CWE-759 使用未加Salt的单向哈希算法

Use of a One-Way Hash without a Salt

结构: Simple

Abstraction: Variant

状态: Incomplete

被利用可能性: unkown

基本描述

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.

扩展描述

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.

相关缺陷

• cwe_Nature: ChildOf cwe_CWE_ID: 916 cwe_View_ID: 699 cwe_Ordinal: Primary

• cwe_Nature: ChildOf cwe_CWE_ID: 916 cwe_View_ID: 1000 cwe_Ordinal: Primary

常见的影响

检测方法

Automated Static Analysis - Binary or Bytecode

Manual Static Analysis - Binary or Bytecode

Manual Static Analysis - Source Code

Automated Static Analysis - Source Code

Automated Static Analysis

Architecture or Design Review

可能的缓解方案

MIT-51 Architecture and Design

策略:

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.

Architecture and Design

策略:

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.

MIT-25 ['Implementation', 'Architecture and Design']

策略:

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.

示例代码

例

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

bad C

bad Java

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).

例

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 Python

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 Python

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

分析过的案例

引用

• REF-291 bcrypt

• REF-292 Tarsnap - The scrypt key derivation function and encryption utility

• REF-293 RFC2898 - PKCS #5: Password-Based Cryptography Specification Version 2.0

• REF-294 How To Safely Store A Password

• REF-295 How Companies Can Beef Up Password Security (interview with Thomas H. Ptacek)

• REF-296 Password security: past, present, future

• REF-297 Our password hashing has no clothes

• REF-298 Should we really use bcrypt/scrypt?

• REF-631 Password Storage Cheat Sheet

• REF-632 Enough With The Rainbow Tables: What You Need To Know About Secure Password Schemes

• REF-633 The Importance of Being Canonical

• REF-634 Password Hashing

• REF-635 Rainbow Hash Cracking

• REF-636 Speed Hashing

• REF-637 Rainbow table

• REF-7 Writing Secure Code

• REF-62 The Art of Software Security Assessment

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