All other ICMP traffic types are denied. For example, the outside host cannot initiate a ping request to the inside host. The outbound ACL is allowing ICMP messages that report various problems. This will allow ICMP tunnelling and data exfiltration.
NAT and PAT
Network Address Translation
Encryption, Encapsulation, and Tunneling
Peer-to-Peer Networking and Tor
Any time that unknown users are provided access to network resources, security is a concern. File-sharing P2P applications should not be allowed on corporate networks. P2P network activity can circumvent firewall protections and is a common vector for the spread of malware.
P2P is inherently dynamic. It can operate by connecting to numerous destination IP addresses, and it can also use dynamic port numbering. Shared files are often infected with malware, and threat actors can position their malware on P2P clients for distribution to other users.
Processor sharing P2P networks donate processor cycles to distributed computational tasks. Cancer research, searching for extraterrestrials, and scientific research use donated processor cycles to distribute computational tasks.
Instant messaging (IM) is also considered to be a P2P application. IM has legitimate value within organizations that have geographically distributed project teams. In this case, specialized IM applications are available, such as the Webex Teams platform, which is more secure than IM uses public servers.