AppSec Blog

Ask the Expert - Johannes Ullrich

Johannes Ullrich is the Chief Research Officer for the SANS Institute, where he is responsible for the SANS Internet Storm Center (ISC) and the GIAC Gold program. Prior to working for SANS, Johannes worked as a lead support engineer for a Web development company and as a research physicist. Johannes holds a PhD in Physics from SUNY Albany and is located in Jacksonville, Florida.

1. There have been so many reports of passwords being stolen lately. What is going on? Is the password system that everyone is using broken?

Passwords are broken. A password is supposed to be a secret you share with a site to authenticate yourself. In order for this to work, the secret may only be known to you and that particular site. This is no longer true if you use the same password with more than one site. Also, the password has to be hard to guess but easy to remember. It is virtually impossible to come up with numerous hard to guess but easy to remember passwords. As a result, users will reuse passwords, or use simple passwords that are easy to guess. Most users would have to remember several hundred hard to guess passwords to make the password system work.

2. What are the best practices today for managing and storing passwords for applications?

As recent breaches have shown, passwords may be leaked via various vulnerabilities. As a first step, you need to ensure that your applications are well written and as secure as possible. Code with access to passwords needs to be reviewed particularly well, and only limited database users should have access to the password data to avoid a SQL injection in one part of your code that doesn't need access to the password data from being escalated to affect password data.

Password data should not be stored in clear text, but encrypted or hashed. Most applications never need to know the clear text form of the password, making cryptographic one way hashes an ideal storage mechanism for passwords. But it is important to recognize that any encryption or hashing scheme is still vulnerable to offline brute force attacks once the data is leaked. After a password database is leaked, you can only delay the attack, but you can not prevent it.

A proper hash algorithm should delay the attacker as much as possible, but should not be too expensive to cause performance problems for the application. Any modern hash algorithm, like sha256 will work, but you may need to apply it multiple times in order to slow down the attacker sufficiently. Other algorithms like PBKDF2 and bcrypt are slower and may be preferred over the rather fast and efficient hash algorithms like sha. But it is important to use an algorithm that is well supported by your development environments. Applying a fast algorithm like sha256 multiple times will in the end be as efficient as using a more expensive algorithm once (or fewer times).

In addition, data should be added to the password before it is hashed. This data is frequently referred to as "salt". If possible, each password should use a different salt. The salt serves two purposes. First of all, multiple users tend to pick the same password. If these password will be hashed, the hashes will be identical. An attacker breaking this hash will be able to access multiple accounts. If a unique "salt" is added first, the password hashes will be different. In addition, by using a sufficiently large salt, brute forcing the password will be harder. One of the most efficient brute force methods is the use of "rainbow tables". Rainbow tables are pre-computed hash databases that are compressed to allow for very efficient storage and queries. Currently, these tables are widely available for strings up to 8 characters long. If one restricts the table to more common dictionary terms, larger tables are available as well. By adding a long unique salt, the string to be hashed will exceed the length commonly found in rainbow tables, and by picking a non-dictionary salt, and attacker would have to create specific rainbow tables for each salt value, making them impractical. The salt is stored in clear with the account information. Sometimes, it is just prepended to the hash. A random salt is ideal, but in many cases, a simple salt may be derived from information like the user id or the time stamp the account was created at.

As a basic summary describing secure password storage: Take the users password, prepend a salt unique to this account, and hash the result as many times as you can afford using a commonly available hashing algorithm.

Over time, you should expect CPUs to become more powerful, and as a result, the number of iterations required to protect your password will increase. Design your application in such a way as to make it easy to change the number of iterations.


3. How can people test or review their apps to make sure that they are handling passwords in a secure way? What should they look for?

Quick Answer: When reviewing an application, make sure that authentication is implemented only once and easily changed later to adopt to a more complex hashing scheme. Developers should be instructed to use these authentication libraries and refrain from writing code that accesses the password database directly.

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