Flutter Security

Flutter Security Skill

Purpose

Provide threat-aware Flutter security recommendations that reduce exploitability risk while remaining operationally feasible. Security is never optional — it is a release gate.

Scope and authority

This skill has global critical override authority for:

• exploitable security risk in any layer
• sensitive data exposure pathways (tokens, PII, credentials)
• secrets and trust-boundary handling
• network transport security
• binary protection and release hardening

If security risk is Critical, this skill overrides convenience, performance, and UX preferences.

When to use

• Designing or reviewing authentication/token lifecycle
• Choosing storage solutions for sensitive data
• Implementing SSL pinning or certificate validation
• Planning obfuscation and release hardening
• Assessing security posture of architecture/build changes
• Root/jailbreak detection requirements
• Logging strategy review (PII leakage risk)

Decision engine workflow

1. Identify assets (tokens, PII, keys, payment data) and trust boundaries.
2. Classify attacker capabilities (physical device, network, app store repackaging).
3. Rank threats by exploitability and business impact.
4. Choose mitigations by risk reduction vs operational cost.
5. Define rollout controls, monitoring, and residual risk.
6. Align with release constraints and incident response readiness.

Branching decision tree

Branch A: risk classification

• Critical exploitability (credentials, PII, payment data exposed):

  • Block release if unresolved
  • Enforce immediate mitigation path
  • Escalate to flutter-build-release

• High but non-blocking (hardening improvements, defense-in-depth):

  • Prioritize near-term remediation
  • Add as release gate for next version

• Medium/Low (informational improvements):

  • Schedule hardening with explicit risk acceptance notes
  • Document residual risk

Branch B: storage security

Sensitive data (tokens, credentials, PII, biometric keys)
→ ALWAYS: flutter_secure_storage (Keychain on iOS, Keystore on Android)
→ NEVER: SharedPreferences, Hive unencrypted, plain sqlite

Semi-sensitive data (user preferences, non-critical settings)
→ OK: SharedPreferences or Hive CE

Highly regulated data (financial, health records)
→ ALWAYS: Drift + SQLCipher (encrypted SQLite)

Branch C: network security

• All apps: HTTPS enforced, no cleartext traffic

  • Android: android:usesCleartextTraffic="false" in manifest
  • iOS: App Transport Security (ATS) enabled

• Financial/health/enterprise apps: Certificate pinning required

  • Implement via custom SecurityContext in Dio
  • Pin leaf certificate + intermediate + rotate backup pins
  • Have pin rotation procedure documented before shipping

• Sensitive API endpoints: Request signing (HMAC) where possible

Branch D: release hardening (mandatory for all production builds)

# Required flags for every release build:
flutter build apk --release \
  --obfuscate \
  --split-debug-info=build/symbols/

flutter build ios --release \
  --obfuscate \
  --split-debug-info=build/symbols/

• Upload symbols to Crashlytics/Sentry for deobfuscated crash reports
• Never commit --split-debug-info output to source control
• API keys: CI secret injection only (GitHub Actions secrets, Fastlane env)
• Build flavors: no production keys in development builds

Quantitative gates

Label each pass | at-risk | fail:

Sensitive data handling rules

Authentication tokens

// ✅ Store in secure storage
const storage = FlutterSecureStorage();
await storage.write(key: 'access_token', value: token);

// ❌ NEVER store in SharedPreferences
final prefs = await SharedPreferences.getInstance();
prefs.setString('access_token', token); // PROHIBITED

Token lifecycle

• Access tokens: in-memory only (Riverpod state) + secure storage for persistence
• Refresh tokens: flutter_secure_storage only
• On logout: explicitly delete from secure storage (storage.deleteAll())
• Token expiry: handle transparently via Dio auth interceptor

Logging rules

// ✅ Safe — no sensitive data
logger.info('User authenticated', {'userId': user.id});

// ❌ PROHIBITED — token in logs
logger.debug('Token: $accessToken');

// ❌ PROHIBITED — PII in logs
logger.info('User email: ${user.email}');

Platform-specific security controls

Android

• android:debuggable="false" in release manifest (enforced by Flutter release build)
• android:allowBackup="false" to prevent ADB backup of sensitive data
• ProGuard/R8 rules for native library protection
• Play Integrity API for app attestation (high-security apps)

iOS

• App Transport Security: no NSAllowsArbitraryLoads exceptions
• Keychain Access Groups for secure token sharing between extensions
• Jailbreak detection for financial apps: flutter_jailbreak_detection

Tradeoff realism

• Interim controls are acceptable if expiry criteria are defined
• Partial hardening is acceptable near release windows when residual risk is transparent
• Do NOT frame risk acceptance as risk elimination
• Certificate pinning has operational cost (pin rotation) — document before implementing

Anti-pattern detection

• Tokens or PII in SharedPreferences → Critical
• API keys in source code or pubspec.yaml → Critical
• android:debuggable="true" in release build → Critical
• Cleartext HTTP traffic in production → Critical
• --obfuscate missing from release build → High
• Debug logging enabled in release builds → High
• Custom crypto without expert review → High
• print() statements with sensitive data → High
• UI-only security checks without server enforcement → Medium
• Biometric authentication bypassed via software → Critical

Uncertainty protocol

• High (≥ 0.80): threat surface clear, assets identified
• Medium (0.60–0.79): incomplete threat model
• Low (< 0.60): missing app context, platform targets, or data classification

If Medium/Low: request threat model completion before finalizing controls.

Cross-skill handoff payload

Use the standard payload from ../../AGENTS.md. Set requesting_skill to flutter-security.

Output contract

Follow global section order from ../../AGENTS.md. Also include:

• Risk summary
• Asset and threat surface map
• Mitigation plan by risk class
• Storage/network/binary controls
• Residual risks and acceptance criteria
• Implementation priorities

Related resources

• references/mobile-threat-model.md
• references/secure-storage-guide.md
• references/ssl-pinning-guide.md
• references/release-hardening-checklist.md
• templates/security-review.md