) against the
Debtor without the Debtor's consent which is not dismissed within sixty (60)
days or (b) by Debtor or against Debtor with Debtor's consent;

                  (f) The levy against $500,000 worth or more of the property
(other than the Collateral) of the Debtor, or any execution, garnishment,
attachment, sequestration or other writ or
<PAGE>
similar proceeding against $500,000 worth or more of property of Debtor which is
not permanently dismissed or discharged within 60 days after the levy;

                  (g) The general assignment by or the filing of application in
any court for a receiver for the Debtor;

                  (h) The dissolution or liquidation of the Debtor;

                  (i) If any or all of this Agreement, the Promissory Note or
any related instrument or agreement or any portion hereof or thereof is asserted
by Debtor or any other party to be unenforceable, void or voidable; or

                  (j) Any lien (except for Permitted Liens and except in favor
of the Secured Party) or levy existing or arising against any of the Collateral.

         8. Rights and Remedies.

                  (a) Upon the occurrence of any Event of Default, Secured
Party, at its option and without the necessity of any presentment, demand or
notice(s) of default, intent to accelerate, demand or acceleration, or any other
notices, all of which are expressly waived by the Debtor, may declare all
Obligations immediately due and payable in full, and Debtor shall thereupon pay
all such Obligations immediately to Secured Party in full; provided that such
acceleration of the Obligations shall occur and be automatic upon the occurrence
of an Event of Default specified in clause (e) of Section 7.

                  (b) Upon the occurrence of any Event of Default, Secured Party
shall have (i) right to bring suit for collection of the Obligations, (ii) right
to collect and foreclose upon any or all of the Collateral and all of the rights
and remedies available to a secured party under the UCC, (iii) all of the other
rights and remedies allowed at law or in equity by applicable laws, ordinances,
statutes, rules, regulations, orders, injunctions, writs, or decrees of any
governmental or political subdivision or agency thereof, or any court or similar
entity established by any such subdivision or agency, and (iv) right to require
the Debtor to assemble the Collateral and make it available to the Secured Party
upon request at a place designated by Secured Party which is reasonably
convenient to both parties. Any and all payments received by Secured Party from
Debtor after an Event of Default and any amounts received by Secured Party from
the liquidation, collection or disposition of Collateral shall be applied first
to costs and expenses incurred by Secured Party in protecting or enforcing its
rights and remedies under this Agreement, the Promissory Note and/or applicable
law, and then to any and all Obligations in such order or manner as Secured
Party determines in its discretion, unless otherwise required by law. Without
limiting Secured Party's rights or Debtor's obligations under this Agreement,
the Promissory Note or any other instrument or agreement, the Debtor shall
remain liable for any deficiency after realization upon the Collateral and
application of the proceeds as set forth above. Debtor hereby grants Secured
Party a power of attorney to execute and deliver as Debtor's attorney-in-fact at
any time upon and after the occurrence of an Event of
<PAGE>
Default any and all instruments and agreements as Secured Party deems desirable
to collect, sell or liquidate any and all Collateral.

All of the foregoing rights and remedies are cumulative.

         9. No Waiver. Neither the failure nor any delay on the part of Secured
Party to exercise any right, power, or privilege shall operate as a waiver of
such right, power, or privilege, nor shall any single or partial exercise of
such right, power, or privilege preclude any other or further exercise thereof
or the exercise of any other right, power, or privilege. No waiver of any
provision in this Agreement shall be effective unless the same shall be in
writing and signed by Secured Party, and then shall be effective only in the
specific instance and for the purpose for which given and to the extent
specified in such writing. No modification or amendment to this Agreement shall
be valid or effective unless the same is in writing and signed by the party
against whom it is sought to be enforced. All rights and remedies of Secured
Party are cumulative.

         10. Additional Obligations of Debtor. Secured Party shall not in any
way be responsible for the performance of any obligations of Debtor under the
Maintenance Agreements and Receivables, or for any failure to do any or all of
the actions for which rights, interests, powers, and authority are herein
granted. Debtor shall defend and indemnify Secured Party against any claims by
anyone against Secured Party alleging any liability of Secured Party arising
under the Maintenance Agreements and Receivables (but not the products covered
by such Maintenance Agreements and Receivables, liability for which shall be
governed by the Asset Purchase Agreement). The failure of Secured Party to take
any of the actions or exercise any right, interest, power or authority granted
to Secured Party hereunder shall not be construed to be a waiver of any such
right, interest, power or authority.

                  Debtor will execute upon the request of Secured Party any and
all further documents, assignments or instruments that Secured Party reasonably
deems appropriate or necessary to evidence or effectuate this Agreement or grant
or confirm the rights and interests given to Secured Party hereunder.

                  Debtor will fulfill and perform each and every obligation of
Debtor under the Maintenance Agreements and Receivables unless the failure to do
so is consistent with prudent business practices and does not materially impair
or reduce the Collateral.

         11. Notices. All notices, requests, demands, or other communications
required or permitted to be given pursuant to this Agreement shall be in writing
and given by (a) personal delivery, (b) expedited delivery service with proof of
delivery, or (c) United States mail, postage prepaid, registered or certified
mail, return receipt requested, sent to the intended addressee at the address on
the first page of this Agreement and shall be deemed to have been received
either, in the case of expedited delivery service or personal delivery, as of
the date of first attempted delivery at the address, or in the case of mail,
upon deposit in a depository receptacle under the care and custody of the United
States Postal Service (provided Secured Party shall not be bound by any notice
until it actually receives the same). Either party shall have the right to
change its address for notice under
<PAGE>
this Agreement to any other location within the continental United States by
notice to the other party of such new address at least thirty (30) days prior to
the effective date of such new address.

         12. GOVERNING LAWS. THIS AGREEMENT, THE PROMISSORY NOTE AND THE OTHER
RELATED DOCUMENTS SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
INTERNAL LAWS OF THE STATE OF ALABAMA.

         13. Severability. If any provision of this Agreement, the Promissory
Note or any related instrument or agreement is held to be illegal, invalid or
unenforceable under present or future laws, such provision shall be fully
severable and ineffective and the remaining provisions of this Agreement, the
Promissory Note or any related agreement shall remain in full force and effect
and shall not be affected by the illegal, invalid or unenforceable provision or
by its severance.

         14. Costs. Debtor shall pay to Secured Party on demand all out of
pocket costs and expenses of Secured Party (including, without limitation, legal
fees, filing fees, recording costs, insurance premiums, taxes (with the
exception of Secured Party's income, excise and franchise taxes) and search
fees) incurred in connection with protecting and/or enforcing Secured Party's
rights and remedies under this Agreement and the Promissory Note which are
incurred by Secured Party after a breach or default under either of them.

         15. Survival. All representations, warranties, covenants, and
agreements made by or on behalf of the Debtor in connection with this Agreement
will survive the execution and delivery of this Agreement and shall be deemed
continuing representations, warranties, covenants and agreements until such time
as the Obligations are indefeasibly paid in full.

         16. Waivers. Debtor waives demand, notice of acceleration, notice of
intent to accelerate, dishonor, notice of protest, all other notices,
presentment, protest, and any and all rights to and of exemption.

         17. Assignment. Any or all of the Obligations, this Agreement, the
Promissory Note and any related agreement and the rights and remedies thereunder
may be assigned or delegated in whole or part by Secured Party (but not by the
Debtor except in connection with the merger of the Debtor) without Debtor's
prior written consent only in connection with (i) a collateral assignment to
Secured Party's primary corporate credit provider, or (ii) a merger,
consolidation or sale of substantially all of the assets of Secured Party.
Debtor agrees not to unreasonably withhold its consent to any other assignment
and/or delegation by Secured Party.

         18. Entire Agreement. This Agreement and the Promissory Note, together
with any documents and instruments executed in connection herewith and any
related instruments and agreements, embody the entire agreement and
understanding between the parties concerning the subject matter hereof and
thereof.

         19. No Third Party Beneficiaries. There are no third party
beneficiaries to this Agreement, the Promissory Note or to any related
instrument or agreement. All conditions to
<PAGE>
Secured Party's obligations under this Agreement are imposed solely and
exclusively for the benefit of Secured Party. Neither the Debtor nor any other
person shall have standing to require satisfaction of any such condition or be
entitled to assume that Secured Party will insist upon strict compliance with
any or all such conditions, and neither the Debtor nor any other person shall,
under any circumstances, be deemed to be a beneficiary of any conditions hereof,
any or all of which conditions may be waived freely, in whole or in part by
Secured Party at any time if, in its sole discretion, Secured Party deems it
advisable so to do.

         20. Modifications. No modification or amendment of this Agreement shall
be effective unless placed in writing and duly executed by Secured Party and
Debtor.

         IN WITNESS WHEREOF, Debtor and Secured Party have executed this
Agreement as of the date first above written.

                                          DEBTOR:

                                          BENTLEY SYSTEMS, INCORPORATED


                                          By:  /s/  David G. Nation
                                             -----------------------------------
                                                   David G. Nation
                                                   Senior Vice President


                                          SECURED PARTY:

                                          INTERGRAPH CORPORATION


                                          By:  /s/ John W. Wilhoitte
                                             -----------------------------------
                                                   John W. Wilhoitte
                                          Title:  Executive Vice President
                                                  ------------------------------
<PAGE>
                                    EXHIBIT A


                                 Permitted Liens



         (a)      PNC Bank, N.A.'s second priority interest in the Collateral,
                  so long as an Intercreditor Agreement acceptable to Secured
                  Party is in effect between PNC Bank, N.A. and Secured Party;

         (b)      liens of carriers, warehousemen, landlords, mechanics,
                  laborers and materialmen arising by law for sums which are (i)
                  not yet due or (ii) being diligently contested in good faith
                  and with respect to which Debtor has set aside sufficient
                  reserves; and

         (c)      liens for taxes which are (i) not yet due or (ii) being
                  diligently contested in good faith by appropriate proceedings
                  and with respect to which Debtor has set aside sufficient
                  reserves.

         (d)      liens or security interests which are subordinate to Secured
                  Party's security interest in the Collateral provided that
                  Debtor is contesting the same pursuant to appropriate
                  proceedings or causes the same to be dismissed promptly.





                                                                 ---------------
                                                                     Initial
<PAGE>
                                   SCHEDULE 1

                            Description of Collateral


Any and all maintenance, license and other agreements and arrangements now or
hereafter entered into by Debtor with customers of Debtor located in the United
States for which revenues are included by Secured Party in the Schedule of
Transferred Maintenance from time to time (as such term is used in the Asset
Purchase Agreement) whereby Debtor agrees to, does or may grant license(s),
provide maintenance, support, upgrades or similar services, rights or property,
in each case only to the extent relating to any of the software products sold to
Debtor by Secured Party and designated by Secured Party as Civil, Raster or
Plotting and any derivatives, upgrades, supplements, variations, redesignations
or modifications of any of them made by Debtor, including without limitation,
the products listed on Schedule 1.1(c) attached hereto, together with all
royalties, accounts, chattel paper, instruments, general intangibles and rights
to payment and performance evidenced thereby, arising therefrom or related to
any or all of the foregoing.



                                                                 ---------------
                                                                     Initial