External Attack Surface Management (EASM)

External attack surface management (EASM) is a process to identify, monitor and secure your external-facing digital assets, including: 

• Public-facing IP addresses
• Domains
• Third-party integrations
• Cloud services 
• Web applications

EASM tools uncover risks such as vulnerabilities, misconfigurations, shadow IT and third-party exposures to decrease external cyber threats.

As cyberattackers increasingly exploit unprotected entry points, EASM helps you identify your exposure from an attacker’s perspective so you can build a proactive and comprehensive cyber defense strategy.

As your external attack surface expands, your organization faces unique EASM challenges:

• Cloud adoption, remote work and third-party SaaS solutions exponentially increase the number of your external-facing assets.
• Employees may deploy unauthorized tools and platforms that bypass IT oversight, creating unmanaged entry points attackers could exploit.
• Cyberattackers can use advanced reconnaissance tools, automation, AI and machine learning to find and exploit security gaps in your digital perimeter.
• Your organization faces increased security and privacy mandates for external asset management, such as General Data Protection Regulation (GDPR), California Consumer Privacy Act (CCPA) and Payment Card Industry Data Security Standards (PCI DSS).

External attack surface management helps you overcome these challenges. It’s an important part of a comprehensive exposure management strategy. Continuous assessment of your external attack surface identifies all your external-facing assets, typically outside security and IT teams’ views, so you can find and close security gaps before attackers exploit them.

Failing to effectively manage your external attack surface increases your risk of data breaches, operational disruptions and reputational damage.

• EASM begins with passive asset discovery. It collects data from public records, DNS, WHOIS databases and certificate transparency logs to identify both known and unknown assets, including historic domains and previously used infrastructure.
• The platform then performs active scanning and fingerprinting, validating discovered assets by analyzing open ports, running services, TLS configurations and web applications to determine asset ownership and potential vulnerabilities.
• Using domain and subdomain enumeration, you can find registered domains, subdomains and cloud-based services while monitoring DNS records for changes to prevent risks like subdomain takeovers.
• EASM tools also map IP ranges and cloud resources. EASM can find externally accessible assets within AWS, Azure, Cloudflare and GCP to reveal misconfigurations, such as exposed storage buckets and improperly secured virtual machines.

Once the system discovers assets, continuous monitoring and risk assessment provide real-time security gap tracking and flagging issues like expired SSL certificates, open ports, publicly accessible databases and exposed admin panels.

The key difference between EASM and IASM is that internal ASM operates within your network, requiring access to scan systems. EASM identifies risks from outside your network using public data sources.

• Internal attack surface management focuses on securing your internal IT environment, including on-prem servers, employee devices and internal apps. It identifies vulnerabilities, misconfigurations and insider threats.
• External ASM (EASM) discovers and protects publicly accessible assets, such as web apps, cloud services and DNS records. It continuously scans for exposed systems, misconfigurations, shadow IT and third-party risks so attackers can’t exploit unknown or unmonitored assets.

Use external attack surface management tools to:

• Automatically find all internet-facing assets, including unmanaged or abandoned resources such as shadow IT and legacy apps.
• Seamlessly connect external attack surface data like configuration management databases (CMDBs), vulnerability management platforms and exposure assessment platforms (EAP).
• Assign vulnerability risk levels based on criticality to focus on your most pressing external threats.
• Detect potential entry points that increase cyber risk, such as open ports, misconfigurations and outdated software attackers could exploit.
• Continuously maintain real-time visibility into attack surface changes, including newly discovered vulnerabilities or exposures.
• Create easy-to-understand reports with tailored mitigation recommendations.

• Identify all public-facing assets like forgotten domains, cloud instances and third-party services, to reduce shadow IT risks.
• Continuously scan for misconfigurations, open ports, exposed databases and weak security settings before attackers exploit them.
• Get a real-time view of your external attack surface to find emerging threats like phishing domains, impersonation attempts and leaked credentials.
• Find and eliminate unnecessary or abandoned assets to shrink your external footprint to make it harder for attackers to find entry points.
• Secure cloud workloads, environments, APIs and storage buckets.
• Support compliance with frameworks like NIST, ISO 27001 and PCI DSS. 
• Get insights to help you prioritize remediation efforts by simulating how attackers discover and target your assets.

To reduce your external attack surface, there are a couple of strategies to implement to help shrink your overall digital footprint.

• Map and inventory your digital assets to understand the full scope of your external attack surface. This includes identifying all internet-facing systems, applications and services. A hardware and software inventory is a foundational principle for frameworks such as NIST or CIS Controls.
• Reduce complexity in your IT environment by removing unnecessary applications, devices and features.
• Regularly scan for vulnerabilities and address misconfigurations promptly. This includes performing security configuration assessments and quantitative risk scoring.

While EASM and cyber asset attack surface management (CAASM) focus on visibility and risk reduction, their scopes differ.

EASM scope

• Focuses on assets visible to external attackers.
• Detects risks like shadow IT, vulnerable web apps and third-party integrations.
• Supported by threat intelligence, helps prioritize external threats and vulnerabilities for remediation.

CAASM scope

• Provides internal visibility across IT, IoT, OT and cloud environments.
• Maps relationships between assets, configurations and identities for comprehensive risk assessments.
• Helps manage vulnerabilities and security controls within your internal infrastructure.

Combining EASM and CAASM gives you a complete picture of both external and internal cyber exposures.

Within the continuous threat exposure management (CTEM) framework, EASM is pivotal for scoping and discovery phases.

Scoping defines your external attack surface boundaries by identifying all assets attackers target. EASM supports this by continuously mapping known and unknown internet-facing assets so you can define and refine your scope based on evolving risks.

Discovery identifies and catalogs external assets, including shadow IT and third-party exposures. EASM enhances discovery with automated reconnaissance, data correlation and threat intelligence to uncover hidden, forgotten or misconfigured assets threat actors could exploit.

Here are some tips to help implement EASM strategies as part of your comprehensive risk management program:

1. Identify your most business-critical external-facing assets to protect.
2. Evaluate EASM tools based on discovery accuracy, monitoring capabilities and how easily they integrate with your existing systems.
3. Ensure the EASM platform seamlessly integrates with your vulnerability assessment and vulnerability management solutions, asset inventory tools and broader security frameworks.
4. Stay ahead of threats by regularly reassessing and adapting to changes in your external attack surface and the evolving threat landscape.
5. Establish workflows between IT, security and compliance teams to streamline risk mitigation.

Implementing EASM as part of a comprehensive exposure management strategy can minimize your attack surface risks and support business resilience.

Interested in learning more about how EASM fits into your cybersecurity framework? Consider partnering with a top external attack surface management vendor like Tenable. Tenable attack surface management tools integrate seamlessly with broader ASM practices and CTEM programs.