Use of a One-Way Hash with a Predictable Salt

Opis

The product uses a one-way cryptographic hash against an input that should not be reversible, such as a password, but the product uses a predictable salt as part of the input.

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.

Skutki

• Access Control

  Bypass Protection Mechanism

Zapobieganie

• 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 [REF-291], scrypt [REF-292], and PBKDF2 [REF-293]. 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.

  Skuteczność: High

• 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.

  Skuteczność: Limited

Przykłady podatności

• CVE-2008-4905 Blogging software uses a hard-coded salt when calculating a password hash.
• CVE-2002-1657 Database server uses the username for a salt when encrypting passwords, simplifying brute force attacks.
• CVE-2001-0967 Server uses a constant salt when encrypting passwords, simplifying brute force attacks.
• CVE-2005-0408 chain: product generates predictable MD5 hashes using a constant value combined with username, allowing authentication bypass.

Słabość nadrzędna

• CWE-916 Use of Password Hash With Insufficient Computational Effort

Kiedy powstaje

• Implementation — REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Dotyczy

Not Language-Specific

Uwagi

As of CWE 4.5, terminology related to randomness, entropy, and predictability can vary widely. Within the developer and other communities, "randomness" is used heavily. However, within cryptography, "entropy" is distinct, typically implied as a measurement. There are no commonly-used definitions, even within standards documents and cryptography papers. Future versions of CWE will attempt to define these terms and, if necessary, distinguish between them in ways that are appropriate for different communities but do not reduce the usability of CWE for mapping, understanding, or other scenarios.

Źródła

1. bcrypt — Johnny Shelley
2. Tarsnap - The scrypt key derivation function and encryption utility — Colin Percival
3. RFC2898 - PKCS #5: Password-Based Cryptography Specification Version 2.0 — B. Kaliski, 5.2 PBKDF2, 2000
4. How To Safely Store A Password — Coda Hale, 2010
5. How Companies Can Beef Up Password Security (interview with Thomas H. Ptacek) — Brian Krebs, 2012
6. Password security: past, present, future — Solar Designer, 2012
7. Our password hashing has no clothes — Troy Hunt, 2012
8. Should we really use bcrypt/scrypt? — Joshbw, 2012
9. Password Storage Cheat Sheet — OWASP
10. Enough With The Rainbow Tables: What You Need To Know About Secure Password Schemes — Thomas Ptacek, 2007
11. The Importance of Being Canonical — Robert Graham, 2009
12. Password Hashing — James McGlinn
13. Rainbow Hash Cracking — Jeff Atwood, 2007
14. Speed Hashing — Jeff Atwood, 2012
15. Rainbow table — Wikipedia, 2009
16. Writing Secure Code — Michael Howard, David LeBlanc, Chapter 9, "Creating a Salted Hash" Page 302, Microsoft Press, 2002
17. The Art of Software Security Assessment — Mark Dowd, John McDonald, Justin Schuh, Chapter 2, "Salt Values", Page 46, Addison Wesley, 2006