What is the main problem with relying solely on firewalls to protect your company's sensitive data?

An IDS complements a firewall but is not required for a firewall to provide meaningful value; firewalls independently block unauthorized network traffic regardless of whether an IDS is present.

Firewall rules and policies can be updated, modified, and reconfigured at any time by administrators, so immutability after initial configuration is not a valid limitation.

Automatic OS patching is a separate security hardening measure that addresses software vulnerabilities and has no bearing on whether a firewall can effectively filter network traffic.

Firewalls only inspect and filter traffic that passes through them, leaving them ineffective against insider threats, social engineering, physical access, VPN tunneling, encrypted malicious payloads, and misconfigured rules. This multi-vector bypass problem is the core limitation of relying solely on firewalls, which is why security frameworks recommend a defense-in-depth strategy with multiple overlapping controls.