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My question is about security groups/firewalls and protecting a virtual private cloud from the external world. Here is a description of VPC default policy for inbound/outbound traffic (on AWS):

Each security group by default contains an outbound rule that allows access to any IP address. It’s important to note that when an instance sends traffic out, the security group will allow reply traffic to reach the instance, regardless of what inbound rules are configured.

I was wondering if there exists an attack vector where a malicious user tries to circumvent the VPC's inbound policy (i.e. block all traffic) by tricking it into thinking that the incoming traffic is a "reply" traffic? Does such attack have a name in the literature?

I can also think of a scenario where a target machine T (within a VPC) sends a request to some valid server V, but the malicious user M sends a malicious response to T (tricking it into believing that it comes from V) before T receives the actual response from V, thence circumventing T's inbound traffic policy.

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    That's why "stateful" firewalls exist. Before stateful firewalls, it would be possible, – schroeder May 23 at 10:17

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