Firewalls are a core part of network security, but penetration testing often shows that breaches succeed because of configuration gaps rather than missing tools. During penetration testing, these gaps are identified by reviewing firewall rules, testing exposed services, and validating how traffic is handled across different access paths. In this blog, we will discuss the firewall gaps commonly observed across on premise and cloud environments.
External and Internal Firewall Gaps
Some firewall gaps affect systems exposed to the internet, such as open ports or public management interfaces. Others affect internal security, such as weak segmentation and trusted internal traffic. Penetration testing evaluates both to understand how attackers can move from initial access to deeper systems.
A. External Firewall Gaps
External firewall gaps are weaknesses in rules or settings that expose systems directly to the internet or external networks. These gaps allow attackers to reach servers, services, VPNs, or firewall management interfaces from outside the network. They are often the first paths tested during penetration testing because they can provide initial access without requiring prior compromise.
1. Unnecessary Open Ports and Services
Penetration testing often finds open ports that are no longer required. These may belong to legacy applications, temporary access, or internal tools exposed by mistake. Each open port increases exposure and gives attackers additional ways to interact with systems.
2. Overly Permissive Firewall Rules
Firewall rules that allow wide access, such as any-to-any traffic or unrestricted outbound connections, are common findings. These rules make it easier for attackers to communicate with external systems once access is gained and reduce control over data movement.
3. Exposed Firewall Management Access
Management interfaces such as SSH or web consoles are sometimes reachable from the internet. Weak access limits or poor restriction settings allow attackers to target the firewall itself rather than the systems it protects.
4. Default Credentials and Weak Admin Settings
Penetration testing often exposes firewalls using default usernames, passwords, or basic admin settings. These login credentials are publicly documented and easy to test. If unchanged, they provide direct access to critical controls.
5. Missing Firewall Updates and Outdated Firmware
Firewalls that are not updated may run outdated firmware containing known software flaws. Firmware is the built-in software that controls how the firewall operates. When it is not kept up to date, attackers can exploit known weaknesses through exposed services or allowed access paths to bypass controls, gain access, or disrupt firewall operation.
6. Cloud Firewall Rules Allowing Public Access
In cloud environments, firewall rules are often reused across multiple systems. Broad rules can expose servers, storage, or services directly to the internet without clear visibility, increasing the attack surface.
7. Weak or Misconfigured VPN Access
VPN settings may allow split tunneling, weak authentication, or broad internal access. These issues allow attackers to use remote access paths to reach internal systems once credentials or devices are compromised.
8. Unnecessary Protocols Allowed
Protocols such as ICMP or unrestricted DNS are sometimes allowed without a clear requirement. These protocols can be used for system discovery or data transfer when not properly limited.
B. Internal Firewall Gaps
Internal firewall gaps exist within the network after access is gained. These weaknesses allow attackers to move between systems, access sensitive resources, or remain unnoticed. They usually involve poor segmentation, weak internal rules, outdated configurations, or limited monitoring, increasing the impact of an initial breach.
1. Poor Internal Network Segmentation
Internal networks are frequently treated as trusted zones. When internal firewall rules are weak or missing, attackers who gain access to one system can move to others with little resistance, increasing overall impact.
2. Old, Unused, and Shadowed Rules
Many firewalls contain rules added years ago that are no longer relevant. Some newer rules never take effect because earlier rules already allow or block the same traffic. These shadowed rules hide insecure access paths and make rule sets harder to manage.
3. Firewall Rules Not Aligned With Current Systems
As systems change, firewall rules are often left behind. Rules may reference old servers or applications that no longer exist, making the rule set complex and easier to misuse.
4. Limited Firewall Logging and Monitoring
Firewalls may log minimal activity or logs may not be reviewed regularly. Without visibility into allowed and blocked traffic, suspicious behavior can continue without detection, delaying response efforts.
5. Lack of Regular Firewall Review and Testing
Firewalls are often treated as set and forget controls. Without regular reviews or validation through testing, small issues remain in place and build up over time, increasing overall exposure.
How Firewall Gaps Combine During Attacks?
Individually, some firewall gaps may appear low risk. During penetration testing, these issues often combine, allowing attackers to gain access, move between systems, and maintain presence over time. Testing highlights how small configuration issues can work together.
How Penetration Testing Identifies Firewall Gaps?
During penetration testing, firewall weaknesses are identified through controlled testing of exposed ports, rule behavior, access paths, and traffic handling. Pentesters perform external and internal scans, review configurations, test remote access systems, and observe how both inbound and outbound traffic is filtered. This approach shows not only what is exposed, but how access can expand after entry.
What’s Next?
Finding firewall gaps is only the first step. The next phase is reducing exposure by fixing weak rules, removing unused access, updating firewall software, and reviewing management controls. Regular penetration testing and configuration reviews help confirm that changes are effective and that new gaps are not introduced as systems and network requirements change.
Peneto Labs has been empanelled by CERT-In to conduct information security auditing services. If you want to keep firewall controls aligned with current systems and reduce exposure, hire Peneto Labs for Configuration Reviews, Firewall Rule Audit, and high quality Penetration Testing.