Detection of SYN-Based Port Scanning Reconnaissance

This threat brief outlines the detection of SYN-based port scanning, a common reconnaissance technique used by adversaries to map network services and identify potential attack surfaces. By sending SYN packets to a high volume of unique destination ports and observing the responses, an actor can identify open ports and available services within a network segment. This behavior is often a precursor to targeted exploitation, as it helps the attacker gain unauthorized access or identify vulnerabilities in critical infrastructure. The detection logic focuses on internal-to-internal traffic, identifying hosts that establish connections with minimal packet exchange across numerous destination ports, which is indicative of automated port scanning tools or manual discovery efforts.

Impact

Successful reconnaissance allows an attacker to build an inventory of reachable services and vulnerable software versions within the internal network. If the attacker successfully identifies misconfigured or unpatched services, they may proceed to perform targeted exploitation, potentially leading to unauthorized data access, privilege escalation, or lateral movement. Continuous internal scanning, if left unchecked, increases the probability of an adversary successfully identifying a path to mission-critical systems.

Recommendation

Prioritize the investigation of internal reconnaissance activity to differentiate between authorized administrative tooling and potential adversary behavior.

• Monitor network traffic logs for high-cardinality connection attempts to destination ports from internal assets.
• Establish an allowlist for known-benign internal security scanners, load balancers, or IT management platforms that perform service availability checks.
• Implement rate limiting on internal SYN packet exchanges to slow down or block automated scanning tools.
• Audit firewall configurations to ensure that only authorized services are reachable across network segments, limiting the efficacy of internal discovery.