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CWE-760: Use of a One-Way Hash with a Predictable Salt

Weakness ID: 760
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ 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.

+ Common Consequences
Access Control

Technical Impact: Bypass protection mechanism

+ Observed Examples
Blogging software uses a hard-coded salt when calculating a password hash.
Database server uses the username for a salt when encrypting passwords, simplifying brute force attacks.
Server uses a constant salt when encrypting passwords, simplifying brute force attacks.
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 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.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.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base916Use of Password Hash With Insufficient Computational Effort
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory958SFP Secondary Cluster: Broken Cryptography
Software Fault Pattern (SFP) Clusters (primary)888
+ References
[R.760.1] [REF-45] Johnny Shelley. "bcrypt". <>.
[R.760.2] [REF-39] Colin Percival. "Tarsnap - The scrypt key derivation function and encryption utility". <>.
[R.760.3] [REF-40] B. Kaliski. "RFC2898 - PKCS #5: Password-Based Cryptography Specification Version 2.0". 5.2 PBKDF2. 2000. <>.
[REF-38] Coda Hale. "How To Safely Store A Password". 2010-01-31. <>.
[REF-41] Brian Krebs. "How Companies Can Beef Up Password Security (interview with Thomas H. Ptacek)". 2012-06-11. <>.
[REF-42] Solar Designer. "Password security: past, present, future". 2012. <>.
[REF-43] Troy Hunt. "Our password hashing has no clothes". 2012-06-26. <>.
[REF-44] Joshbw. "Should we really use bcrypt/scrypt?". 2012-06-08. <>.
[REF-46] OWASP. "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. <>.
[REF-48] Robert Graham. "The Importance of Being Canonical". 2009-02-02. <>.
[REF-49] James McGlinn. "Password Hashing". <>.
[REF-50] Jeff Atwood. "Rainbow Hash Cracking". 2007-09-08. <>.
[REF-51] Jeff Atwood. "Speed Hashing". 2012-04-06. <>.
[REF-52] "Rainbow table". Wikipedia. 2009-03-03. <>.
[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.
+ Content History
Submission DateSubmitterOrganizationSource
2009-03-03Internal CWE Team
Modification DateModifierOrganizationSource
2009-10-29CWE Content TeamMITREInternal
updated Observed_Examples, Relationships
2010-02-16CWE Content TeamMITREInternal
updated References
2011-03-29CWE Content TeamMITREInternal
updated Observed_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated References, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations, References
2013-02-21CWE Content TeamMITREInternal
updated Description, Potential_Mitigations, References, Relationships, Type
2014-02-18CWE Content TeamMITREInternal
updated Potential_Mitigations, References
2014-07-30CWE Content TeamMITREInternal
updated Relationships
2017-01-19CWE Content TeamMITREInternal
updated Relationships

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Page Last Updated: May 05, 2017