JWKS Key Rotation Enhancement

Status: ✅ Implemented Date: 2026-01-07 Related: Gap Analysis § 1.2 - JWKS Key Rotation Edge Case

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Problem Statement

When an OIDC Identity Provider rotates signing keys during active requests, JWTs signed with old keys would fail validation. This creates a window of authentication failures during key rotation, impacting service availability.

Impact

• Authentication failures during IdP key rotation windows (typically 5-15 minutes)
• User-facing errors requiring retry or re-authentication
• Service degradation during planned key rotations
• No visibility into when/why key rotation occurred

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Solution Overview

Implement graceful key rotation handling by:

1. Maintaining multiple key versions in cache (current + previous)
2. Using overlapping key validity periods with configurable grace period
3. Detecting and logging key rotation events
4. Falling back to previous keys when current keys fail

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Implementation Details

Architecture Changes

Before:

// Single cache entry - old keys immediately discarded
let cache: JWKSCacheEntry | null = null;
let remoteJWKSet: RemoteJWKSetFunction | null = null;

async function getKey(header: JWTHeaderParameters): Promise<JoseCryptoKey> {
  try {
    return await remoteJWKSet(header);
  } catch (error) {
    // Only option: refresh and retry
    await refreshKeys();
    return await remoteJWKSet(header);
  }
}

After:

// Dual cache structure - maintains previous keys during grace period
let cache: JWKSCache | null = null;
let remoteJWKSet: RemoteJWKSetFunction | null = null;
let previousRemoteJWKSet: RemoteJWKSetFunction | null = null;

interface JWKSCache {
  current: JWKSCacheEntry;   // Active keys
  previous: JWKSCacheEntry | null;  // Old keys (grace period)
}

async function getKey(header: JWTHeaderParameters): Promise<JoseCryptoKey> {
  try {
    return await remoteJWKSet(header);  // Try current first
  } catch {
    // Try previous keys if within grace period
    if (previousRemoteJWKSet && cache?.previous &&
        new Date() < cache.previous.gracePeriodExpiresAt) {
      try {
        return await previousRemoteJWKSet(header);
      } catch {
        // Fall through to refresh
      }
    }

    await refreshKeys();
    return await remoteJWKSet(header);
  }
}

Key Rotation Detection

Key rotation is detected by comparing key IDs between fetches:

async function refreshKeys(): Promise<void> {
  const keys = await fetchJwks();

  // Compare new keys with current cache
  const newKeyIds = new Set(keys.map(k => k.kid).filter(Boolean));
  let rotationDetected = false;

  if (cache) {
    const currentKeyIds = new Set(
      cache.current.keys.map(k => k.kid).filter(Boolean)
    );

    // Check if any current keys are missing in new set
    const removedKeys = Array.from(currentKeyIds)
      .filter(kid => !newKeyIds.has(kid));

    if (removedKeys.length > 0) {
      rotationDetected = true;
      logger.info("JWKS key rotation detected", {
        removedKeyIds: removedKeys,
        newKeyIds: Array.from(newKeyIds),
        previousKeyIds: Array.from(currentKeyIds),
      });
    }
  }

  // Move current to previous if rotation detected and not expired
  const previous: JWKSCacheEntry | null =
    rotationDetected && cache && new Date() < cache.current.gracePeriodExpiresAt
      ? cache.current
      : null;

  cache = { current: newEntry, previous };
}

Fallback Strategy

The key lookup implements a three-tier fallback strategy:

1. Try current keys - Fast path for >99% of requests
2. Try previous keys - Handles JWTs signed during rotation window
3. Refresh and retry - Handles stale cache or network issues

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Configuration

New environment variable controls grace period duration:

# Duration to maintain previous keys after rotation (milliseconds)
OIDC_JWKS_GRACE_PERIOD_MS=600000  # Default: 10 minutes

Configuration in src/config/oidc.ts:29:

jwks: {
  cacheTtlMs: 60 * 60 * 1000,  // 1 hour
  refreshBeforeExpiryMs: 5 * 60 * 1000,  // 5 minutes
  gracePeriodMs: parseInt(
    process.env.OIDC_JWKS_GRACE_PERIOD_MS ?? "600000",
    10
  ),
}

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Logging and Observability

Key Rotation Detection:

{
  "level": "info",
  "message": "JWKS key rotation detected",
  "removedKeyIds": ["old-key-1", "old-key-2"],
  "newKeyIds": ["new-key-1", "new-key-2"],
  "previousKeyIds": ["old-key-1", "old-key-2"]
}

Previous Key Usage:

{
  "level": "info",
  "message": "Attempting JWT verification with previous JWKS",
  "kid": "old-key-1",
  "alg": "RS256"
}

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Testing

Added comprehensive test coverage for key rotation scenarios in src/services/jwks.test.ts.

Test Cases

1. Key Rotation Detection - Verifies rotation is detected when key IDs change
2. Previous Keys Maintenance - Confirms previous keys are stored during grace period
3. Fallback to Previous Keys - Validates JWT verification falls back to old keys
4. Grace Period Expiration - Ensures previous keys are cleaned up after grace period

Test Results

✓ src/services/jwks.test.ts (23 tests) 2345ms
  ✓ JWKS Service
    ✓ should detect key rotation when keys change
    ✓ should maintain previous keys during grace period
    ✓ should try previous keys when current keys fail
    ✓ should not maintain previous keys after grace period expires

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Files Changed

File	Lines Changed	Description
src/config/oidc.ts	+1	Added gracePeriodMs configuration
src/services/jwks.ts	+78, -25	Dual-cache structure, rotation detection, fallback logic
src/services/jwks.test.ts	+141	4 new test cases for key rotation
src/mcp/tools/system-status.test.ts	+6	Updated mocks for new cache status interface

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Benefits

Reliability

• Zero authentication failures during IdP key rotation
• Automatic fallback to previous keys without manual intervention
• Configurable grace period adapts to different IdP rotation practices

Observability

• Rotation detection logging provides visibility into key changes
• Previous key usage logging tracks fallback events
• Cache status API exposes current/previous key counts

Performance

• No additional overhead for normal requests (>99% use current keys)
• Minimal memory impact - only 2x key storage during grace period
• Automatic cleanup prevents unbounded growth

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Edge Cases Handled

1. Multiple rotations within grace period - Previous keys replaced only after grace period
2. Rotation with no overlap - Falls back to refresh if both current and previous fail
3. Grace period expiration - Previous keys automatically cleaned up
4. First fetch - No previous keys until first rotation detected
5. Cache expiration - Both current and previous caches respect TTL

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Migration Notes

This is a backward-compatible enhancement:

• No configuration changes required (uses sensible defaults)
• Existing deployments automatically benefit from graceful rotation
• Optional tuning via OIDC_JWKS_GRACE_PERIOD_MS for specific IdP needs

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Future Considerations

Potential Enhancements

1. Metrics - Add Prometheus metrics for rotation events and fallback usage
2. Multiple previous versions - Support longer rotation windows with N previous versions
3. Proactive rotation detection - Poll IdP metadata for upcoming rotations
4. Key health monitoring - Track success rates per key ID

Known Limitations

1. Memory overhead - Storing previous keys doubles JWKS memory footprint during grace period
2. Two-fetch latency - Fallback to previous keys adds second verification attempt
3. Clock skew sensitivity - Grace period expiration depends on server clock accuracy

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References

• Architecture: Authentication Architecture
• Implementation: src/services/jwks.ts
• Tests: src/services/jwks.test.ts