Source Port Manipulation Scanner

Many network devices, firewalls, and legacy systems implement rules that permit or prioritize traffic based on the source port of the request. For instance, DNS requests typically come from port 53, HTTP from port 80, or SMTP from port 25. Misconfigured systems may allow traffic to bypass filtering or rate-limiting mechanisms simply because it originates from a "trusted" source port. This behavior can be exploited to gather unauthorized information or access protected services. These heuristics are often overlooked in modern security audits, despite their impact on firewall and IDS evasion techniques.

This scanner checks whether specific ports on a target host respond differently when scanned using a manipulated source port — for example, sending a SYN packet from port 53 or 443. It uses the Nmap engine to conduct a stealth TCP SYN scan across a set of commonly abused ports while enforcing a spoofed source port. The scan evaluates whether any unusual responses or open ports are detected due to this manipulation. Such behavior may indicate weak firewall rule logic, port-based trust assumptions, or legacy configuration weaknesses.

The scanner targets a predefined set of TCP ports commonly associated with essential services: 80, 443, 21, 22, 25, 3389, 445, 110, 139, 3306, and more. By sending requests from a fixed source port (e.g., 53), it identifies whether the destination system treats the probe differently than a standard scan. If the service responds positively — i.e., returns a SYN-ACK — it is considered suspicious and flagged. These findings help uncover potentially exploitable trust assumptions in perimeter defenses. The scanner uses Nmap with strict timeouts and retries to ensure efficient scanning.

Improper handling of source-port-based trust can lead to information disclosure, unauthorized access, or traffic prioritization vulnerabilities. Attackers may use this method to bypass rate-limiting, escalate privileges, or fingerprint internal services by impersonating known protocols. Systems with relaxed filtering policies based on expected port behavior are especially vulnerable. Detecting these misconfigurations early allows defenders to apply proper firewall hardening and enforce protocol-independent access controls. Eliminating source port assumptions is a critical step toward robust network segmentation and security posture.

REFERENCES

• https://nmap.org/book/man-bypass-firewalls-ids.html