Feature flag security best practices

You wouldn’t grant every engineer SSH access to your production database, nor would you allow unauthenticated API calls to modify your payment gateways. Yet, organizations often treat feature flags (which can toggle entire microservices, expose beta features, or act as kill switches) as simple configuration variables rather than critical infrastructure. Securing feature flags requires the same rigor you apply to your identity provider or CI/CD pipeline.

TL;DR

• Feature flags on the client side are inherently insecure/tamperable and must never replace server-side authorization checks.
• Treat your feature flag management system as a Tier-1 control plane with strict Role-Based Access Control (RBAC) and least privilege principles.
• Segregate tokens by environment and type; backend tokens effectively grant remote control over your application and must be rotated as secrets.
• Production changes typically require “four-eyes” approval workflows to prevent accidental outages or malicious state changes.
• Audit logs are not optional; you must be able to reconstruct the exact timeline of who changed a flag state and why.

Treat feature flags as a privileged control plane

The most common security operational failure regarding feature flags is underestimating their power. Feature toggles have evolved into complex control planes that manage releases, operations, and permissions. When you decouple deployment from release, the feature flag dashboard becomes your new release console.

Security and compliance standards like NIST SP 800-53 (specifically controls AC-6 and CM-3) govern least privilege and configuration change control. These standards apply directly to your flagging infrastructure. That’s why securing write access to your feature flagging system is critical. Depending on your setup, a single misconfigured toggle could be as impactful as a malicious code deployment.

You must implement strict user management and access controls. Grant permissions based on the principle of least privilege. A developer might need write access in testing environments but should only hold read access in production. Administrative functions, such as creating new projects or generating API tokens, belong to a restricted group of leads or release managers.

Never trust the client

One of the most persistent misunderstandings in feature flag security is the difference between visibility and authorization.

Client-side flags are evaluated in a browser or mobile app and exist solely for user experience (UX). They determine whether a button renders or a navigation item appears. They do not secure the underlying API endpoint or data resource. A knowledgeable user can inspect the network traffic, find the feature flag payload, and modify the JavaScript state to “enable” a hidden feature in their browser.

If you control a sensitive administrative feature with a flag, hiding the button in the UI is insufficient. You must enforce the check on the server.

The Open Web Application Security Project (OWASP) guidelines on transaction authorization state clearly that authorization must be enforced server-side. When you design a feature gated by a flag, ensure the backend validates the flag status before processing the request, regardless of what the frontend sends.

Secure your integration tokens

Feature flag systems rely on API tokens to sync configurations between the control server and your applications. These tokens are often the weak link in the security chain. If a backend token leaks, an attacker can potentially retrieve your entire configuration, including targeting rules for unreleased features or commercially sensitive rollout plans.

Practice rigorous API token hygiene:

• Separate client and server tokens: Do not use a server-side token in a client-side application. Server tokens often have broader permissions or access to PII-laden targeting rules that should never be exposed to the public internet.
• Scope tokens strictly: A token used by a billing microservice does not need access to the marketing website’s project. Scope tokens to specific projects and environments.
• Rotate credentials: Treat long-lived tokens as a risk. Establish a rotation policy where tokens are refreshed periodically, and immediately revoke tokens if a developer leaves or a repository is compromised.

OWASP’s Secrets Management Cheat Sheet advises centralizing the storage of secrets and logging all access. Injecting these tokens as environment variables via CI/CD pipelines is safer than hardcoding them into the application source.

Harden network boundaries

Even with valid tokens, you should limit where your flag system accepts requests from. If your feature flag service exposes a frontend API, it likely relies on Cross-Origin Resource Sharing (CORS) headers to tell browsers which domains are allowed to read the response.

A default configuration often sets Access-Control-Allow-Origin to * (wildcard), allowing any domain to request your feature flags. While acceptable for public information, this default setting is dangerous for enterprise applications. Restrict CORS settings to your specific domains and subdomains.

For backend execution, internalize the traffic where possible. Using a proxy or edge-evaluator can keep flag evaluation traffic within your private network or VPC, ensuring that sensitive evaluation data never traverses the public internet. Using a private proxy or edge-evaluator aligns with security and compliance use cases where maintaining strict data residency and network isolation is required.

Enforce change management in production

Flipping a flag in production is a release event. It carries the same risk as pushing a hotfix. Therefore, it requires the same governance.

To mitigate incidents caused by ad hoc changes to production flags, enforce a “four-eyes principle” for critical environments. Under this policy, no single individual can initiate and approve a change to a production flag without a secondary review.

Requiring a secondary review mirrors the pull request model developers already use for code. One engineer proposes a state change, such as ramping a rollout to 50 percent, and a second engineer or product manager reviews the impact and approves it. A review step prevents “fat-finger” errors and ensures that a rogue actor cannot silently disable security features.

Organizations using change requests reduce their blast radius significantly. Formal change governance creates a natural paper trail for compliance audits, proving that every production change went through authorized channels.

Maintain immutable audit logs

When an incident occurs, the first questions are always: “What changed?” and “Who changed it?“

Feature flag systems must produce an immutable audit log of every action. Every entry should include:

• The identity of the user or service account.
• The specific action taken (e.g., enabled flag, updated segment, deleted token).
• The timestamp.
• The “before” and “after” state of the configuration.
• The source IP address.

NIST Audit and Accountability controls (AU-2) emphasize the necessity of retrievable records for forensic analysis. An event log allows security teams to correlate application anomalies with configuration changes. If your application spikes in error rates at 2:00 PM, and the log shows a feature flag served to 100% of users at 1:59 PM, you have your root cause.

Ensure your team knows how to access and export these logs. During a security event, you may need to ingest event data into a SIEM (Security Information and Event Management) tool for deeper analysis.

Manage lifecycle and technical debt

Stale feature flags are a security liability. A flag left in the codebase for months or years creates a dormant code path that is rarely tested but remains executable.

If an attacker discovers a way to force an old flag to evaluate as “true,” they might reactivate deprecated logic that contains unpatched vulnerabilities. This risk is why modern incident remediation often focuses on flag governance. For instance, following a significant outage in June 2025, Google Cloud’s incident report listed a key remediation to “enforce all changes to critical binaries to be feature flag protected and disabled by default.” This shift demonstrates that flags are now viewed as essential safety barriers, not just developer conveniences.

Establish a lifecycle policy for flags:

• Short-lived flags: Release toggles and experiments should be archived and removed from code immediately after the rollout or experiment concludes.
• Long-lived flags: Operational kill switches and permission toggles should be documented and audited quarterly to ensure they are still necessary.

Proactively organize and archive feature flags to keep your attack surface small.

Handle user data with care

Feature flags often target specific users based on attributes like email, ID, or region. Consequently, your flag evaluation context contains Personally Identifiable Information (PII).

If your architecture sends this context to a third-party cloud provider for evaluation, you are transmitting user data outside your trust boundary. This can violate GDPR, HIPAA, or SOC 2 data residency requirements.

Privacy-conscious organizations prefer local evaluation. In this model, the flag rules (logic) are downloaded to the application or a private proxy, and the user data (context) is evaluated locally in memory. The PII never leaves your server.

If you must log evaluation data for debugging, verify that PII is redacted or hashed. Your logging practices should help you debug deployment issues without creating a data leak.

Establishing a secure feature architecture

Security is not a feature you add later; it is an architectural requirement. As feature flags enable faster deployments and finer-grained control, they naturally become a target. Securing them requires a shift in perspective where you view flags not as temporary hacks, but as permanent production infrastructure. 

Unleash approaches this challenge by focusing on data privacy and enterprise governance. By offering self-hosted and Edge deployment options, Unleash ensures that sensitive user data never leaves your infrastructure during evaluation. Combined with strict RBAC, SSO integration, and built-in change request workflows, the platform provides the control plane security required by regulated industries like banking and healthcare. Securing your flags means securing your release process, and that starts with the right foundation.

FAQS about feature flag security

Can feature flags be used for managing permissions?

Yes, but with caveats. Feature flags can manage “permissioning toggles,” such as enabling ‘Op-level’ features for a specific set of beta users. However, the actual enforcement of access rights must happen on the server side using the application’s authorization logic, not just by hiding the UI element via a client-side flag.

How do I secure client-side feature flags?

You secure client-side flags by assuming they are public. Do not put sensitive data (like secret keys or future pricing data) in the payload of a feature flag sent to the browser. Additionally, enable strict CORS policies to ensure only your specific domains can query your flag service, preventing unauthorized sites from scraping your configuration.

What is the risk of stale feature flags?

Stale flags represent untested code paths and technical debt. If a flag is left in the code after a feature is fully launched, it creates a risk where a malicious actor or accidental configuration change could re-enable old, buggy, or vulnerable code features that the team believes were removed.

How can I prevent developers from changing flags in production?

Implement Role-Based Access Control (RBAC) and environment-specific permissions. Developers should have write access in development environments but only read access in production. Production changes should be routed through a change management workflow or approval process to ensure accountability.

Should I encrypt feature flag data?

Yes, data should be encrypted in transit using TLS/HTTPS to prevent interception. For highly sensitive systems, organizations often choose self-hosted feature flag platforms to keep the entire data persistence layer within their own encrypted private networks, ensuring no third party can access the configuration rules or user targeting data.