CWE-788: Access of Memory Location After End of Buffer
Presentation Filter:
 DescriptionDescription Summary The software reads or writes to a buffer using an index or pointer that references a memory location after the end of the buffer.
Extended Description This typically occurs when a pointer or its index is decremented to a position before the buffer; when pointer arithmetic results in a position before the buffer; or when a negative index is used, which generates a position before the buffer. Common Consequences
Demonstrative ExamplesExample 1 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. (Bad Code) Example
Language: C 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 */
validate_addr_form(user_supplied_addr);
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, 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 2 In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed: (Bad Code) Example
Language: C 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 3 This example applies an encoding procedure to an input string and stores it into a buffer. (Bad Code) Example
Language: C char * copy_input(char *user_supplied_string){ int i, dst_index;
char *dst_buf = (char*)malloc(4*sizeof(char) *
MAX_SIZE);
if ( MAX_SIZE <= strlen(user_supplied_string) ){
die("user string too long, die evil hacker!");
}
dst_index = 0;
for ( i = 0; i < strlen(user_supplied_string); i++
){
if( '&' == user_supplied_string[i] ){
dst_buf[dst_index++] = '&';
dst_buf[dst_index++] = 'a';
dst_buf[dst_index++] = 'm';
dst_buf[dst_index++] = 'p';
dst_buf[dst_index++] = ';';
}
else if ('<' == user_supplied_string[i] ){
/* encode to < */
}
else dst_buf[dst_index++] =
user_supplied_string[i];
}
return dst_buf;
} The programmer attempts to encode the ampersand character in the user-controlled string, however the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands. Example 4 In the following C/C++ example the method processMessageFromSocket() will get a message from a socket, placed into a buffer, and will parse the contents of the buffer into a structure that contains the message length and the message body. A for loop is used to copy the message body into a local character string which will be passed to another method for processing. (Bad Code) Example Languages: C and C++ int processMessageFromSocket(int socket) { int success;
char buffer[BUFFER_SIZE];
char message[MESSAGE_SIZE];
// get message from socket and store into buffer
//Ignoring possibliity that buffer >
BUFFER_SIZE
if (getMessage(socket, buffer, BUFFER_SIZE) > 0)
{
// place contents of the buffer into message
structure
ExMessage *msg = recastBuffer(buffer);
// copy message body into string for
processing
int index;
for (index = 0; index < msg->msgLength;
index++) {
message[index] = msg->msgBody[index];
}
message[index] = '\0';
// process message
success = processMessage(message);
}
return success;
} However, the message length variable from the structure is used as the condition for ending the for loop without validating that the message length variable accurately reflects the length of message body. This can result in a buffer over read by reading from memory beyond the bounds of the buffer if the message length variable indicates a length that is longer than the size of a message body (CWE-130). Observed Examples
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Page Last Updated:
May 05, 2017
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