Exercise 3: Two Ways a Port Forward Becomes a Security Hole — Possible Solution
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EXAMPLE 1: A FORGOTTEN OPEN TUNNEL
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Per this chapter, "forwarding a sensitive internal service and then
forgetting the tunnel is still open" is a real risk. As long as the
tunnel remains active, the forwarded service stays reachable through
it - if the tunnel is left running long after the diagnostic work is
actually finished, it's an unnecessary, unmonitored access path to a
sensitive internal service that nobody is actively using or watching,
extending the window during which that access could be misused.

EXAMPLE 2: BINDING TO 0.0.0.0 INSTEAD OF LOCALHOST
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Per this chapter, "explicitly binding a forward to 0.0.0.0 instead of
the (usual, safe) localhost-only default - accidentally exposing the
forwarded port to your entire local network rather than just your own
machine." The default forwarded port is only reachable from the local
machine itself; overriding that default to bind more broadly makes the
forwarded service reachable by anyone else on the same local network,
not just the person who set up the tunnel - turning a personal,
single-machine tunnel into something effectively shared with everyone
nearby, likely without any of them being an authorized user of the
underlying service.

WHY BOTH EXAMPLES SHOW THE SAME UNDERLYING RISK
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In both cases, the tunnel exists specifically to grant careful,
intentional, temporary access to a normally-restricted service - and
in both cases, the mistake extends that access beyond its intended
scope (in time, for the forgotten tunnel; in reach, for the 0.0.0.0
bind) without anyone deciding that expansion was actually warranted.

WHY THIS WORKS AS AN ANSWER
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It explains both specific examples the chapter names, and explains the
concrete mechanism by which each one expands access beyond what was
actually intended, rather than treating either as a vague, unexplained
"risk."