Fail open describes a system that defaults to allowing access or continuing operations when it encounters a failure. Instead of shutting down or blocking everything, a fail-open system behaves as if it weren’t there at all. The concept applies across physical security, networking, and software development, and the core idea is always the same: when something breaks, the door stays unlocked rather than locked.
How Fail Open Works
Every system that controls access needs a rule for what happens when the system itself stops working. Fail open is one answer to that question: keep things flowing. A fail-open firewall that crashes will let internet traffic pass through to the internal network without inspection. A fail-open electronic lock will unlock when it loses power. A fail-open authentication check in software will grant access if the authorization code throws an unexpected error.
The logic behind choosing fail open is straightforward. Some situations demand that access or availability never stops, even if the system protecting it goes down. Availability is prioritized over verification.
Physical Security: Doors and Locks
The most intuitive example is a building door with an electronic lock. Electromagnetic locks are typically fail-open (often called “fail safe” in the physical security industry). When power is cut, the magnet releases and the door unlocks. This matters most during emergencies. If a fire knocks out the building’s electrical system, every exit needs to open so people can get out. NFPA 101, the Life Safety Code, addresses this directly in its requirements for means of egress.
These locks usually include a release mechanism, like panic hardware, a push-button actuator, or a trigger tied to the fire alarm and sprinkler system. The key principle: people must always be able to leave a building, even when electronics fail. Fail-secure locks, which stay locked during a power outage, are generally reserved for doors where quick entrance during an emergency isn’t needed.
Firewalls and Network Security
In networking, fail open means a security appliance like a firewall or intrusion prevention system lets traffic pass uninspected when it crashes or loses power. Early network administrators sometimes preferred this behavior because a failed firewall that blocked all traffic would take down the entire network. For organizations where uptime is critical, a few minutes of uninspected traffic felt like a better outcome than a complete outage.
The risk is obvious. During those minutes (or longer), traffic flows into the internal network without any authentication or filtering. An attacker who can trigger or detect that failure state gets a window of completely unguarded access. This is why many security teams now favor fail-closed configurations for firewalls, paired with redundancy and failover systems that switch traffic to a backup appliance instead.
Fail Open in Software
In software development, fail-open bugs are a common and dangerous security flaw. They typically happen when an authorization check doesn’t account for every possible error condition. If the code that verifies whether a user is allowed to perform an action encounters an unexpected exception and doesn’t explicitly block the request, execution continues and the user gets through. The system has effectively failed open by accident.
This is particularly risky because it can be invisible. The application keeps running, users keep getting responses, and nothing appears broken in monitoring. But unauthorized users may be performing actions they shouldn’t have access to. Well-written authorization code treats any unexpected result as a denial, which is the fail-closed approach.
Fail Open vs. Fail Closed
The two approaches represent opposite answers to the same question: what matters more when the system breaks?
- Fail open prioritizes availability. Users, traffic, or people can still get through. The cost is that protections disappear during the failure.
- Fail closed prioritizes security. Nothing gets through until the system is restored. The cost is a complete outage for anyone depending on that access.
Neither is universally correct. The right choice depends on what breaks your operation faster: downtime or unprotected access. A hospital exit door should fail open because a locked exit during a fire can kill people. A vault door should fail closed because the contents are more valuable than immediate access. A payment processing gateway might fail closed to prevent unauthorized transactions, while a content delivery network might fail open to keep websites loading.
The Availability vs. Security Tradeoff
Choosing between fail open and fail closed is fundamentally a risk management decision. Organizations need to ask which scenario causes more damage: losing service entirely, or operating without protections for a period of time.
Fail-open systems recover faster. When the issue is resolved, service returns to normal with minimal disruption, often in minutes. Fail-closed systems can take much longer to restore, especially in networked environments where cached data, propagation delays, and dependent systems all need to catch up. Recovery times of hours or even days aren’t unusual for complex fail-closed configurations.
Some organizations use a hybrid approach: allow fail-open behavior for a limited window (say, 48 hours) and then force a hard shutdown if the underlying problem hasn’t been fixed. This caps the exposure while still providing short-term availability. The goal is to avoid a situation where a fail-open system quietly runs unprotected indefinitely because nobody noticed the failure.
In practice, the strongest setups avoid the binary choice altogether by using redundancy. Rather than letting a firewall fail open or fail closed, traffic automatically reroutes to a backup appliance. Rather than unlocking a door permanently, a backup power supply keeps the lock functional. The fail-open vs. fail-closed question matters most when redundancy isn’t available or itself fails.