> For the complete documentation index, see [llms.txt](https://docs.fusion.vectra.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fusion.vectra.ai/detection-models/library/reconnaissance/synscan_external_internal.md).

# synscan\_external\_internal

**Explanation**

The synscan\_external\_internal NDM looks for SYN scanning, an indication that an attacker is attempting to map out a network by sending multiple SYN requests to various endpoints to determine open ports. The event is triggered when multiple sources from external IP addresses are initiating SYN requests against internal IP addresses in a short period of time.

**What to Look For**

To examine the results of the synscan\_external\_internal event, monitor for a large number of SYN requests from multiple sources (external IP addresses) targeting internal IP addresses in a short period of time. Analyze the network traffic to identify the source of the SYN requests and determine if they are legitimate or malicious. If malicious, take immediate action to block the attacker and remediate any vulnerabilities that were exploited.

**Related MITRE ATT\&CK Categories**

[Reconnaissance: Active Scanning, Technique T1595 - Enterprise](https://attack.mitre.org/techniques/T1595)


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