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CWE-759: Use of a One-Way Hash without a Salt

 
Use of a One-Way Hash without a Salt
Weakness ID: 759 (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 does not also use a 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.

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.

+ Common Consequences
ScopeEffect

Technical Impact: Bypass protection mechanism; Gain privileges / assume identity

If an attacker can gain access to the hashes, then the lack of a salt makes it easier to conduct brute force attacks using techniques such as rainbow tables.

+ Demonstrative Examples

Example 1

In both of these examples, a user is logged in if their given password matches a stored password:

(Bad Code)
Example Language:
unsigned char *check_passwd(char *plaintext) {
ctext = simple_digest("sha1",plaintext,strlen(plaintext), ... );
//Login if hash matches stored hash
if (equal(ctext, secret_password())) {
login_user();
}
}
(Bad Code)
Example Language: Java 
String plainText = new String(plainTextIn);
MessageDigest encer = MessageDigest.getInstance("SHA");
encer.update(plainTextIn);
byte[] digest = password.digest();
//Login if hash matches stored hash
if (equal(digest,secret_password())) {
login_user();
}

This code does not provide a salt to the hashing function, thus increasing the chances of an attacker being able to reverse the hash and discover the original password. Note this code also exhibits CWE-328 (Reversible One-Way Hash).

Example 2

In this example, a new user provides a new username and password to create an account. The program hashes the new user's password then stores it in a database.

(Bad Code)
Example Language: Python 
def storePassword(userName,Password):
hasher = hashlib.new('md5')
hasher.update(Password)
hashedPassword = hasher.digest()
# UpdateUserLogin returns True on success, False otherwise
return updateUserLogin(userName,hashedPassword)

While it is good to avoid storing a cleartext password, the program does not provide a salt to the hashing function, thus increasing the chances of an attacker being able to reverse the hash and discover the original password if the database is compromised.

Fixing this is as simple as providing a salt to the hashing function on initialization:

(Good Code)
Example Language: Python 
def storePassword(userName,Password):
hasher = hashlib.new('md5',b'SaltGoesHere')
hasher.update(Password)
hashedPassword = hasher.digest()
# UpdateUserLogin returns True on success, False otherwise
return updateUserLogin(userName,hashedPassword)

Note that regardless of the usage of a salt, the md5 hash is no longer considered secure, so this example still exhibits CWE-327.

+ Observed Examples
ReferenceDescription
Router does not use a salt with a hash, making it easier to crack passwords.
Router does not use a salt with a hash, making it easier to crack passwords.
+ 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 compared to intentionally-fast functions such as MD5. 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.759.1], scrypt [R.759.2], and PBKDF2 [R.759.3]. 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: Architecture and Design

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.

Phases: Implementation; Architecture and Design

When using industry-approved techniques, use them correctly. Don't cut corners by skipping resource-intensive steps (CWE-325). These steps are often essential for preventing common attacks.

+ Background Details

In cryptography, salt refers to some random addition of data to an input before hashing to make dictionary attacks more difficult.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory816OWASP Top Ten 2010 Category A7 - Insecure Cryptographic Storage
Weaknesses in OWASP Top Ten (2010) (primary)809
ChildOfCategoryCategory8662011 Top 25 - Porous Defenses
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory903SFP Cluster: Cryptography
Software Fault Pattern (SFP) Clusters (primary)888
ChildOfWeakness BaseWeakness Base916Use of Password Hash With Insufficient Computational Effort
Research Concepts (primary)1000
+ References
[R.759.1] [REF-45] Johnny Shelley. "bcrypt". <http://bcrypt.sourceforge.net/>.
[R.759.2] [REF-39] Colin Percival. "Tarsnap - The scrypt key derivation function and encryption utility". <http://www.tarsnap.com/scrypt.html>.
[R.759.3] [REF-40] B. Kaliski. "RFC2898 - PKCS #5: Password-Based Cryptography Specification Version 2.0". 5.2 PBKDF2. 2000. <http://tools.ietf.org/html/rfc2898>.
[REF-38] Coda Hale. "How To Safely Store A Password". 2010-01-31. <http://codahale.com/how-to-safely-store-a-password/>.
[REF-41] Brian Krebs. "How Companies Can Beef Up Password Security (interview with Thomas H. Ptacek)". 2012-06-11. <http://krebsonsecurity.com/2012/06/how-companies-can-beef-up-password-security/>.
[REF-42] Solar Designer. "Password security: past, present, future". 2012. <http://www.openwall.com/presentations/PHDays2012-Password-Security/>.
[REF-43] Troy Hunt. "Our password hashing has no clothes". 2012-06-26. <http://www.troyhunt.com/2012/06/our-password-hashing-has-no-clothes.html>.
[REF-44] Joshbw. "Should we really use bcrypt/scrypt?". 2012-06-08. <http://www.analyticalengine.net/2012/06/should-we-really-use-bcryptscrypt/>.
[REF-46] OWASP. "Password Storage Cheat Sheet". <https://www.owasp.org/index.php/Password_Storage_Cheat_Sheet>.
[REF-47] 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-48] Robert Graham. "The Importance of Being Canonical". 2009-02-02. <http://erratasec.blogspot.com/2009/02/importance-of-being-canonical.html>.
[REF-49] James McGlinn. "Password Hashing". <http://phpsec.org/articles/2005/password-hashing.html>.
[REF-50] Jeff Atwood. "Rainbow Hash Cracking". 2007-09-08. <http://www.codinghorror.com/blog/archives/000949.html>.
[REF-51] Jeff Atwood. "Speed Hashing". 2012-04-06. <http://www.codinghorror.com/blog/2012/04/speed-hashing.html>.
[REF-52] "Rainbow table". Wikipedia. 2009-03-03. <http://en.wikipedia.org/wiki/Rainbow_table>.
[R.759.6] [REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 9, "Creating a Salted Hash" Page 302. 2nd Edition. Microsoft. 2002.
[R.759.7] [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.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2009-03-03Internal CWE Team
Modifications
Modification DateModifierOrganizationSource
2009-10-29MITREInternal
updated Relationships
2010-02-16MITREInternal
updated References
2011-03-29MITREInternal
updated Observed_Examples
2011-06-01MITREInternal
updated Common_Consequences
2011-06-27MITREInternal
updated Common_Consequences, Demonstrative_Examples, Potential_Mitigations, Related_Attack_Patterns, Relationships
2011-09-13MITREInternal
updated Potential_Mitigations, References, Relationships
2012-05-11MITREInternal
updated References, Related_Attack_Patterns, Relationships
2012-10-30MITREInternal
updated Demonstrative_Examples, Potential_Mitigations, References
2013-02-21MITREInternal
updated Description, Potential_Mitigations, References, Relationships, Type
2014-02-18MITREInternal
updated Potential_Mitigations, References
Page Last Updated: June 23, 2014