The software performs a calculation that generates incorrect or unintended results that are later used in security-critical decisions or resource management.
Extended Description
When software performs a security-critical calculation incorrectly, it might lead to incorrect resource allocations, incorrect privilege assignments, or failed comparisons among other things. Many of the direct results of an incorrect calculation can lead to even larger problems such as failed protection mechanisms or even arbitrary code execution.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
Language-independent
Common Consequences
Scope
Effect
Availability
Technical Impact: DoS: crash / exit /
restart
If the incorrect calculation causes the program to move into an
unexpected state, it may lead to a crash or impairment of
service.
If the incorrect calculation is used in the context of resource allocation, it could lead to an out-of-bounds operation (CWE-119) leading to a crash or even arbitrary code execution. Alternatively, it may result in an integer overflow (CWE-190) and / or a resource consumption problem (CWE-400).
Access Control
Technical Impact: Gain privileges / assume
identity
In the context of privilege or permissions assignment, an incorrect
calculation can provide an attacker with access to sensitive
resources.
Access Control
Technical Impact: Bypass protection
mechanism
If the incorrect calculation leads to an insufficient comparison (CWE-697), it may compromise a protection mechanism such as a validation routine and allow an attacker to bypass the security-critical code.
Likelihood of Exploit
High
Detection Methods
Manual Analysis
This weakness can be detected using tools and techniques that require
manual (human) analysis, such as penetration testing, threat modeling,
and interactive tools that allow the tester to record and modify an
active session.
Specifically, manual static analysis is useful for evaluating the correctness of allocation calculations. This can be useful for detecting overflow conditions (CWE-190) or similar weaknesses that might have serious security impacts on the program.
Effectiveness: High
These may be more effective than strictly automated techniques. This
is especially the case with weaknesses that are related to design and
business rules.
Demonstrative Examples
Example 1
The following image processing code allocates a table for
images.
This code intends to allocate a table of size num_imgs, however as num_imgs grows large, the calculation determining the size of the list will eventually overflow (CWE-190). This will result in a very small list to be allocated instead. If the subsequent code operates on the list as if it were num_imgs long, it may result in many types of out-of-bounds problems (CWE-119).
Example 2
This code attempts to calculate a football team's average number of
yards gained per touchdown.
(Bad Code)
Example
Language: Java
...
int touchdowns = team.getTouchdowns();
int yardsGained = team.getTotalYardage();
System.out.println(team.getName() + " averages " + yardsGained /
touchdowns + "yards gained for every touchdown scored");
...
The code does not consider the event that the team they are querying
has not scored a touchdown, but has gained yardage. In that case, we
should expect an ArithmeticException to be thrown by the JVM. This could
lead to a loss of availability if our error handling code is not set up
correctly.
Example 3
This example attempts to calculate the position of the second byte
of a pointer.
(Bad Code)
Example
Language: C
int *p = x;
char * second_char = (char *)(p + 1);
In this example, second_char is intended to point to the second byte
of p. But, adding 1 to p actually adds sizeof(int) to p, giving a result
that is incorrect (3 bytes off on 32-bit platforms). If the resulting
memory address is read, this could potentially be an information leak.
If it is a write, it could be a security-critical write to unauthorized
memory-- whether or not it is a buffer overflow. Note that the above
code may also be wrong in other ways, particularly in a little endian
environment.
Potential Mitigations
Phase: Implementation
Understand your programming language's underlying representation and
how it interacts with numeric calculation. Pay close attention to byte
size discrepancies, precision, signed/unsigned distinctions, truncation,
conversion and casting between types, "not-a-number" calculations, and
how your language handles numbers that are too large or too small for
its underlying representation.
Phase: Implementation
Strategy: Input Validation
Perform input validation on any numeric input by ensuring that it is
within the expected range. Enforce that the input meets both the minimum
and maximum requirements for the expected range.
Phase: Implementation
Use the appropriate type for the desired action. For example, in
C/C++, only use unsigned types for values that could never be negative,
such as height, width, or other numbers related to quantity.
Phase: Architecture and Design
Strategies: Language Selection; Libraries or Frameworks
Use languages, libraries, or frameworks that make it easier to handle
numbers without unexpected consequences.
Examples include safe integer handling packages such as SafeInt (C++)
or IntegerLib (C or C++).
Phase: Implementation
Strategy: Compilation or Build Hardening
Examine compiler warnings closely and eliminate problems with
potential security implications, such as signed / unsigned mismatch in
memory operations, or use of uninitialized variables. Even if the
weakness is rarely exploitable, a single failure may lead to the
compromise of the entire system.
Phase: Testing
Use automated static analysis tools that target this type of weakness.
Many modern techniques use data flow analysis to minimize the number of
false positives. This is not a perfect solution, since 100% accuracy and
coverage are not feasible.
Phase: Testing
Use dynamic tools and techniques that interact with the software using
large test suites with many diverse inputs, such as fuzz testing
(fuzzing), robustness testing, and fault injection. The software's
operation may slow down, but it should not become unstable, crash, or
generate incorrect results.
[REF-17] Michael Howard, David LeBlanc
and John Viega. "24 Deadly Sins of Software Security". "Sin 7: Integer Overflows." Page 119. McGraw-Hill. 2010.
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Signed Integer Boundaries", Page
220.. 1st Edition. Addison Wesley. 2006.
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May 05, 2017
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