Predictable Exact Value from Previous Values
Draft Base
Structure: Simple
Description
An exact value or random number can be precisely predicted by observing previous values.
Common Consequences 1
Scope: Other
Impact: Varies by Context
Potential Mitigations 3
Increase the entropy used to seed a PRNG.
Phase: Architecture and DesignRequirements
Strategy: Libraries or Frameworks
Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems. Consult FIPS 140-2 Annex C ("Approved Random Number Generators").
Phase: Implementation
Use a PRNG that periodically re-seeds itself using input from high-quality sources, such as hardware devices with high entropy. However, do not re-seed too frequently, or else the entropy source might block.
Observed Examples 4
CVE-2002-1463Firewall generates easily predictable initial sequence numbers (ISN), which allows remote attackers to spoof connections.
CVE-1999-0074Listening TCP ports are sequentially allocated, allowing spoofing attacks.
CVE-1999-0077Predictable TCP sequence numbers allow spoofing.
CVE-2000-0335DNS resolver uses predictable IDs, allowing a local user to spoof DNS query results.
References 2
FIPS PUB 140-2: SECURITY REQUIREMENTS FOR CRYPTOGRAPHIC MODULES
Information Technology Laboratory, National Institute of Standards and Technology
25-05-2001
ID: REF-267
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Modes of Introduction
Architecture and Design
Implementation
Related Weaknesses
Taxonomy Mapping
- PLOVER
Notes
MaintenanceAs 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.