COMMON WEAKNESS ENUMERATIONIncomplete

CWE-839

Numeric Range Comparison Without Minimum Check

The product checks a value to ensure that it is less than or equal to a maximum, but it does not also verify that the value is greater than or equal to the minimum.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

The product checks a value to ensure that it is less than or equal to a maximum, but it does not also verify that the value is greater than or equal to the minimum.

Extended description

Some products use signed integers or floats even when their values are only expected to be positive or 0. An input validation check might assume that the value is positive, and only check for the maximum value. If the value is negative, but the code assumes that the value is positive, this can produce an error. The error may have security consequences if the negative value is used for memory allocation, array access, buffer access, etc. Ultimately, the error could lead to a buffer overflow or other type of memory corruption. The use of a negative number in a positive-only context could have security implications for other types of resources. For example, a shopping cart might check that the user is not requesting more than 10 items, but a request for -3 items could cause the application to calculate a negative price and credit the attacker's account.

Terminology

Alternate terms and usages

Signed comparison

The "signed comparison" term is often used to describe when the product uses a signed variable and checks it to ensure that it is less than a maximum value (typically a maximum buffer size), but does not verify that it is greater than 0.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

Risk

What does successful exploitation lead to?

Modify Application DataExecute Unauthorized Code or Commands

Affected security scopes: Integrity, Confidentiality, Availability

An attacker could modify the structure of the message or data being sent to the downstream component, possibly injecting commands.
DoS: Resource Consumption (Other)

Affected security scopes: Availability

in some contexts, a negative value could lead to resource consumption.
Modify MemoryRead Memory

Affected security scopes: Confidentiality, Integrity

If a negative value is used to access memory, buffers, or other indexable structures, it could access memory outside the bounds of the buffer.

Defence

How is it prevented and fixed?

Implementation

Enforcement by Conversion

Detail
If the number to be used is always expected to be positive, change the variable type from signed to unsigned or size_t.

Implementation

Input Validation

Detail
If the number to be used could have a negative value based on the specification (thus requiring a signed value), but the number should only be positive to preserve code correctness, then include a check to ensure that the value is positive.

Verification

How is it detected?

Automated Static Analysis

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)