CWE-331: Insufficient Entropy
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Edit Custom FilterThe product uses an algorithm or scheme that produces insufficient entropy, leaving patterns or clusters of values that are more likely to occur than others.
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Example 1 This code generates a unique random identifier for a user's session. (bad code)
Example Language: PHP
function generateSessionID($userID){
srand($userID); }return rand(); Because the seed for the PRNG is always the user's ID, the session ID will always be the same. An attacker could thus predict any user's session ID and potentially hijack the session. This example also exhibits a Small Seed Space (CWE-339). Example 2 The following code uses a statistical PRNG to create a URL for a receipt that remains active for some period of time after a purchase. (bad code)
Example Language: Java
String GenerateReceiptURL(String baseUrl) {
Random ranGen = new Random(); }ranGen.setSeed((new Date()).getTime()); return(baseUrl + ranGen.nextInt(400000000) + ".html"); This code uses the Random.nextInt() function to generate "unique" identifiers for the receipt pages it generates. Because Random.nextInt() is a statistical PRNG, it is easy for an attacker to guess the strings it generates. Although the underlying design of the receipt system is also faulty, it would be more secure if it used a random number generator that did not produce predictable receipt identifiers, such as a cryptographic PRNG. Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.
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Maintenance
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.
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