CWE 131

Automated Static Analysis

This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.

Automated static analysis generally does not account for environmental considerations when reporting potential errors in buffer calculations. This can make it difficult for users to determine which warnings should be investigated first. For example, an analysis tool might report buffer overflows that originate from command line arguments in a program that is not expected to run with setuid or other special privileges.

Effectiveness: High

Automated Dynamic Analysis

This weakness can be detected using 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.

Manual Analysis

Manual analysis can be useful for finding this weakness, but it might not achieve desired code coverage within limited time constraints. This becomes difficult for weaknesses that must be considered for all inputs, since the attack surface can be too large.

Black Box

Evidence of this weakness can be detected using 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.

Effectiveness: Moderate

Phase: Implementation

If you allocate a buffer for the purpose of transforming, converting, or encoding an input, make sure that you allocate enough memory to handle the largest possible encoding. For example, in a routine that converts "&" characters to "&" for HTML entity encoding, you will need an output buffer that is at least 5 times as large as the input buffer.

Phase: Implementation

Understand your programming language's underlying representation and how it interacts with numeric calculation (CWE-681). 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

When processing structured incoming data containing a size field followed by raw data, ensure that you identify and resolve any inconsistencies between the size field and the actual size of the data (CWE-130).

Phase: Implementation

When allocating memory that uses sentinels to mark the end of a data structure - such as NUL bytes in strings - make sure you also include the sentinel in your calculation of the total amount of memory that must be allocated.

Phase: Implementation

Use sizeof() on the appropriate data type to avoid CWE-467.

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. This will simplify your sanity checks and will reduce surprises related to unexpected casting.

Phase: Architecture and Design

Strategy: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

Use libraries or frameworks that make it easier to handle numbers without unexpected consequences, or buffer allocation routines that automatically track buffer size.

Examples include safe integer handling packages such as SafeInt (C++) or IntegerLib (C or C++).

This is a broad category. Some examples include: (1) simple math errors, (2) incorrectly updating parallel counters, (3) not accounting for size differences when "transforming" one input to another format (e.g. URL canonicalization or other transformation that can generate a result that's larger than the original input, i.e. "expansion").

This level of detail is rarely available in public reports, so it is difficult to find good examples.

This weakness may be a composite or a chain. It also may contain layering or perspective differences.

This issue may be associated with many different types of incorrect calculations (CWE-682), although the integer overflow (CWE-190) is probably the most prevalent. This can be primary to resource consumption problems (CWE-400), including uncontrolled memory allocation (CWE-789). However, its relationship with out-of-bounds buffer access (CWE-119) must also be considered.