TL;DR
Caching sensitive data can speed up your application but introduces security risks. This guide explains how to cache safely using short timeouts, encryption, and careful key management.
1. Understand the Risks
Caching stores copies of data. If that data is sensitive (passwords, personal details, financial information), a compromised cache can lead to serious breaches. Key risks include:
- Data Exposure: Unauthorized access to cached data.
- Replay Attacks: Using old cached data after it should be invalid.
- Cache Poisoning: Replacing legitimate cached data with malicious content.
2. Minimise Cache Lifetime (TTL)
The shorter the time a piece of sensitive data is stored in the cache, the smaller the window for attackers. Use short Time-To-Live (TTL) values.
- Consider your application: How often does the data change?
- Typical TTLs: For highly sensitive data, consider TTLs of seconds or minutes rather than hours or days.
# Example (Redis) - set expiry to 60 seconds
SET my_sensitive_data "some value" EX 60
3. Encryption at Rest and in Transit
Always encrypt sensitive data before storing it in the cache, even if you trust the caching server itself.
- Encryption Algorithms: Use strong encryption algorithms like AES-256.
- Key Management: Securely store and manage your encryption keys – never hardcode them! Consider using a key management service (KMS).
- HTTPS/TLS: Ensure all communication between your application and the caching server is encrypted with HTTPS/TLS.
# Example (Python) - encrypting data before storing
from cryptography.fernet import Fernet
key = b'YOUR_ENCRYPTION_KEY'
f = Fernet(key)
token = f.encrypt(b"my sensitive data")
4. Secure Cache Keys
Cache keys should not reveal sensitive information or be easily guessable.
- Hashing: Use a cryptographic hash function (e.g., SHA-256) to generate cache keys from user IDs or other identifiers.
- Salting: Add a random salt to the data before hashing to prevent rainbow table attacks.
# Example (JavaScript) - generating a secure key
import crypto from 'crypto';
const userId = 123;
const salt = crypto.randomBytes(16).toString('hex');
const dataToHash = userId + salt;
const hash = crypto.createHash('sha256').update(dataToHash).digest('hex');
const cacheKey = 'user_' + hash;
5. Access Control
Restrict access to the caching server and cached data.
- Firewalls: Only allow necessary connections to the caching server.
- Authentication/Authorization: Implement strong authentication and authorization mechanisms for accessing the cache.
6. Cache Invalidation
Ensure data is removed from the cache when it becomes invalid.
- Event-Driven Invalidation: When sensitive data changes, trigger an immediate cache invalidation event.
- Regular Checks: Implement background processes to periodically check for stale data and remove it.
# Example (Redis) - deleting a key
DEL my_sensitive_data
7. Monitoring and Auditing
Regularly monitor your caching server for suspicious activity.
- Access Logs: Review access logs to identify unauthorized attempts to access cached data.
- Performance Metrics: Monitor cache hit rates and latency to detect anomalies.