Predictable from Observable State
Draft Base
Structure: Simple
Description
A number or object is predictable based on observations that the attacker can make about the state of the system or network, such as time, process ID, etc.
Common Consequences 1
Scope: Other
Impact: Varies by Context
This weakness could be exploited by an attacker in a number ways depending on the context. If a predictable number is used to generate IDs or keys that are used within protection mechanisms, then an attacker could gain unauthorized access to the system. If predictable filenames are used for storing sensitive information, then an attacker might gain access to the system and may be able to gain access to the information in the file.
Potential Mitigations 3
Phase: Implementation
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.
Demonstrative Examples 1
ID : DX-45
This code generates a unique random identifier for a user's session.Code Example:
Bad
PHP
phpBecause 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 (Small Seed Space in PRNG).
Observed Examples 4
CVE-2002-0389Mail server stores private mail messages with predictable filenames in a world-executable directory, which allows local users to read private mailing list archives.
CVE-2001-1141PRNG allows attackers to use the output of small PRNG requests to determine the internal state information, which could be used by attackers to predict future pseudo-random numbers.
CVE-2000-0335DNS resolver library uses predictable IDs, which allows a local attacker to spoof DNS query results.
CVE-2005-1636MFV. predictable filename and insecure permissions allows file modification to execute SQL queries.
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.