Mergers & acquisitions (M&A) present a powerful growth lever, opening up new markets, new product lines, fresh talent, and expanded capabilities. But they also bring one of the least visible and most dangerous risks: cybersecurity. In particular, the external attack surface (all the assets exposed to the internet) becomes larger and more complex during an M&A event. If left unmanaged, this expanded surface is an open invitation to attacker activity.

For a security manager, M&A isn’t just a business transaction; it’s a risk amplifier. In the course of due diligence, integration planning, and post-deal operations, the external attack surface changes rapidly and often unpredictably. To stay ahead, you must combine visibility, in-depth testing (beyond standard scans), automation, and a mindset shift from reactive to proactive.

In this blog we’ll cover:

• The overall risk landscape in M&A and why the external attack surface matters.
• A precise definition of “external attack surface” in this context and what you should test.
• The timing and logic of when to test from pre-deal, through integration, into the operational post-deal phase.

M&A and cyber risk

M&A activity is rebounding in 2025, with deal values climbing and transaction complexity increasing with Bloomberg notes that total M&A deal value passed US $3.5 trillion globally in this year. At the same time, the cyber-threat environment is more aggressive: external exposures, cloud-first architectures, hybrid systems, and AI-driven attack tools are all raising the stakes.

For acquiring organizations, that means every deal brings not just the opportunity of addition, but the inheritance of risk, including exposures, vulnerabilities, unknown assets, and the legacy of past incidents. As Anjali Das, partner and co-chair of Wilson Elser's national cybersecurity and data privacy practice, put it in a recent article: “the buyer steps into the target’s existing cybersecurity posture, including its vulnerabilities, past breaches and latent threats.”

Cybersecurity is no longer an IT issue tucked away in the annex of a deal book. According to research by Forescout, 53% of respondents experienced critical cybersecurity issues that put the M&A deal in jeopardy. It now carries direct implications for deal value, post-deal integration cost, regulatory risk and reputational damage. For example:

• Undisclosed cyber incidents may become deal-breakers. 
• A weak or unknown external attack surface can introduce hidden liabilities that will materially affect the transaction.
• Regulatory regimes (data protection, breach notification, third-party risk) mean that post-deal exposures can trigger fines, lawsuits, or remediation costs that the acquiring company absorbs.

Despite all this, one of the greatest gaps in M&A cyber-risk assessment is the external attack surface. Why? Several reasons:

• The target’s asset inventory may not be accurate or up-to-date (shadow IT, unsanctioned cloud services, orphaned domains).
• The external attack surface tends to expand during integration (new endpoints, merged networks, third-party exposures) and is rarely fully measured before or after the deal.
• Many due diligence processes focus on internal controls, compliance checklists, breach history, but not the real “front door” exposures (internet-facing applications, API endpoints, misconfigured cloud services, brand impersonation domains).
• Traditional vulnerability scanning or patch-check tools miss the broader discovery and mapping needed to see the full external surface.

External risk factors during M&A

Exploitation of vulnerabilities is the second biggest root cause of data breaches, according to Verizon’s Data Breach Investigation Report. Attackers scan the internet for reachable assets, probe for weaknesses, and exploit anything that is misconfigured, outdated, or unmonitored. In a merger or acquisition, the number of external assets inevitably increases as two organizations combine their networks, domains, and cloud environments.

Each new or inherited system adds potential entry points that need to be discovered and secured. Without a full inventory and continuous testing, it is impossible to know what has been inherited or newly exposed during integration. These blind spots often become the first targets for threat actors. From a business perspective, an unmonitored external exposure can translate into post-deal remediation costs, regulatory penalties, and reputational harm, all of which diminish the expected value of the transaction.

Managing risk before, during and after M&A

Effective management of the external attack surface requires attention at every stage of an M&A transaction. The following phases outline how external attack surface testing should be applied from due diligence through ongoing operations to reduce risk and support a smooth integration.

Pre-deal or due diligence phase

The due diligence stage is when the acquiring organization has the best chance to identify risk before it becomes a liability. A complete assessment provides a realistic picture of the target’s internet-facing assets and their condition. This includes reconnaissance, exposure mapping, and vulnerability scanning across all domains, IPs, cloud environments, and third-party systems. 

Discovering unmanaged assets, legacy servers, or outdated services during this stage can reveal security debt that would otherwise remain hidden until after the acquisition. Findings from external testing can inform valuation, indemnities, and negotiation, giving the acquiring company leverage to factor remediation costs into the deal.

Integration planning phase

Once the deal is announced and integration planning begins, the risk of exposure rises. Systems start connecting, shared access is established, and data begins to flow between the two organizations. External testing during this phase helps align both environments and prevent the accidental introduction of new vulnerabilities. This phase is also the right time to define a shared standard for external exposure management, so that both sides follow the same security baseline as integration proceeds.

Post-signing and centralization of visibility

After signing, the acquiring company’s security team must centralize visibility across all inherited and existing external assets. Without a unified view, blind spots are inevitable. Fragmented inventories, separate monitoring systems, and inconsistent policies leave room for attackers to exploit overlooked systems. 

Centralizing visibility means consolidating all external asset data into one dashboard, aligning monitoring tools, and assigning ownership for every discovered asset. This step establishes the foundation for consistent detection and remediation processes. Once visibility is centralized, the team can measure progress, assign accountability, and start reducing exposure in a controlled and transparent way.

Post-merger and operational phase

When integration moves into daily operations, external attack surface testing becomes a continuous process rather than a one-time event. The environment continues to evolve as new systems are deployed, cloud accounts are merged, and business units adapt to new workflows. Periodic re-testing and automated external monitoring help identify newly exposed assets or configuration drift before they turn into incidents. Ongoing measurement of remediation time and exposure reduction shows whether integration is strengthening or weakening security over time.

M&A requires breadth and depth of testing

Basic vulnerability scanning or security rating services cannot uncover the full range of exposures that arise during M&A. Traditional tools tend to focus on known systems and predictable patterns, but hidden assets and misconfigurations are what make M&A environments vulnerable. Security assessments need to consider the full breadth of the attack surface to provide a level of assurance needed to identify and prioritize risks that are both inherited and newly created. 

Each entity has its own technology stack, its own naming conventions, and its own approach to cloud deployment. Without a thorough examination of the combined external perimeter, it is easy to miss small vulnerabilities that can lead to major breaches. The purpose of in-depth testing is to find those weak points early and provide clear direction for remediation.

These are some of the security assessment activities that should be completed as part of the M&A due diligence process:

• Identify all externally reachable assets, including domains, sub-domains, IP addresses, and endpoints, and map their relationships while uncovering unknown or unmanaged assets.
• Assess each reachable asset to determine what services are exposed, such as web servers, databases, APIs, or remote access, and verify whether they are properly secured with authentication, patches, and correct configuration.
• Conduct external vulnerability scans, check for misconfigurations, and evaluate cloud-specific rules for assets such as S3 buckets, IAM roles, and container endpoints.
• Analyze how an externally reachable asset could provide a route to more critical internal systems, taking into account misconfigurations, weak identity controls, and legacy systems.
• Review domains and services associated with the company, its sub-brands, partners, and acquired entities to detect typosquatting and other forms of impersonation that attackers might exploit during integration.
• Maintain ongoing visibility after the deal closes, monitoring for newly added or changed assets and performing regular testing to detect exposures or misconfigurations.

The scale and complexity of M&A

Completing a full external security assessment is labor and time intensive. Each technology stack may require its own form of testing and interpretation. For example, legacy on-premises web servers demand different validation steps than modern containerized cloud services. Conducting these tests manually across hundreds or thousands of assets can take weeks, delaying integration and exposing both entities to unnecessary risk.

Automation is the only practical way to address this scale. Tools that automate discovery and testing allow teams to continuously map all internet-facing assets, track changes as they occur, and flag exposures in real time. However, each environment has unique technologies, configurations, and risks, so meaningful results require contextual understanding. Automated tools have traditionally struggled to understand the unique context of each environment.

This is a challenge that can be solved by building LLMs into the AI assessment agents. Contextual, agentic AI can adapt its testing logic to the type of asset, the technology in use, and the business function it supports. Rather than running a single generic script across all systems, it applies the right test with awareness of context. 

Automating M&A external exposure testing

In-depth external exposure testing has become a necessity in modern mergers and acquisitions. The external perimeter of both entities represents the true front line of risk, where attackers look first and where hidden vulnerabilities are most likely to exist.

A structured, automated, and context-aware approach allows organizations to discover, assess, and secure their combined external environments before those risks can jeopardize the deal or create an incident. Centralizing visibility, enforcing governance, and adopting automation with contextual AI all contribute to a more efficient and resilient integration process.

Ultimately, proactive external testing is a financial safeguard that protects deal value, ensures operational continuity, and reinforces trust between stakeholders. To learn more about how Hadrian can support M&A external exposure testing, book a meeting with one of our security experts.

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