COMMON WEAKNESS ENUMERATIONDraft

CWE-135

Incorrect Calculation of Multi-Byte String Length

The product does not correctly calculate the length of strings that can contain wide or multi-byte characters.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

The product does not correctly calculate the length of strings that can contain wide or multi-byte characters.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

There are several ways in which improper string length checking may result in an exploitable condition. All of these, however, involve the introduction of buffer overflow conditions in order to reach an exploitable state. The first of these issues takes place when the output of a wide or multi-byte character string, string-length function is used as a size for the allocation of memory. While this will result in an output of the number of characters in the string, note that the characters are most likely not a single byte, as they are with standard character strings. So, using the size returned as the size sent to new or malloc and copying the string to this newly allocated memory will result in a buffer overflow. Another common way these strings are misused involves the mixing of standard string and wide or multi-byte string functions on a single string. Invariably, this mismatched information will result in the creation of a possibly exploitable buffer overflow condition.

Risk

What does successful exploitation lead to?

Execute Unauthorized Code or Commands

Affected security scopes: Integrity, Confidentiality, Availability

This weakness may lead to a buffer overflow. Buffer overflows often can be used to execute arbitrary code, which is usually outside the scope of a program's implicit security policy. This can often be used to subvert any other security service.
Read MemoryDoS: Crash, Exit, or RestartDoS: Resource Consumption (CPU)DoS: Resource Consumption (Memory)

Affected security scopes: Availability, Confidentiality

Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash. Other attacks leading to lack of availability are possible, including putting the program into an infinite loop.
Read Memory

Affected security scopes: Confidentiality

In the case of an out-of-bounds read, the attacker may have access to sensitive information. If the sensitive information contains system details, such as the current buffer's position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.

Defence

How is it prevented and fixed?

Implementation

Input Validation

Detail
Always verify the length of the string unit character.

Implementation

Libraries or Frameworks

Detail
Use length computing functions (e.g. strlen, wcslen, etc.) appropriately with their equivalent type (e.g.: byte, wchar_t, etc.)

Verification

How is it detected?

Automated Static Analysis

High
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.)