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.
Mail server stores private mail messages with predictable filenames in a world-executable directory, which allows local users to read private mailing list archives.
PRNG 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.
MFV. predictable filename and insecure permissions allows file modification to execute SQL queries.
Related Attack Patterns (CAPEC)
N/A
Attack TTPs
N/A
Modes of Introduction
Phase
Note
Architecture and Design
N/A
Implementation
REALIZATION: This weakness is caused during implementation of an architectural security tactic.
Common Consequences
Impact: Varies by Context — Notes: 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
Implementation: Increase the entropy used to seed a PRNG. (N/A)
Architecture and Design: 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"). (N/A)
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. (N/A)
Applicable Platforms
None (Not Language-Specific, Undetermined)
Demonstrative Examples
Intro: This code generates a unique random identifier for a user's session.
Body: 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.
function generateSessionID($userID){ srand($userID); return rand(); }
Notes
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.