Weak Cryptographic Hash: User-Controlled Seed

Methods that generate cryptographic hashes should not be called with a user input seed value.

Weak Cryptographic Hash: User-Controlled Seed issues occur when:

1. Data enters a program through an untrusted source.

2. The user-controlled data is included within the seed, or used entirely as the seed within a cryptographic hash function.


As with many software security vulnerabilities, Weak Cryptographic Hash: User-Controlled seed is a means to an end, not an end in and of itself. At its root, the vulnerability is straightforward: an attacker passes malicious data to an application, and the data is then used as all or part of the seed in a cryptographic hash function.

A user-controlled seed can enable various attacks:

1. The attacker might use this vulnerability to specify an empty seed for the data being hashed. An attacker can quickly hash the data being hashed using a number of different hashing algorithms to leak information about the hashing implementation used within your application. This could make "cracking" other data values easier by limiting the particular variant of hash used.
2. If an attacker can manipulate other users' seeds, or trick other users into using an empty seed, then they could compute "rainbow tables" for the application and easily determine the hashed values.

Example 1: The following code uses a user-controlled seed for password hashing:

    ...
    String seed = prop.getProperty("seed");
    Hashing.murmur3_32_fixed(Integer.parseInt(seed)).hashString("foo", StandardCharsets.UTF_8);
    ...

The code in Example 1 runs successfully, but anyone who can get to this functionality can manipulate the seed used to hash the password by modifying the property seed. After the program ships, it can be difficult to undo an issue regarding user-controlled seeds, because you cannot know whether a password hash had its seed determined by a malicious user.