CWE-324: Use of a Key Past its Expiration Date

What is Use of a Key Past its Expiration Date?

• Overview: The "Use of a Key Past its Expiration Date" vulnerability occurs when a cryptographic key or password is used beyond its intended validity period. This diminishes security because the longer a key is used, the more likely it is to be cracked or compromised.

• Exploitation Methods:

• Attackers can exploit this vulnerability by using extended timeframes to conduct brute force or other cracking attacks on outdated keys.
• Common attack patterns include increasing the computational effort over time to guess or derive the key, especially if it has been used for a prolonged period.

• Security Impact:

• Direct consequences include unauthorized access to sensitive data protected by the expired key.
• Potential cascading effects involve further system compromises if the key is used across multiple systems or services.
• Business impact can include loss of data integrity, legal liabilities, and damage to reputation due to unauthorized data access.

• Prevention Guidelines:

• Implement code-level checks to ensure that keys are not used past their expiration date by automatically invalidating them.
• Follow security best practices such as key rotation and regularly updating cryptographic keys.
• Utilize recommended tools and frameworks that support key expiration management and automated key rotation schedules to ensure timely updates.

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Technical Details

Likelihood of Exploit: Low

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import datetime
from cryptography.fernet import Fernet

class EncryptionService:
    def __init__(self):
        # Key generated on initialization, but never updated or checked for expiration
        self.key = self._generate_key()

    def _generate_key(self):
        return Fernet.generate_key()

    def encrypt(self, message):
        f = Fernet(self.key)
        return f.encrypt(message.encode())

    def decrypt(self, token):
        f = Fernet(self.key)
        return f.decrypt(token).decode()

# Using the service without checking if the key has expired
service = EncryptionService()
token = service.encrypt("Sensitive Data")
print(service.decrypt(token))

Explanation of Vulnerability

• The code generates a cryptographic key upon initialization and uses it indefinitely without checking if it has expired. This increases the risk of the key being compromised over time, as it is used for encryption and decryption without any expiration policy. Long-term use of the same key can lead to vulnerabilities if the key is compromised.

How to fix Use of a Key Past its Expiration Date?

To fix this vulnerability, you should implement a policy to regularly rotate cryptographic keys and check for their validity before use. Here are some best practices:

1. Key Rotation: Implement a mechanism to rotate keys periodically based on a predefined schedule or policy.
2. Key Expiration: Store the creation or expiration date of the keys and enforce usage only if the key is still valid.
3. Automated Checks: Before performing encryption or decryption, check if the key is expired, and update it if necessary.

Fixed Code Example

import datetime
from cryptography.fernet import Fernet

class EncryptionService:
    def __init__(self):
        self.key, self.key_expiration = self._generate_key()  # Initialize key and expiration date
    
    def _generate_key(self):
        key = Fernet.generate_key()
        expiration = datetime.datetime.now() + datetime.timedelta(days=30)  # Key expires in 30 days
        return key, expiration

    def _is_key_expired(self):
        return datetime.datetime.now() > self.key_expiration  # Check if the key is expired

    def _update_key(self):
        self.key, self.key_expiration = self._generate_key()  # Update key and expiration date

    def encrypt(self, message):
        if self._is_key_expired():  # Check key expiration before encryption
            self._update_key()
        f = Fernet(self.key)
        return f.encrypt(message.encode())

    def decrypt(self, token):
        if self._is_key_expired():  # Check key expiration before decryption
            self._update_key()
        f = Fernet(self.key)
        return f.decrypt(token).decode()

# Using the service with key expiration checks
service = EncryptionService()
token = service.encrypt("Sensitive Data")
print(service.decrypt(token))

Explanation of Fix

• Key Generation and Expiration: A key is generated along with an expiration date set to 30 days from creation. This ensures the key is not used indefinitely.
• Expiration Check: Before encryption or decryption, the service checks if the key is expired. If it is, a new key is generated and the expiration date is updated.
• Automated Key Management: By automating key expiration checks and updates, the service mitigates the risk associated with using an outdated key, enhancing security.

These changes ensure that the service does not use a cryptographic key past its expiration date, thereby reducing the risk of compromise due to prolonged key usage.