CWE-806: Buffer Access Using Size of Source Buffer

Description:
The product uses the size of a source buffer when reading from or writing to a destination buffer, which may cause it to access memory that is outside of the bounds of the buffer.

Extended Description:
When the size of the destination is smaller than the size of the source, a buffer overflow could occur.

Affected languages: Memory-Unsafe, C, C++
Parent weaknesses: CWE-805

Consequences:
  - Availability: Modify Memory
  - Integrity: Read Memory
  - Access Control: Bypass Protection Mechanism

Detection methods:
  - 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 havin
  - Automated Dynamic Analysis: Use tools that are integrated during
	     compilation to insert runtime error-checking mechanisms
	     related to memory safety errors, such as AddressSanitizer
	     (ASan) for C/C++ [REF-1518].

Mitigations:
  - [Architecture and Design] Use an abstraction library to abstract away risky APIs. Examples include the Safe C String Library (SafeStr) by Viega, and the Strsafe.h library from Microsoft. This is not a complete solution, since many buffer overflows are not related to strings.
  - [Operation] Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detectio
  - [Implementation] Programmers should adhere to the following rules when allocating and managing their applications memory: Double check that your buffer is as large as you specify. When using functions that accept a number of bytes to copy, such as strncpy(), be aware that if the destination buffer size is equal to t
  - [Operation] Run or compile the software using features or extensions that randomly arrange the positions of a program's executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.   
		   Examples include Address Space

Demonstrative examples:
In the following example, the source character string is copied to the dest character string using the method strncpy.
However, in the call to strncpy the source character string is used within the sizeof call to determine the number of characters to copy. This will create a buffer overflow as the size of the source character string is greater than the dest character string. The dest character string should be used within the sizeof call to ensure that the correct number of characters are copied, as shown below.
... char source[21] = "the character string"; char dest[12]; strncpy(dest, source, sizeof(source)-1); ...
... char source[21] = "the character string"; char dest[12]; strncpy(dest, source, sizeof(dest)-1); ...

In this example, the method outputFilenameToLog outputs a filename to a log file. The method arguments include a pointer to a character string containing the file name and an integer for the number of characters in the string. The filename is copied to a buffer where the buffer size is set to a maximum size for inputs to the log file. The method then calls another method to save the contents of the buffer to the log file.
However, in this case the string copy method, strncpy, mistakenly uses the length method argument to determine the number of characters to copy rather than using the size of the local character string, buf. This can lead to a buffer overflow if the number of characters contained in character string pointed to by filename is larger then the number of characters allowed for