SQL Injection Attacks and Defense - 2009

176 Chapter 4 • Exploiting SQL Injection
Are You Owned?
Using Hash Functions to
Store Passwords in Your Database
The scenario that was just illustrated, in which a few queries have been enough to
retrieve a list of usernames and passwords unencrypted (in clear text), is not as unusual
as you might think. During our penetration tests and security assessments, we (the
book’s authors) have come across plenty of cases in which vulnerable applications had
passwords and other sensitive data stored in clear text.
The danger of storing users’ passwords in clear text poses other dangers:
Because human beings have the tendency to reuse the same password for several different
online services, a successful attack such as the one described might pose a threat not only
to the users’ accounts on victim.com, but also to other areas of their online identity, such
as online banking and private e-mail. And victim.com might even be liable for these
additional break-ins, depending on the specific laws of the country where it resides!
Therefore, if you are responsible for a Web application or a database that handles
the credentials of your users, make sure that such credentials are always stored
using a cryptographic hash function. A cryptographic hash function transforms an
arbitrary value (in our case, the user’s password) into a fixed-length string (called the
hash value). This function has several mathematical properties, but here we are mostly
interested in two of them:
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Given a hash value, it is extremely difficult to construct a value that
generates it.
The probability that two different values generate the same hash value is
extremely low.
Storing the hash value of the password instead of the password itself still allows
users to authenticate, because it’s enough to calculate the hash value of the password
they provide and compare it with the stored hash value. However, it provides a security
advantage, because if the list of hash values is captured, the attacker would not be able
to convert them back to the original passwords without going through a brute-force
attack.
When choosing the right hashing algorithm, do not rely on MD5, as several weaknesses
have been found over the years. SHA1 provides a far higher level of security
against attacks, and its more recent variants (such as SHA256 and SHA512) even more
so, thanks to a longer hash value length. Using such an algorithm will not protect you
against SQL injection attacks (fear not—we wrote Chapter 8 and Chapter 9 for that),
but will greatly protect your customers in case the data falls into the wrong hands.