Description
The product reads or writes to a buffer using an index or pointer that references a memory location after the end of the buffer.
This typically occurs when a pointer or its index is incremented to a position after the buffer; or when pointer arithmetic results in a position after the buffer.
Potential Impact
Confidentiality
Read Memory
Integrity, Availability
Modify Memory, DoS: Crash, Exit, or Restart
Integrity
Modify Memory, Execute Unauthorized Code or Commands
Demonstrative Examples
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
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 the function 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).
In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:
Bad
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).
This example applies an encoding procedure to an input string and stores it into a buffer.
Bad
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.
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
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 (msgLength) from the structure is used as the condition for ending the for loop without validating that msgLength accurately reflects the actual length of the message body (CWE-606). If msgLength indicates a length that is longer than the size of a message body (CWE-130), then this can result in a buffer over-read by reading past the end of the buffer (CWE-126).
Detection Methods
- Fuzzing High — Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption,
- Automated Static Analysis High — 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 having to execute it. Typically, this is done by building a model of data flow and control flow, then sea
- Automated Dynamic Analysis Moderate — 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].
Real-World CVE Examples
| CVE ID | Description |
|---|---|
| CVE-2009-2550 | Classic stack-based buffer overflow in media player using a long entry in a playlist |
| CVE-2009-2403 | Heap-based buffer overflow in media player using a long entry in a playlist |
| CVE-2009-0689 | large precision value in a format string triggers overflow |
| CVE-2009-0558 | attacker-controlled array index leads to code execution |
| CVE-2008-4113 | OS kernel trusts userland-supplied length value, allowing reading of sensitive information |
| CVE-2007-4268 | Chain: integer signedness error (CWE-195) passes signed comparison, leading to heap overflow (CWE-122) |
Related Weaknesses
Taxonomy Mappings
- OMG ASCRM: ASCRM-CWE-788 —
Frequently Asked Questions
What is CWE-788?
CWE-788 (Access of Memory Location After End of Buffer) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product reads or writes to a buffer using an index or pointer that references a memory location after the end of the buffer.
How can CWE-788 be exploited?
Attackers can exploit CWE-788 (Access of Memory Location After End of Buffer) to read memory. This weakness is typically introduced during the Implementation phase of software development.
How do I prevent CWE-788?
Follow secure coding practices, conduct code reviews, and use automated security testing tools (SAST/DAST) to detect this weakness early in the development lifecycle.
What is the severity of CWE-788?
CWE-788 is classified as a Base-level weakness (Medium abstraction). It has been observed in 6 real-world CVEs.