External Network Penetration Testing Methodology

Review Cobalt pentest methodologies for external networks (includes instances of Microsoft Office 365).

External network penetration testing is a process in which a tester uses simulated attacks to identify potential security vulnerabilities in an external network.

We follow an industry-standard methodology primarily based on the Open Source Security Testing Methodology Manual (OSSTMM).

External network penetration testing methodology process

Penetration testing of an external network includes the following stages:

When testing Microsoft Office 365 instances, pentesters look into data security and encryption and verify access controls, in addition to testing the network that is hosting the services that are in scope.

The Cobalt security assessment team carries out testing without the following, unless it’s required as part of the pentest scope:

  • Detailed network or infrastructure diagrams
  • Any accounts or additional user information

However, you’re welcome to add network diagrams and other details when describing your asset.

Target Scope Reconnaissance

Cobalt pentesters search for all information that a malicious user might find. For example, to connect to the internet, you typically have shared some information:

  • To receive email, you need to post a mail server address.
  • To set up a web server, you need to post its URL and more.

During the initial phase of testing, pentesters determine what information is publicly available. They examine the following:

  • Your corporate website. Cobalt pentesters evaluate your website in ways that could interest a potential attacker, including:
    • Locations (URLs)
    • Contact details, such as phone numbers, emails, or physical addresses
    • Domain information
    • Links to other servers within an organization
    • Other companies with links to your website
    • Information on the security policy of your organization
  • Other web locations and databases. Cobalt pentesters search for information on your asset from other websites and databases, especially anything related to publicly traded companies. Pentesters then evaluate what information the organization makes public, in particular anything that goes beyond what’s required by local laws. They also evaluate news articles and press releases for more clues about your security policy.
  • Your domain names. Cobalt pentesters use “whois” databases to identify the domains that you own. These domains provide information about your network infrastructure.
  • Public records. Public records about your organization may include information about the people responsible for administering your domain, such as their name, address, or phone number. Attackers may use social engineering to get additional information, such as the details of hardware and software purchases. It also gives clues about where the best place to target an attack may be.


During this testing phase, pentesters use multiple tools, such as:

  • Nmap
  • DirBuster
  • Shodan
  • Censys

With these tools, pentesters can find:

  • IP addresses
  • Networks
  • Open ports
  • Webcams
  • Printers
  • Other devices connected to the internet

With this information, our pentesters can identify potential weaknesses, such as:

  • Accidental leaks of sensitive information
  • Open network ports
  • Unsecured network devices
  • Unpatched software
  • Leaked or exposed assets on pastebins

Service Discovery

After gathering all available information, our pentesters probe the resources belonging to the targeted organization. These tests involve several stages:

As certain vulnerabilities and exploits could paralyze, damage, or alter the content of the network, our pentesters do not perform these attacks. They do make note of the possible risks. For example, our pentesters won’t run exploits that:

  • Disable certain services
  • Deny service from outside systems
  • May affect customers (such as with a Denial of Service (DoS) attack)
  • Disable the ability of an organization to function

Port Scans

Pentesters perform a complete port scan on the IP address ranges for your asset. From this information, pentesters can identify public-facing machines and resources, along with their functionality.

For example, the following services require access to the outside world to function:

  • Firewalls
  • Mail servers
  • Office 365 servers
  • Web servers
  • FTP servers

All of these services leave characteristic signatures that a port scan can detect.

Further Investigation

Based on the results of the initial port scan, our pentesters work to identify:

  • The types of applications running on externally exposed machines
  • Version numbers for identified software
  • Operating systems on which the software runs

In some cases, an externally exposed machine may have open services that don’t have functions associated with them. Pentesters can identify and target them for testing.

Vulnerability Scans

Cobalt pentesters follow up by identifying vulnerabilities in the external-facing portion of the network. Their goal is to penetrate external endpoints and gain access to the internal LAN and the organization’s resources.

If a potential attacker achieves this goal, an organization could face:

  • Leaks of sensitive or confidential information from the organization’s network. The exfiltration of trade secrets or internal communication data could damage the affected organization. Such leaks could include:
    • Personnel records
    • Payment data
    • Other financial records
  • Attackers who use the mail gateway or website as the source of spam email. Other sites may blacklist the organization’s domain and automatically reject legitimate email correspondence.
  • Service disruptions, to the point when organization’s resources become unavailable, either temporarily or permanently.
  • Defacing of the website. Attackers may even substitute their own version of the website where current or potential customers sign in. The organization could lose credibility or even potential customers.

Manual Assessment

During manual assessment, Cobalt pentesters examine specific resources that they identified. In most cases, pentesters focus on visibly open services:

  • Web servers
  • FTP servers
  • Email servers
  • Firewalls
  • Routers
  • DNS servers
  • Other services that are in place on the external IP address range, including Office 365 services

While pentesters perform checks based on the specifics of a given situation, a common scenario involves examining the following:

Domain Name System (DNS)

For organizations to use the internet, their network users need the ability to query DNS servers. Some organizations have their own DNS server, and some rely on external DNS servers. If an organization’s internal DNS server fails, this could cause their internet connection to go down.

Attackers may also obtain internal knowledge from a DNS server, such as:

  • How the domain sends and receives email
  • Website locations

Let’s look at an example of a serious DNS configuration error that may occur. When an organization allows unknown internet users to perform a DNS zone transfer, an attacker may get access to valuable information about the network.


All connections to the internet typically go through a border router managed by the Internet Service Provider (ISP). However, sometimes routers remain unpatched for an extended period, or default user accounts remain active.

We locate all visible routers, establish the manufacturer and operating system (OS), then check for potential vulnerabilities. Our tests include:

  • Checking software, to make sure your routers are patched and up to date
  • Default user accounts such as admin
  • Attempts to access the router using various databases of well-known default passwords and settings


A firewall is designed to be the main gateway to an organization, with rules to protect internal resources. An attacker may get access to the firewall technology, so we don’t recommend treating it as an “out-of-the-box” solution. An organization should configure a firewall for the specific needs of their business, and keep it up to date through patching and maintenance.

Our pentesters look for configuration errors that could leave a path into the corporate LAN. Pentesters attempt to perform firewall attacks, such as:

  • Buffer overflows
  • IP spoofing
  • Corrupted IP packets
  • Attacks against open services

Web Servers

Web servers are vulnerable to defacement attacks, or could be used as a launching pad for further attacks against internal networks.

Cobalt pentesters scan all web servers (client side) for potential exploits and vulnerabilities, such as:

  • Poor patching policy
  • Default installation

Email Servers

Cobalt pentesters check SMTP, POP3, and IMAP on the mail gateway for open relay vulnerabilities. Your mail servers should:

  • Accept mail only for the organization’s domains
  • Not relay mail for other domains

Attackers could exploit an open relay to flood the mail server with spam, which could lead to the domain being blacklisted.

Pentesters examine the mail server using a variety of methods, such as sending emails to non-existent domains.

Remote Sites and Virtual Private Network (VPN)

Corporate network infrastructures may require connections to several other subsidiary offices around the globe over a VPN. While VPNs support secure communication, they may be vulnerable to the same configuration problems as firewalls, because firewalls handle the VPN.

An improperly configured VPN to a subsidiary site could be an attack vector to the main corporate network.

Verifying the Use of Secure Versions

As researchers discover vulnerabilities and security flaws in software, software vendors release patches for their products. Our pentesters search for outdated and unpatched versions of software. They run tests against published and patched exploits.

Older versions have lower security thresholds and leave data vulnerable. According to the SANS Institute, some of the most common vulnerabilities are based on outdated versions of Office 365. Cobalt can perform pentests for Office 365 instances.

Ensuring the Security of Legacy Protocols

Cobalt pentesters can test legacy protocols such as POP3, IMAP, and SMTP for known vulnerabilities. They verify if the use of these protocols is secured against documented security flaws.

Additional Testing

Cobalt pentesters use various custom and publicly available tools throughout a pentest, such as:

  • Port scanners
  • Automated vulnerability scanners
  • HTTP proxies
  • Exploits
  • Custom scripts
  • Security applications

Reporting, Triaging, and Retesting

Cobalt pentesters report and triage all vulnerabilities during the assessment. You can review details of all findings, in real time, through the Cobalt platform. In these findings, as well as in any report, our pentesters include detailed information on how you can:

  • Remediate each finding
  • Improve your overall security posture

You can remediate findings during and after the pentest. Then you can submit findings for retest. Our pentesters test the updated components and retest issues to ensure that there are no security-related residual risks.

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Last modified December.12.2023