What Is Load Balancer?

A load balancer distributes traffic across multiple servers for availability and scale. Its security relevance is incidental to that purpose and considerable: it is frequently the point where TLS terminates, the first device hostile traffic reaches, and the place where several security capabilities are most naturally applied.

TLS termination makes it a sensitive system

Where TLS terminates at the load balancer, that device holds the private keys and sees every request in plaintext. It becomes one of the most valuable targets in the architecture, and its administrative access deserves treatment accordingly.

It also creates a decision about traffic behind it. Re-encrypting to the backend protects internal traffic and costs performance; leaving it in plaintext is common and means anyone with access to that internal segment sees everything. In regulated environments the second option is frequently a finding.

Security capabilities it naturally hosts

Because all traffic passes through it, the load balancer is a practical place for rate limiting, connection limits, request filtering and health-based removal of failing backends. Many deployments run a web application firewall at this layer for the same reason.

It also hides backend topology. Clients see one endpoint rather than the servers behind it, which removes information an attacker would otherwise use.

The recurring misconfigurations

Management interfaces exposed to the internet, which has led to serious compromises of load balancing appliances. Outdated TLS versions and weak cipher suites, because the configuration was set years ago and never revisited. Health check endpoints that leak version or environment information. And preserved client address headers that backends trust without validation — allowing address-based controls to be bypassed by forging the header.

It is also a denial-of-service control point

Because everything passes through it, the load balancer is where connection limits, rate limiting and request throttling are most effectively applied. Protecting backends from being overwhelmed is part of its function rather than an addition to it.

Health checking contributes to the same outcome. Removing a failing backend automatically prevents a partial failure becoming a total one, provided the check tests something meaningful rather than merely confirming the port is open.

A health check that returns success whenever the web server responds will happily keep sending traffic to an instance whose database connection has failed, which is a common and frustrating source of intermittent outages.

Session handling is worth reviewing alongside the security configuration. Where the balancer maintains affinity between a client and a backend, the mechanism used — a cookie, an address, an inserted header — can leak infrastructure detail or be manipulated to direct a client to a chosen backend.

A common mistake worth avoiding

Where to start

Check whether your backend servers accept connections that did not come through the load balancer, and whether its management interface is reachable from anywhere but a management network.

See network security. GuardsArm runs security program reviews.