CWE-806ΒΆ

Buffer Access Using Size of Source Buffer. [Improper-Control-Of-A-Resource-Through-Its-Lifetime]

Required inputs: IR

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. When the size of the destination is smaller than the size of the source, a buffer overflow could occur.
Demonstrative Examples
Example 1

In the following example, the source character string is copied to the dest character string using the method strncpy.

Example Language:C
    ...
    char source[21] = "the character string";
    char dest[12];
    strncpy(dest, source, sizeof(source)-1);
    ...

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.

Example Language:C
    ...
    char source[21] = "the character string";
    char dest[12];
    strncpy(dest, source, sizeof(dest)-1);
    ...
Example 2

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.

Example Language:C
    #define LOG_INPUT_SIZE 40

    // saves the file name to a log file
    int outputFilenameToLog(char *filename, int length) {
        int success;

        // buffer with size set to maximum size for input to log file
        char buf[LOG_INPUT_SIZE];

        // copy filename to buffer
        strncpy(buf, filename, length);

        // save to log file
        success = saveToLogFile(buf);

        return success;
    }

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 the local character string. The string copy method should use the buf character string within a sizeof call to ensure that only characters up to the size of the buf array are copied to avoid a buffer overflow, as shown below.

Example Language:C
    ...
    // copy filename to buffer
    strncpy(buf, filename, sizeof(buf)-1);
    ...
Excerpts from CWE [https://cwe.mitre.org], Copyright (C) 2006-2026, the MITRE Corporation. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

not_using_size_of_target

Not using size of target buffer

None

False

using_size_of_source

Using size of source buffer

None

False

Options