Security Design Principles - Sensitive Data

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- J.D. Meier, Alex Mackman, Michael Dunner, Srinath Vasireddy, Ray Escamilla and Anandha Murukan


Contents

Do Not Store Secrets if You Can Avoid It.

Storing secrets in software in a completely secure fashion is not possible. An administrator, who has physical access to the server, can access the data. For example, it is not necessary to store a secret when all you need to do is verify whether a user knows the secret. In this case, you can store a hash value that represents the secret and compute the hash using the user-supplied value to verify whether the user knows the secret.


Do Not Store Secrets in Code

Do not hard code secrets in code. Even if the source code is not exposed on the Web server, it is possible to extract string constants from compiled executable files. A configuration vulnerability may allow an attacker to retrieve the executable.


Do Not Store Database Connections, Passwords, or Keys in Plaintext

Avoid storing secrets such as database connection strings, passwords, and keys in plaintext. Use encryption and store encrypted strings.


Avoid Storing Secrets in the LSA

Avoid the LSA because your application requires administration privileges to access it. This violates the core security principle of running with least privilege. Also, the LSA can store secrets in only a restricted number of slots. A better approach is to use DPAPI, available on Microsoft Windows® 2000 and later operating systems.


Use DPAPI for Encrypting Secrets

To store secrets such as database connection strings or service account credentials, use DPAPI. The main advantage to using DPAPI is that the platform system manages the encryption/decryption key and it is not an issue for the application. The key is either tied to a Windows user account or to a specific computer, depending on flags passed to the DPAPI functions.

DPAPI is best suited for encrypting information that can be manually recreated when the master keys are lost, for example, because a damaged server requires an operating system re-install. Data that cannot be recovered because you do not know the plaintext value, for example, customer credit card details, require an alternate approach that uses traditional symmetric key-based cryptography such as the use of triple-DES.


Retrieve Sensitive Data on Demand

The preferred approach is to retrieve sensitive data on demand when it is needed instead of persisting or caching it in memory. For example, retrieve the encrypted secret when it is needed, decrypt it, use it, and then clear the memory (variable) used to hold the plaintext secret. If performance becomes an issue, consider the following options:

  • Cache the encrypted secret.
  • Cache the plaintext secret.

Cache the Encrypted Secret

Retrieve the secret when the application loads and then cache the encrypted secret in memory, decrypting it when the application uses it. Clear the plaintext copy when it is no longer needed. This approach avoids accessing the data store on a per request basis.

Cache the Plaintext Secret

Avoid the overhead of decrypting the secret multiple times and store a plaintext copy of the secret in memory. This is the least secure approach but offers the optimum performance. Benchmark the other approaches before guessing that the additional performance gain is worth the added security risk.


Encrypt the Data or Secure the Communication Channel.

If you are sending sensitive data over the network to the client, encrypt the data or secure the channel. A common practice is to use SSL between the client and Web server. Between servers, an increasingly common approach is to use IPSec. For securing sensitive data that flows through several intermediaries, for example, Web service Simple Object Access Protocol (SOAP) messages, use message level encryption.


Do Not Store Sensitive Data in Persistent Cookies

Avoid storing sensitive data in persistent cookies. If you store plaintext data, the end user is able to see and modify the data. If you encrypt the data, key management can be a problem. For example, if the key used to encrypt the data in the cookie has expired and been recycled, the new key cannot decrypt the persistent cookie passed by the browser from the client.


Do Not Pass Sensitive Data Using the HTTP-GET Protocol

You should avoid storing sensitive data using the HTTP-GET protocol because the protocol uses query strings to pass data. Sensitive data cannot be secured using query strings and query strings are often logged by the server.

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