CWE-760: Use of a One-Way Hash with a Predictable Salt
Use of a One-Way Hash with a Predictable Salt
Weakness ID: 760 (Weakness Base)
Status: Incomplete
Description
Description Summary
The software uses a one-way cryptographic hash against an input that should not be reversible, such as a password, but the software uses a predictable salt as part of the input.
Extended Description
This makes it easier for attackers to pre-compute the hash value using dictionary attack techniques such as rainbow tables, effectively disabling the protection that an unpredictable salt would provide.
It should be noted that, despite common perceptions, the use of a good salt with a hash does not sufficiently increase the effort for an attacker who is targeting an individual password, or who has a large amount of computing resources available, such as with cloud-based services or specialized, inexpensive hardware. Offline password cracking can still be effective if the hash function is not expensive to compute; many cryptographic functions are designed to be efficient and can be vulnerable to attacks using massive computing resources, even if the hash is cryptographically strong. The use of a salt only slightly increases the computing requirements for an attacker compared to other strategies such as adaptive hash functions. See CWE-916 for more details.
chain: product generates predictable MD5 hashes
using a constant value combined with username, allowing authentication
bypass.
Potential Mitigations
Phase: Architecture and Design
Use an adaptive hash function that can be configured to change the
amount of computational effort needed to compute the hash, such as the
number of iterations ("stretching") or the amount of memory required.
Some hash functions perform salting automatically. These functions can
significantly increase the overhead for a brute force attack, far more
than standards such as MD5, which are intentionally designed to be fast.
For example, rainbow table attacks can become infeasible due to the high
computing overhead. Finally, since computing power gets faster and
cheaper over time, the technique can be reconfigured to increase the
workload without forcing an entire replacement of the algorithm in use.
Some hash functions that have one or more of these desired properties include bcrypt [R.760.1], scrypt [R.760.2], and PBKDF2 [R.760.3]. While there is active debate about which of these is the most effective, they are all stronger than using salts with hash functions with very little computing overhead.
Note that using these functions can have an impact on performance, so
they require special consideration to avoid denial-of-service attacks.
However, their configurability provides finer control over how much CPU
and memory is used, so it could be adjusted to suit the environment's
needs.
Effectiveness: High
Phase: Implementation
If a technique that requires extra computational effort can not be
implemented, then for each password that is processed, generate a new
random salt using a strong random number generator with unpredictable
seeds. Add the salt to the plaintext password before hashing it. When
storing the hash, also store the salt. Do not use the same salt for
every password.
Effectiveness: Limited
Be aware that salts will not reduce the workload of a targeted attack
against an individual hash (such as the password for a critical person),
and in general they are less effective than other hashing techniques
such as increasing the computation time or memory overhead. Without a
built-in workload, modern attacks can compute large numbers of hashes,
or even exhaust the entire space of all possible passwords, within a
very short amount of time, using massively-parallel computing and GPU,
ASIC, or FPGA hardware.
Background Details
In cryptography, salt refers to some random addition of data to an input
before hashing to make dictionary attacks more difficult.
Thomas Ptacek. "Enough With The Rainbow Tables: What You Need To Know About
Secure Password Schemes". 2007-09-10. <http://www.securityfocus.com/blogs/262>.
[REF-11] M. Howard and
D. LeBlanc. "Writing Secure Code". Chapter 9, "Creating a Salted Hash" Page
302. 2nd Edition. Microsoft. 2002.
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 2, "Salt Values", Page 46.. 1st Edition. Addison Wesley. 2006.
Description
