The concept of a security perimeter used to be simple: protect the walls of your internal network, and you’re safe. But that perimeter no longer exists. The way companies work today, using cloud services, supporting remote employees, and relying heavily on outside vendors, has made the boundaries much harder to define and control.
As a result, the external attack surface has become the new frontline in cybersecurity. It includes every exposed asset an attacker can discover and exploit, whether it belongs to your organization or to a third-party vendor in your supply chain.
This shift in the cyber perimeter presents a pressing challenge: how can organizations secure what they don’t directly control—but are still responsible for?
What Is the External Attack Surface?
The external attack surface is everything an attacker can see, scan, and target from outside your network. Unlike internal systems, these assets are publicly exposed, often unintentionally, and they represent the first point of contact for threat actors.
From Firewalls to Footprints—How the Perimeter Has Changed
In the past, security teams could draw clear boundaries: on-premises servers, user devices, internal firewalls. But cloud migration, hybrid work, and the rise of SaaS platforms have made those boundaries blur.
Key changes include:
- Cloud infrastructure: Services like AWS, Azure, and GCP introduce dynamic and often misconfigured assets.
- Remote work: Home networks and personal devices increase the number of external access points.
- Third-party integrations: Vendors bring their own infrastructure risks into your ecosystem.
Today, what attackers see and can access extends far beyond your internal perimeter.
Components of the Modern External Attack Surface
Let’s break it down. Here are the core components making up a modern organization’s external footprint:
| Component | Description |
| Domains & Subdomains | Public-facing web assets, often forgotten or orphaned |
| Cloud Storage & Buckets | Misconfigured cloud environments (e.g., open S3 buckets) |
| Web Applications & APIs | Exposed interfaces that may contain vulnerabilities |
| IP Addresses & Ports | Open ports, unpatched servers, and unused services vulnerable to scanning |
| Certificates & DNS Records | Expired certs or outdated DNS entries that attackers can exploit |
| Vendor Infrastructure | Assets belonging to third parties but connected to your business operations |
| Shadow IT | Unapproved SaaS or devices created without IT oversight |
Each of these components expands your attack surface, especially when unmanaged, misconfigured, or simply forgotten.
In many cases, organizations don’t even realize these assets are visible to attackers. That’s what makes them so dangerous.
Why Traditional Internal Controls Aren’t Enough
Security tools have long focused on protecting what’s inside the network: endpoint detection, email filtering, firewalls, and access control. But attackers aren’t knocking on your internal doors first, they’re scanning what’s already visible from the outside.
When external assets are left out of your monitoring scope, you create blind spots that traditional internal controls can’t detect or defend.
The Blind Spots in Perimeter-Centric Security Models
Here’s why perimeter-focused security falls short in the modern threat landscape:
- External assets aren’t always inventoried – Many organizations don’t have a complete list of exposed domains, cloud services, or IPs.
- Vendor infrastructure lies outside your control – Even when third parties introduce risk, most companies can’t monitor their assets.
- Internal controls don’t detect surface-level vulnerabilities – Misconfigurations like open ports or expired certificates may never be flagged by internal tools.
- Threat intelligence is limited to known signatures – Most internal systems only detect known threats, not emerging external exposures.
In other words, you can’t protect what you don’t know exists.
How Attackers Exploit What You Don’t See
Threat actors use the same tools as penetration testers but with far worse intentions. Here’s how they capitalize on external exposures:
- Scanning for open ports and vulnerable services using automated tools like Shodan, Censys, and Masscan
- Targeting expired SSL certificates, subdomain takeovers, and forgotten DNS records
- Pivoting through third-party vendors to access downstream data or systems
- Harvesting credentials from breached external systems and using them in credential stuffing attack
How Sling Secures the External Attack Surface
Most organizations don’t fail because they’re unaware of cybersecurity risks; they fail because they lack visibility. Especially when it comes to assets they don’t own but rely on, like those managed by third parties.
Sling was purpose-built to address this gap. It continuously monitors what attackers can see, giving organizations an actionable view of both their own and their vendors’ exposed infrastructure, before it’s exploited.
1. Automatic Discovery of Vendor-Owned Assets
Sling maps both your infrastructure and your vendors’ public-facing assets like forgotten subdomains, open ports, and exposed cloud services, without needing access to their internal systems. This ensures nothing visible to attackers goes unnoticed.
2. Continuous External Monitoring
Sling scans continuously and identifies changes like new domains, reactivated IPs, or expired SSL certificates, as soon as they appear, helping you act before attackers do.
3. Cyber Threat Intelligence (CTI)
Sling enriches each risk with real-world threat context:
- Are attackers scanning this asset?
- Is it mentioned on the darknet?
- Has this vulnerability been exploited recently?
5. Prioritization Based on Attacker Behavior
Sling ranks issues based on how likely they are to be exploited, like open ports, leaked credentials, initial access, etc.
6. Risk Transparency Across Your Ecosystem
You gain visibility into not just your digital footprint but also your vendors’ external risks, letting you detect, investigate, and act without waiting for disclosures or updates from their side.
Actionable Next Steps for Cybersecurity Teams
Securing the external attack surface means adopting a proactive, continuous, and risk-based approach, especially when third-party assets are in play. Here’s how to get started:
1. Map Your External Footprint (Including Vendors)
Develop a comprehensive inventory of all exposed assets:
- Domains, subdomains, and cloud services
- IPs and open ports
- Vendor-owned infrastructure linked to your organization
Automated tools like Sling eliminate blind spots by discovering assets attackers can already see.
2. Monitor Continuously
External environments change fast. Establish continuous monitoring to detect:
- Newly exposed assets or reactivated infrastructure
- Certificate expirations, open ports, or misconfigurations
- Shifts in your vendors’ digital presence
This gives you an early warning system before attackers exploit new exposures.
3. Focus on What Attackers Are Most Likely to Target
Trying to fix every vulnerability isn’t realistic. What matters is knowing which exposures are most likely to be exploited, and acting on those first.
Sling helps by ranking external risks based on real-world attacker behavior, not just technical severity. It highlights things like:
- Services attackers are actively scanning (e.g., exposed RDP or SSH)
- Subdomains that can be hijacked or abused
- Vendor infrastructure connected to known credential leaks
- Assets that show up in threat actor activity or dark web listings
This gives your team a clear starting point and helps avoid wasting time on low-impact issues.
4. Engage Vendors with Clear Expectations
Treat third-party exposure as part of your own risk. Strengthen vendor management by:
- Setting contractual requirements for risk response
- Sharing exposure reports and requiring timely remediation
- Tracking performance over time
Sling helps provide the visibility needed to make this process actionable.
5. Use Threat Intelligence to Inform Action
Integrate threat intel into your security workflow to validate urgency. For example:
- Is the asset part of an active exploit campaign?
- Has it been indexed by scanners or listed on dark web markets?
Sling enriches findings with real-world context, so your team focuses where it matters.
Security teams are often focused on what’s happening inside the network, but most attacks today don’t start there. They begin with what’s exposed: a forgotten subdomain, an open port on a vendor’s server, or leaked credentials tied to an unused login page. These are small gaps, but they’re all it takes.
This isn’t just about more scanning. It’s about visibility into what your organization and your vendors are unknowingly exposing. If you’re not tracking your external attack surface in real time, you’re already behind. Sling helps you close that visibility gap before attackers take advantage of it.