CWE-135: Incorrect Calculation of Multi-Byte String Length
Incorrect Calculation of Multi-Byte String Length
Weakness ID: 135 (Weakness Base)
Status: Draft
Description
Description Summary
The software does not correctly calculate the length of strings that can contain wide or multi-byte characters.
Time of Introduction
Implementation
Applicable Platforms
Languages
C
C++
Common Consequences
Scope
Effect
Integrity
Confidentiality
Availability
Technical Impact: Execute unauthorized code or
commands
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.
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.
Confidentiality
Technical Impact: Read memory
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 buffers position in memory, this knowledge
can be used to craft further attacks, possibly with more severe
consequences.
Demonstrative Examples
Example 1
The following example would be exploitable if any of the commented
incorrect malloc calls were used.
Always verify the length of the string unit character.
Phase: Implementation
Strategy: Libraries or Frameworks
Use length computing functions (e.g. strlen, wcslen, etc.)
appropriately with their equivalent type (e.g.: byte, wchar_t,
etc.)
Other Notes
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. Again, if a language subject to these flaws must be
used, the most effective mitigation technique is to pay careful attention to
the code at implementation time and ensure that these flaws do not
occur.
Description
