Material Event (Form 8-K)

As previously announced, on September 24, 2025, Integral Ad Science Holding Corp., a Delaware corporation ("IAS" or the "Company"), entered into an Agreement and Plan of Merger (the "Merger Agreement" and the transactions contemplated thereby, the "Merger") with Igloo Group Parent, Inc., a Delaware corporation ("Parent"), and Igloo Group Acquisition Company, Inc., a Delaware corporation and a wholly owned subsidiary of Parent. On November 7, 2025, IAS filed a definitive information statement on Schedule DEFM14C (the "Definitive Information Statement"), as such may be supplemented from time to time, with the U.S. Securities and Exchange Commission (the "SEC").

Supplemental Disclosures

While IAS believes that the disclosures set forth in the Definitive Information Statement comply fully with all applicable laws, IAS has determined voluntarily to supplement certain disclosures in the Definitive Information Statement with certain supplemental disclosures set forth below (the "Supplemental Disclosures"). Nothing in the Supplemental Disclosures shall be deemed an admission of the legal merit, necessity or materiality under applicable laws of any of the disclosures set forth herein.

All page references used herein refer to pages in the Definitive Information Statement before any additions or deletions resulting from the Supplemental Disclosures, and capitalized terms used below, unless otherwise defined, have the meanings set forth in the Definitive Information Statement. Underlined and boldedtext shows text being added to a referenced disclosure in the Definitive Information Statement and stricken-throughtext shows text being deleted from a referenced disclosure in the Definitive Information Statement. This Current Report on Form 8-K is incorporated into, and amends and/or supplements, the Definitive Information Statement as provided herein. Except as specifically noted herein, the information set forth in the Definitive Information Statement remains unchanged.

Supplemental Disclosures to Definitive Information Statement

1.      The section of the Definitive Information Statement entitled "The Merger-Background of the Merger" is hereby amended and supplemented as follows:

A.      By amending and restating the fifth full paragraph on page 24 of the Definitive Information Statement in its entirety as follows:

On January 29, 2025, representatives of Jefferies shared a preliminary version of IAS's the2025 Annual Operating Plan prepared by members of IAS management, including 3-year projections and revenue forecasts (the ''InitialFebruary 2025 Forecasts''as discussed in further detail in the section of this information statement entitled ''The Merger-Certain Company Financial Forecasts'' beginning on page 42) with the Company Board for review.

B.      By amending and restating the sixth full paragraph on page 24 of the Definitive Information Statement in its entirety as follows:

On February 4, 2025, the Company Board held a special meeting via videoconference with representatives of Jefferies, members of IAS senior management and representatives of Kirkland to discuss the InitialFebruary 2025 Forecasts. Following discussion, the Company Board instructed IAS management to make certain minor adjustments to the Initial February 2025 Forecasts prior to sharing with potential counterparties in connection with the 2025 Process.

Over the course of the next several days, IAS management updated the Initial February 2025 Forecasts in accordance with the Company Board's instructions at the February 4 Board meeting. The updated version of the Initial February 2025 Forecasts (the "February 2025 Forecasts" as discussed in further detail in the section of this information statement entitled "The Merger-Certain Company Financial Forecasts" beginning on page 42) were reviewed and approved by the Board on February 8, 2025 to be shared with Following discussion, the Company Board authorized sharing of the February 2025 Forecasts with potential transaction counterparties in connection with thestrategic review 2025 Pprocess whom had executed acceptable confidentiality agreements.

C.      By amending and restating the last paragraph on page 29 of the Definitive Information Statement in its entirety as follows:

On September 87, 2025, certain members ofthe Company Board held a special meeting by videoconference with, among others, representatives of Kirkland, representatives of Jefferies and members of IAS's management in attendance. Representatives of Kirkland toreviewed with the Company Boardcertain key terms of the initial draft of the merger agreement (the ''Draft Merger Agreement'') proposed to be shared with Novacap. The Draft Merger Agreement, among other things, (i) contained customary representations, warranties and covenants for a transaction involving a financial sponsor buyer, (ii) provided for customary termination rights of IAS, including to enter into an acquisition agreement for a superior proposal prior to receipt of the requisite shareholder approval of the transaction (subject to compliance with the terms of the agreement and the payment of a termination fee), and (iii) included ''hell or high water'' obligations with respect to achieving required regulatory approvals. The Draft Merger Agreement contemplated that the Consenting Stockholders, who together held the requisite percentage of voting securities of IAS to approve a change of control transaction in accordance with the DGCL and IAS's organizational documents, would deliver a written consent approving the potential transaction after signing of the Merger Agreement. Following such discussion, the members ofthe Company Board in attendanceapproved the distribution of the Draft Merger Agreement to Novacap.

2.      The section of the Definitive Information Statement entitled "The Merger-Certain Company Financial Forecasts" is hereby amended and supplemented as follows:

A.      By amending and restating the third full paragraph on page 42 of the Definitive Information Statement in its entirety as follows:

In connection with its consideration of its stand-alone prospects and potential strategic transactions available to IAS and the process leading up to the evaluation of the Merger, IAS's management developed, among other things, certain forward-looking forecasts with respect to fiscal years 2024 through 2027 which were prepared by IAS management and reviewed and approved for use by the Company Board on September 29, 2024, which were further refined by IAS management and shared with the Company Board on October 5, 2024 (the "2024 Forecasts") (as described in the section entitled "-Background of the Merger" beginning on page 20) in connection with the Company Board's evaluation of the strategic review process, and, subsequently provided to representatives of Jefferies. Subsequently, at the direction of the Company Board, due to the passage of time and changes in market conditions and related performance and outlook, IAS's management provided the Company Board and representatives of Jefferies with updated financial forecasts with respect to fiscal years 2025 through 2027 which were reviewed and approved by the Company Board on February 48, 2025 (the "February 2025 Forecasts") in connection with the Company Board's evaluation of its stand-alone prospects and the strategic review process as well as for use by representatives of Jefferies in connection with its financial analyses. In August 2025, IAS's management provided the Company Board and representatives of Jefferies with updated financial forecasts with respect to fiscal years 2025 through 2029 which were reviewed and approved by the Company Board on August 14,2025 (the "August 2025 Forecasts" and together with the 2024 Forecasts and the February 2025 Forecasts, the "Forecasts") in connection with the Company Board's evaluation of the proposed Merger as well as for use by representatives of Jefferies in connection with its financial analyses and opinion (as described in the section entitled "Opinion of Jefferies LLC").