CYBS Cybersource (MM)

SUMMARY OF THE JOINT PROXY STATEMENT/PROSPECTUS

This summary highlights selected information from this joint proxy statement/prospectus and may not contain all of the information that is important to you. You should carefully read this entire joint proxy statement/prospectus and the other documents to which this joint proxy statement/prospectus refers or that are incorporated by reference herein in order to understand fully the merger and the matters being considered at the special meetings. The merger agreement is attached as Annex A to this joint proxy statement/prospectus. CyberSource and Authorize.Net encourage you to read the merger agreement in its entirety as it is the legal document that governs the merger and related transactions. You may obtain the information incorporated by reference in this joint proxy statement/prospectus without charge by following the instructions in the section entitled “Where You Can Find More Information” on page 115. Unless we have stated otherwise, all references in this joint proxy statement/prospectus to CyberSource are to CyberSource Corporation, all references to Authorize.Net are to Authorize.Net Holdings, Inc., all references to Merger Corp. are to Congress Acquisition-Sub, Inc., all references to Merger LLC are references to Congress Acquisition Sub 1, LLC, all references to Lightbridge are to Lightbridge, Inc., which was the former name of Authorize.Net, and all references to the merger agreement are to the Agreement and Plan of Reorganization, dated as of June 17, 2007, by and among CyberSource, Authorize.Net, Merger Corp. and Merger LLC, a copy of which is attached as Annex A to this joint proxy statement/prospectus. Under the terms of the merger agreement, (i) Merger Corp. will merge with and into Authorize.Net, with Authorize.Net as the surviving corporation, and (ii) immediately thereafter, Authorize.Net will merge with and into Merger LLC, with Merger LLC as the surviving company. When we refer to the “merger” in this joint proxy statement/prospectus, we are referring to both mergers described in this paragraph. Immediately after the consummation of the mergers, Merger LLC will change its name to Authorize.Net LLC.

General

The Companies

CyberSource Corporation

1295 Charleston Road

Mountain View, California 94043

(650) 965-6000

http://www.cybersource.com

CyberSource provides secure electronic payment and risk management solutions to organizations that process orders for goods and services over the Internet. CyberSource’s payment solutions allow eCommerce merchants to accept a wide range of online payment options, from credit cards and electronic checks to global payment options and emerging payment types. CyberSource’s risk and compliance management tools address complexities common to online merchants such as credit card fraud, online tax requirements and export controls. CyberSource’s reporting and management tools facilitate the automation of the flow of complex eCommerce processes, such as recurring billing and payment reconciliation. CyberSource partners with and connects to a large network of payment processors and other payment service providers to offer merchants a single source solution.

CyberSource was founded in 1994 and initially launched software.net, an online software store, and built the payment processing capability to support software.net. In 1996, CyberSource created a second business focused on providing payment processing services to third parties and ran two separate business units: the online software store and the payment processing business. On December 31, 1997, the two business units were split into two separate corporations through the spin-off of the payment processing business into a separate corporation initially named, Internet Commerce Services Corporation. After the online software store changed its name to

software.net Corporation (and later to Beyond.com Corporation), Internet Commerce Services Corporation changed its name to CyberSource Corporation and continues to operate today under that name. CyberSource became a publicly traded company in June 1999.

Congress Acquisition-Sub, Inc.

c/o CyberSource Corporation

1295 Charleston Road

Mountain View, California 94043

(650) 965-6000

Congress Acquisition Sub 1, LLC

c/o CyberSource Corporation

1295 Charleston Road

Mountain View, California 94043

(650) 965-6000

Congress Acquisition-Sub, Inc. and Congress Acquisition Sub 1, LLC are wholly owned subsidiaries of CyberSource, and both entities were formed on June 12, 2007 in the State of Delaware. Neither Congress Acquisition-Sub, Inc. nor Congress Acquisition Sub 1, LLC has engaged in any operations. Each exists solely to facilitate this transaction. Therefore, although Congress Acquisition-Sub, Inc. and Congress Acquisition Sub 1, LLC will both be parties to the merger, we generally refer only to CyberSource when we discuss the merger in this joint proxy statement/prospectus.

Authorize.Net Holdings, Inc.

293 Boston Post Road West, Suite 220

Marlborough, Massachusetts 01752

(508) 229-3200

http://www.authorize.net

Authorize.Net (formerly, Lightbridge, Inc.) develops, markets and supports products and services primarily for businesses that sell products or services online. Authorize.Net’s IP-based Payment Processing solutions offer products and services to merchants in both the Card Not Present (e-commerce and mail order/telephone order) and Card Present (retail point-of-sale and mobile devices) segments of the U.S. credit card transaction processing market. In addition, the Payment Processing Services include an electronic check payment processing solution for merchants, eCheck.Net ^® . The Payment Processing solutions are designed to provide secure transmission of transaction data over the Internet and manage submission of this payment information to the credit card and Automated Clearing House processing networks. The Company provides its Payment Processing solutions primarily through a network of outside sales partners, Independent Sales Organizations and merchant bank partners.

Authorize.Net was incorporated in June 1989 under the Laws of the State of Delaware and became a publicly traded company in September 1996. Authorize.Net changed its name from Lightbridge, Inc. to Authorize.Net Holdings, Inc. in April 2007.

Market Price and Dividend Information (See page 19)

CyberSource common stock is listed on the Nasdaq Global Market under the symbol “CYBS,” and Authorize.Net common stock is listed on the Nasdaq Global Market under the symbol “ANET.” On June 15, 2007, the last full trading day before the public announcement of the merger, the closing sale price per share of CyberSource common stock on the Nasdaq Global Market was $12.90 and the closing sale price per share of

Authorize.Net common stock on the Nasdaq Global Market was $16.73. On September 19, 2007, the last full trading day before the printing date of this joint proxy statement/prospectus, the closing sale price per share of CyberSource common stock on the Nasdaq Global Market was $11.79 and the closing sale price per share of Authorize.Net common stock on the Nasdaq Global Market was $17.89.

THE MEETINGS

CyberSource Special Meeting (See page 32)

Under the rules of the Nasdaq Global Market, CyberSource stockholders must vote to approve the issuance of CyberSource securities pursuant to the merger in order for CyberSource and Authorize.Net to complete the merger. According to Delaware General Corporate law, as well as the rules of the Nasdaq Global Market, CyberSource stockholders must vote to approve the amendment to the CyberSource amended and restated certificate of incorporation. According to the rules of the Nasdaq Global Market, CyberSource stockholders must approve the amendment to the CyberSource Amended and Restated 1999 Stock Option Plan.

The CyberSource special meeting will be held at 10:00 a.m. local time on October 23, 2007, at the offices of CyberSource, located at 1295 Charleston Road, Mountain View, CA 94043. At the stockholder meeting, CyberSource stockholders will be asked to vote to (a) approve the issuance of CyberSource securities pursuant to the merger, (b) to approve the amendment to the CyberSource amended and restated certificate of incorporation, (c) to approve the amendment to the 1999 Stock Option Plan and (d) to grant discretionary authority to the management of CyberSource to vote your shares to adjourn the special meeting to a date not later than November 22, 2007, in order to solicit additional proxies if there appears to be insufficient votes for approval of the share issuance of CyberSource common stock pursuant to the merger. The amendment to the CyberSource amended and restated certificate of incorporation will increase the number of authorized shares of CyberSource common stock to 125 million shares so that CyberSource will have enough shares of common stock available to consummate the merger. The amendment to the 1999 Stock Option Plan will extend the term of the plan an additional three years and will increase the number of shares that can be awarded under the plan to 15.5 million shares. Such additional shares will be used, in part, for issuance to Authorize.Net employees who will become CyberSource employees upon the completion of the merger.

Record holders of CyberSource common stock at the close of business on September 14, 2007, which is the record date for the CyberSource meeting, may vote, or submit a proxy to vote, at the CyberSource meeting. CyberSource stockholders who return a proxy may revoke it at any time before the vote at the CyberSource meeting by sending a later-dated proxy or a written notice revoking the proxy to the secretary of CyberSource or by attending the meeting and voting in person.

Authorize.Net Stockholder Meeting (See page 32)

Under the rules of the Delaware General Corporate Law and the Nasdaq Global Market, Authorize.Net stockholders must vote to adopt and approve the merger agreement in order for Authorize.Net and CyberSource to consummate the merger.

The Authorize.Net special meeting will be held 10:00 a.m. local time on October 25, 2007, at the offices of Authorize.Net, located at 293 Boston Post Road West, Suite 220, Marlborough, Massachusetts 01752. At the special meeting, Authorize.Net stockholders will be asked to vote to adopt and approve the merger agreement, referred to herein as the merger proposal, and to grant discretionary authority to the management of Authorize.Net to vote your shares to adjourn the special meeting to a date not later than November 24, 2007, to solicit additional proxies if there are not sufficient votes for approval of the merger proposal.

Record holders of Authorize.Net common stock at the close of business on September 17, 2007, which is the record date for the Authorize.Net special meeting, may vote, or submit a proxy to vote, at the Authorize.Net special meeting. Authorize.Net stockholders who return a proxy may revoke it at any time before the vote at the Authorize.Net special meeting by sending a later-dated proxy or a written notice revoking the proxy to the secretary of Authorize.Net or by attending the meeting and voting in person.

Recommendation of Boards of Directors

CyberSource’s Board of Director’s Recommendation to CyberSource Stockholders (See page 33)

After careful consideration, CyberSource’s board of directors unanimously approved and adopted the merger agreement, determined that the merger and the issuance of CyberSource securities in the merger are in the best interests of CyberSource and its stockholders and unanimously determined to recommend that CyberSource stockholders vote to approve the issuance of CyberSource securities pursuant to the merger. In addition, the board of directors also unanimously approved the amendment to the CyberSource amended and restated certificate of incorporation and the amendment to the 1999 Stock Option Plan. The amendment to the CyberSource amended and restated certificate of incorporation is required so that CyberSource has enough authorized shares of common stock to issue to the Authorize.Net stockholders upon the consummation of the merger. The board of directors unanimously determined to recommend that CyberSource stockholders vote to approve the two amendments.

THE CYBERSOURCE BOARD UNANIMOUSLY RECOMMENDS THAT CYBERSOURCE STOCKHOLDERS VOTE “FOR” THE PROPOSAL TO APPROVE THE ISSUANCE OF CYBERSOURCE SECURITIES TO THE AUTHORIZE.NET STOCKHOLDERS PURSUANT TO THE MERGER AGREEMENT. THE CYBERSOURCE BOARD ALSO UNANIMOUSLY RECOMMENDS THAT CYBERSOURCE STOCKHOLDERS VOTE “FOR” THE PROPOSAL TO APPROVE THE AMENDMENT TO THE CERTIFICATE OF INCORPORATION AND “FOR” THE PROPOSAL TO APPROVE THE AMENDMENT TO THE CYBERSOURCE AMENDED AND RESTATED 1999 STOCK OPTION PLAN. THE CYBERSOURCE BOARD OF DIRECTORS FURTHER UNANIMOUSLY RECOMMENDS THAT CYBERSOURCE STOCKHOLDERS VOTE “FOR” THE PROPOSAL TO AUTHORIZE ADJOURNMENT OF THE SPECIAL MEETING TO SOLICIT ADDITIONAL PROXIES IF NECESSARY.

In approving the merger agreement and making its recommendation, the CyberSource board of directors consulted with CyberSource’s senior management and CyberSource’s financial and legal advisors and considered a number of strategic, financial and other considerations referred to under “ The Merger—CyberSource’s Reasons for the Merger ; CyberSource’s Board of Directors’ Recommendations ” beginning on page 46.

Authorize.Net’s Board of Director’s Recommendation to Authorize.Net Stockholders (See page 34)

After careful consideration, Authorize.Net’s board of directors unanimously approved the merger agreement and the merger, determined that the merger is in the best interests of Authorize.Net and its stockholders, and unanimously determined to recommend that Authorize.Net stockholders vote to adopt and approve the merger agreement as required under Delaware law.

THE AUTHORIZE.NET BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS THAT AUTHORIZE.NET STOCKHOLDERS VOTE “FOR” THE PROPOSAL TO ADOPT AND APPROVE THE MERGER AGREEMENT AND THE ADJOURNMENT PROPOSAL.

In approving the merger agreement and making its recommendation, the Authorize.Net board of directors consulted with Authorize.Net’s senior management and Authorize.Net’s financial and legal advisors and considered a number of strategic, financial and other considerations referred to under “ The Merger—Authorize.Net’s Reasons for the Merger ; Authorize.Net’s Board of Directors’ Recommendations ” beginning on page 47.

Vote Required (See page 34)

CyberSource . The affirmative vote of a majority of the outstanding shares of CyberSource common stock on September 14, 2007, the record date for the CyberSource special meeting, is required for approval of the amendment to the CyberSource amended and restated certificate of incorporation to increase the number of authorized shares of CyberSource common stock from 50 million to 125 million, referred to as the certificate amendment proposal. Approval of the certificate amendment proposal is required to complete the merger. Therefore, if a CyberSource stockholder does not send in a completed proxy, or attend the stockholder meeting to vote shares of CyberSource common stock in person, the effect will be the same as a vote against the certificate amendment proposal and the merger. The affirmative vote of a majority of the votes cast in person or by proxy at the CyberSource special meeting is required to approve the proposals relating to the issuance of CyberSource common stock to Authorize.Net stockholders in the merger and the amendment to 1999 Stock Option Plan to increase the number of shares awarded under the plan from 11.0 million to 15.5 million. The affirmative vote of a majority of votes cast in person or by proxy, whether or not a quorum is present, at the CyberSource special meeting, is required for stockholders to approve the proposal to adjourn the CyberSource special meeting.

Authorize.Net . The affirmative vote of a majority of the outstanding shares of Authorize.Net common stock on September 17, 2007, the record date for the Authorize.Net stockholder meeting, is required to approve the merger proposal. Approval of the merger proposal is required to complete the merger. Therefore, if an Authorize.Net stockholder does not send in a completed proxy, or attend the stockholder meeting to vote shares of Authorize.Net common stock in person, the effect will be the same as a vote against the merger. The affirmative vote of a majority of votes cast in person or by proxy, whether or not a quorum is present at the Authorize.Net special meeting, is required for stockholders to approve the proposal to adjourn the Authorize.Net special meeting.

THE MERGER

The Merger (See page 40)

CyberSource and Authorize.Net are proposing a business combination transaction pursuant to the terms of the merger agreement. Under the terms of the merger agreement, (a) Merger Corp. will merge with and into Authorize.Net, with Authorize.Net as the surviving corporation, and (b) immediately thereafter, Authorize.Net will merge with and into Merger LLC, with Merger LLC as the surviving company. Following the merger, Merger LLC will change its name to Authorize.Net LLC and operate as a wholly owned subsidiary of CyberSource.

As a result of the merger, each share of Authorize.Net common stock issued and outstanding (excluding shares held by CyberSource, Authorize.Net and their respective subsidiaries) will be converted into the right to receive (a) 1.1611 shares of CyberSource common stock, referred to as the exchange ratio, and (b) a pro rata share of $125 million in the form of cash. Holders of Authorize.Net common stock will not receive any fractional shares of CyberSource common stock in the merger. Instead, the total number of shares of CyberSource common stock that each Authorize.Net stockholder will receive in the merger will be rounded down to the nearest whole number, and CyberSource will pay cash for any resulting fractional share of CyberSource common stock that an Authorize.Net stockholder otherwise would be entitled to receive. The amount of cash payable for a fractional share of CyberSource common stock will be determined by multiplying such fraction of a share by the closing

price for CyberSource common stock on the trading day immediately prior to the closing date of the merger. The value, but not the number, of shares of CyberSource common stock that Authorize.Net stockholders will receive in the merger will vary depending upon the market price of CyberSource common stock at the time of the consummation of the merger. In addition, the amount of cash consideration to be paid per share of Authorize.Net common stock will not be determined until the consummation of the merger. On June 15, 2007, the last full trading day before the public announcement of the merger, the cash consideration per share would have been $4.45. The exchange ratio is described in more detail in the section titled “ The Merger—General Description of the Merger and Conversion of Authorize.Net Common Stock ” beginning on page 40.

Former Authorize.Net stockholders are currently expected to own approximately 48% of the outstanding CyberSource common shares after the merger, based on shares outstanding as of September 14, 2007, the record date of the CyberSource special meeting.

Opinion of Authorize.Net’s Financial Advisor (See page 49)

On June 15, 2007, Jefferies Broadview, a division of Jefferies & Company, Inc. (“Jefferies Broadview”), delivered to the Authorize.Net board of directors its opinion to the effect that, as of that date and based upon and subject to the various considerations and assumptions set forth in its opinion, the merger consideration to be received by holders of Authorize.Net common stock pursuant to the merger agreement (consisting of (a) 1.1611 shares of CyberSource common stock per share of Authorize.Net common stock and (b) cash consideration per share of Authorize.Net common stock equal to $125 million divided by the number of shares of Authorize.Net common stock issued and outstanding immediately prior to the effective time of the merger (excluding shares held by CyberSource, Authorize.Net or any of their respective subsidiaries)) was fair, from a financial point of view, to those holders. The full text of Jefferies Broadview’s opinion, which sets forth the assumptions made, matters considered and limitations on the scope of review undertaken by Jefferies Broadview in rendering its opinion, is attached to this joint proxy statement/prospectus as Annex F . Authorize.Net encourages its stockholders to read the Jefferies Broadview opinion carefully and in its entirety. Jefferies Broadview’s opinion was provided to the Authorize.Net board of directors in connection with its consideration of the merger and addresses only the fairness, from a financial point of view and as of the date of Jefferies Broadview’s opinion, of the merger consideration to be received by the holders of Authorize.Net common stock pursuant to the merger agreement and does not address any other aspect of the merger. Jefferies Broadview’s opinion does not constitute a recommendation as to how any holder of shares of Authorize.Net common stock should vote on the merger or any matter related thereto.

Opinion of CyberSource’s Financial Advisor (See page 57)

Goldman, Sachs & Co. (“Goldman Sachs”) delivered its opinion to CyberSource’s board of directors that, as of June 17, 2007, and based upon and subject to the factors and assumptions set forth therein, the merger consideration in the aggregate to be paid by CyberSource in respect of each share of Authorize.Net common stock pursuant to the merger agreement was fair from a financial point of view to CyberSource.

The full text of the written opinion of Goldman Sachs, dated June 17, 2007, which sets forth assumptions made, procedures followed, matters considered and limitations on the review undertaken in connection with the opinion, is attached to this joint proxy statement/prospectus as Annex E . Goldman Sachs provided its opinion for the information and assistance of CyberSource’s board of directors in connection with its consideration of the transaction. The Goldman Sachs opinion does not constitute a recommendation as to how any holder of CyberSource common stock should vote with respect to the transaction or any other matter. Pursuant to an engagement letter between CyberSource and Goldman Sachs, CyberSource has agreed to pay Goldman Sachs a transaction fee of $3 million, all of which is only payable upon consummation of the transaction.

Interests of Authorize.Net’s Officers and Directors in the Merger (See page 64)

When considering the recommendation of Authorize.Net’s board of directors, you should be aware that Authorize.Net directors and executive officers have interests in the merger that are different from, or are in addition to, your interests. Some of the directors and officers of Authorize.Net participate, or will participate, in employment and severance agreements and other arrangements and have continuing indemnification against liabilities that provide them with interests in the merger that are different from, or are in addition to, your interests. For example, CyberSource will make cash payments to certain directors and officers in cancellation of their Authorize.Net stock options. Based on the closing price of CyberSource’s common stock on the Nasdaq Global Market on June 29, 2007, the last trading day before the end of the fiscal quarter ended June 30, 2007, the aggregate cash payment would have been approximately $21 million. The actual amount of the cash payment will be based on the average closing price of CyberSource’s common stock on the Nasdaq Global Market for the 10 trading days immediately preceding the closing of the merger and the number of outstanding shares of Authorize.Net’s common stock at the closing of the merger.

Following the merger, Robert Donahue, currently a member of the board of directors of Authorize.Net, will become a member of the board of directors of CyberSource.

Pursuant to an employment agreement by and between Roy Banks, president of Authorize.Net’s principal operating subsidiary, and CyberSource, Mr. Banks will become president of Authorize.Net LLC after the merger and will receive cash compensation, stock options, severance benefits and other consideration as part of his continued employment.

The members of the Authorize.Net board of directors knew about these additional interests, and considered them, when they approved the merger and the merger agreement. As a result of these interests, these directors and officers may be more likely to vote to approve the merger agreement and the merger than if they did not hold these interests. You should consider whether these interests may have influenced these directors and officers to support or recommend the merger.

See the section entitled “ The Merger—Interests of CyberSource’s and Authorize.Net’s Officers and Directors in the Merger ” on page 64.

Material United States Federal Income Tax Consequences of the Merger (See page 67)

It is intended that the merger will be treated as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code (the “Code”) for United States federal income tax purposes and that no gain or loss will be recognized by CyberSource, Authorize.Net, Congress Acquisition-Sub, Inc. or Congress Acquisition Sub 1, LLC solely as a result of the merger. If the merger is treated as a reorganization, a stockholder of Authorize.Net who holds his, her or its shares of Authorize.Net as capital assets and who receives shares of CyberSource common stock and cash in the merger in exchange for all of the Authorize.Net shares actually owned by the stockholder will not recognize any loss as a result of the merger, but will recognize gain (if any) equal to the lesser of (a) the amount of cash so received and (b) the amount of gain realized (i.e., the amount by which the sum of the amount of cash so received and the fair market value on the date of the merger of the shares of CyberSource common stock received (including any fractional share) exceeds the stockholder’s adjusted federal income tax basis for the Authorize.Net shares surrendered). Although the matter is not free from doubt, Authorize.Net stockholders should be required to treat any recognized gain as dividend income (which may be eligible for preferential tax rates) unless the gain qualifies for capital gain treatment under Section 302 of the Code. Authorize.Net stockholders should consult their own tax advisors as to whether their receipt of cash qualifies for dividend or capital gain treatment under the Code.

In connection with the filing of the registration statement on Form S-4, of which this joint proxy statement/prospectus forms a part, CyberSource and Authorize.Net will receive legal opinions from Morrison & Foerster LLP and Foley Hoag LLP, respectively, to the effect that the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code and that CyberSource and Authorize.Net will each be treated as a party to the reorganization within the meaning of Section 368(b) of the Code. These signed legal opinions will be filed as exhibits to the registration statement on Form S-4 of which this joint proxy statement/prospectus forms a part.

The legal opinions described above are based upon certain representations, assumptions and limitations, and are not binding on the Internal Revenue Service or a court.

For a more detailed description of the anticipated tax consequences of the merger, see the section of this joint proxy statement/prospectus entitled “ Material United States Federal Income Tax Consequences ” beginning on page 67.

Accounting Treatment (See page 71)

CyberSource will account for the merger using the purchase method of accounting in accordance with United States generally accepted accounting principles. As the sole owner of Authorize.Net following the merger, CyberSource will be required to include Authorize.Net’s operating results under “Operating Revenues” and “Operating Expenses” on CyberSource’s consolidated financial statements.

Regulatory Approvals (See page 71)

Other than the SEC declaring CyberSource’s registration statement on Form S-4 relating to this transaction (of which this joint proxy statement/prospectus is a part) effective and approvals under the HSR Act, neither CyberSource nor Authorize.Net believes that any additional material governmental filings are required with respect to the transaction.

Under the HSR Act and the rules and regulations promulgated thereunder, certain transactions, including the merger, may not be consummated unless certain waiting period requirements have expired or been terminated. CyberSource and Authorize.Net each filed a Pre-Merger Notification and Report Form pursuant to the HSR Act with the Department of Justice and the Federal Trade Commission. Effective July 30, 2007, the Federal Trade Commission granted early termination of the waiting period under the HSR Act.

Appraisal Rights (See page 72)

Authorize.Net stockholders will have appraisal rights under Delaware law in connection with the merger. If the merger is consummated, any Authorize.Net stockholder who has not voted shares of Authorize.Net common stock in favor of the merger agreement has the right to demand appraisal of, and to be paid the fair market value for, such shares of Authorize.Net common stock in lieu of the cash and CyberSource securities provided for in the merger agreement. The value of the Authorize.Net common stock for this purpose will exclude any actual or expected impact on value arising from the merger. In order for the holder of Authorize.Net common stock to exercise its right to an appraisal, such holder must deliver to Authorize.Net a written demand for an appraisal of the shares of Authorize.Net common stock prior to the time the vote is taken on the merger at the Authorize.Net special meeting as provided by Delaware law.

Exchange of Stock Certificates (See page 73)

After the merger is completed, Authorize.Net stockholders will receive a letter of transmittal from American Stock Transfer & Trust Company, as exchange agent, that will provide instructions on how to exchange their Authorize.Net stock certificates for new CyberSource stock certificates.

Holders of uncertificated shares of Authorize.Net common stock (holders whose shares are held in book entry) will automatically be issued uncertificated (book entry) shares of CyberSource common stock as soon as possible after the completion of the merger. In addition, the exchange agent will mail to Authorize.Net stockholders a check in the amount (after giving effect to any required tax withholdings) of any cash payable in lieu of fractional shares of CyberSource common stock.

Expenses (See page 73)

Except for the termination fees payable by CyberSource and Authorize.Net in certain circumstances and certain filing fees required by the Hart-Scott-Rodino Antitrust Improvements Act of 1976 (the “HSR Act”), each company has agreed to pay all expenses it incurs in connection with the merger, whether or not the merger is completed.

Limitation on Authorize.Net’s Ability to Consider Other Acquisition Proposals (See page 79)

Authorize.Net has agreed, unless it receives a superior acquisition proposal as defined in the merger agreement, not to take any action (a) to solicit, initiate or encourage any proposal for a merger or other business combination involving Authorize.Net or (b) involving the acquisition or purchase of a 15% or greater equity interest in, or 15% or more of the assets of, Authorize.Net or any of its subsidiaries, prior to the completion of the merger or the earlier termination of the merger agreement in accordance with its terms.

Treatment of Authorize.Net Stock Options by CyberSource (See page 81)

Upon the closing of the merger, each outstanding option award held by employees of Authorize.Net who become employees of the combined company, with the exception of Roy Banks, the president of Authorize.Net’s principal operating subsidiary, will be assumed by CyberSource and converted into an option to purchase a certain number of shares of CyberSource common stock as described in this paragraph. These options will be exercisable on the same terms and conditions as set forth in Authorize.Net’s stock option plan. Each assumed option will be converted into an option to purchase 1.1611 shares of CyberSource common stock plus an additional number of shares equal to the quotient obtained by dividing the cash exchange ratio by the stock option ratio, with the result rounded down to the nearest number of whole shares. The cash exchange ratio is determined by dividing $125 million by the number of shares of Authorize.Net common stock issued and outstanding immediately prior to the effective time of the merger but excluding shares of Authorize.Net common stock held by CyberSource, Authorize.Net or any of their subsidiaries. The stock option ratio is the average closing price of CyberSource’s common stock on the Nasdaq Global Market for the 10 trading days immediately preceding the closing of the merger. The per share exercise price for the shares of CyberSource common stock issuable upon exercise of such assumed CyberSource stock option will be equal to the quotient determined by dividing the per share exercise price for such Authorize.Net stock by the option exchange ratio, rounded up to the nearest whole cent. The option exchange ratio is equal to 1.1611 plus the quotient obtained by dividing the cash exchange ratio by the stock option ratio.

Upon the closing of the merger, each outstanding option award held by non-employee members of Authorize.Net’s board of directors, any Authorize.Net employee who is not offered employment by the combined company and Mr. Banks, who will become an employee of the combined company, will be cancelled and converted into a right to receive a cash payment. The cash payment for each vested option will be equal to the

cash exchange ratio, as defined in the preceding paragraph, plus an amount determined by multiplying 1.1611 by the difference between (a) the average closing price of CyberSource common stock for the 10 trading days immediately preceding the closing of the merger and (b) the exercise price to exercise the corresponding vested option and acquire the corresponding share of Authorize.Net common stock.

Conditions to Completion of the Merger (See page 82)

CyberSource and Authorize.Net will complete the merger when all of the conditions to completion of the merger are satisfied or waived, including, among other things, approval of the merger proposal by the stockholders of Authorize.Net, approval of the issuance of CyberSource securities pursuant to the merger agreement by the CyberSource stockholders, and waiver or material performance of closing obligations of the parties under the merger agreement.

Termination of the Merger Agreement (See page 83)

CyberSource and Authorize.Net may terminate the merger agreement and decide not to proceed with the merger at any time before completion if both parties agree. Either CyberSource or Authorize.Net may terminate the merger agreement and decide not to proceed with the merger at any time before the completion of the merger if:

Authorize.Net may terminate the merger agreement and decide not to proceed with the merger before the merger is completed if:

CyberSource may terminate the merger agreement and decide not to proceed with the merger before the merger is completed if:

Effect of Termination and Termination Fees (See page 84)

In general, if the merger agreement is terminated, neither CyberSource nor Authorize.Net will have any liability to the other under the merger agreement, except for liabilities and damages resulting from (a) breaches by a party of any of its representations, warranties or covenants, (b) failure of either party to fulfill a condition to the performance of the obligations of the other party or (c) any obligation to pay a termination fee or the fees and expenses of the other party.

Authorize.Net will be required to pay a $17 million termination fee to CyberSource if the merger agreement is terminated:

CyberSource will be required to pay a $17 million termination fee to Authorize.Net if the merger agreement is terminated:

Restrictions on the Ability to Sell CyberSource Common Stock and Delisting and Deregistration of Authorize.Net Common Stock (See page 85)

All shares of CyberSource common stock received by an Authorize.Net stockholder in connection with the merger will be freely transferable, unless the stockholder is considered an “affiliate” of either CyberSource or Authorize.Net under the federal securities laws. CyberSource will be entitled to place appropriate legends on the certificates evidencing any CyberSource common stock to be received by affiliates of Authorize.Net in the merger.

If the merger is completed, Authorize.Net common stock will no longer be listed on the Nasdaq Global Market and will be deregistered under the Securities Exchange Act of 1934, as amended, and Authorize.Net will no longer file periodic reports with the SEC.

Differences in the Rights of Holders of Authorize.Net and CyberSource Common Stock (See page 96)

Holders of Authorize.Net common stock will receive CyberSource securities and become a stockholder of CyberSource after the merger. Their rights as a CyberSource stockholder will continue to be governed by Delaware law, but rather than being governed by Authorize.Net’s certificate of incorporation and bylaws, their rights will be governed by CyberSource’s amended and restated certificate of incorporation and bylaws.

Where You Can Find More Information (See page 115)

Both CyberSource and Authorize.Net are required to file annual, quarterly and current reports, proxy statements and other information with the SEC. You may read and copy the registration statement of which this joint proxy statement/prospectus is a part and any other documents the parties have filed at the SEC’s Public Reference Room at 100 F Street, N.E., Washington D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the Public Reference Room. SEC filings are also available to the public at the website maintained by the SEC at http://www.sec.gov .

Please do not send your stock certificates at this time.