Buffer Access with Incorrect Length Value

The software uses a sequential operation to read or write a buffer, but it uses an incorrect length value that causes it to access memory that is outside of the bounds of the buffer.


When the length value exceeds the size of the destination, a buffer overflow could occur.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

void host_lookup(char *user_supplied_addr){

  struct hostent *hp;
  in_addr_t *addr;
  char hostname[64];
  in_addr_t inet_addr(const char *cp);

  /*routine that ensures user_supplied_addr is in the right format for conversion */

  addr = inet_addr(user_supplied_addr);
  hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);
  strcpy(hostname, hp->h_name);


This function allocates a buffer of 64 bytes to store the hostname under the assumption that the maximum length value of hostname is 64 bytes, however there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then we may overwrite sensitive data or even relinquish control flow to the attacker.

Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).

Example Two

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

int returnChunkSize(void *) {

  /* if chunk info is valid, return the size of usable memory,

  * else, return -1 to indicate an error


int main() {
  memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

Example Three

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

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.

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

Example Four

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.

#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.

// copy filename to buffer
strncpy(buf, filename, sizeof(buf)-1);

See Also

Memory Buffer Errors

Weaknesses in this category are related to the handling of memory buffers within a software system.

SEI CERT C Coding Standard - Guidelines 06. Arrays (ARR)

Weaknesses in this category are related to the rules and recommendations in the Arrays (ARR) section of the SEI CERT C Coding Standard.

CERT C++ Secure Coding Section 06 - Arrays and the STL (ARR)

Weaknesses in this category are related to rules in the Arrays and the STL (ARR) section of the CERT C++ Secure Coding Standard. Since not all rules map to specific we...

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

CWE Cross-section

This view contains a selection of weaknesses that represent the variety of weaknesses that are captured in CWE, at a level of abstraction that is likely to be useful t...

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