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
All
Common Consequences
Scope
Effect
Availability
If the incorrect calculation causes the program to move into an
unexpected state, it may lead to a crash or impairment of
service.
Integrity
Availability
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
In the context of privilege or permissions assignment, an incorrect
calculation can provide an attacker with access to sensitive
resources.
Other
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
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)
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, taken from CWE-462, attempts to calculate the position
of the second byte of a pointer.
(Bad Code)
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
Description
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.
Implementation
Perform input validation on any numeric inputs by ensuring that they
are within the expected range.
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.
Implementation
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++).
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.
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.
Description
