The product reads or writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.
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 beginning of the valid memory location, or when a negative index is used.

Resultant

Implementation

Confidentiality
Read Memory
For an out-of-bounds read, the attacker may have access to sensitive information. If the sensitive information contains system details, such as the current buffer's position in memory, this knowledge can be used to craft further attacks, possibly with more severe consequences.

Integrity
Availability
Modify Memory
DoS: Crash, Exit, or Restart
Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.

Integrity
Modify Memory
Execute Unauthorized Code or Commands
If the corrupted memory can be effectively controlled, it may be possible to execute arbitrary code. If the corrupted memory is data rather than instructions, the system will continue to function with improper changes, possibly in violation of an implicit or explicit policy.

Fuzzing
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, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.
High

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].
Moderate
Crafted inputs are necessary to
reach the code containing the error, such as generated
by fuzzers. Also, these tools may reduce performance,
and they only report the error condition - not the
original mistake that led to the
error.

In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.

char* trimTrailingWhitespace(char *strMessage, int length) {
char *retMessage;char *message = malloc(sizeof(char)*(length+1));

// copy input string to a temporary string
char message[length+1];int index;for (index = 0; index < length; index++) {message[index] = strMessage[index];}message[index] = '\0';

// trim trailing whitespace
int len = index-1;while (isspace(message[len])) {message[len] = '\0';len--;}

// return string without trailing whitespace
retMessage = message;return retMessage;
}

However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.

The following example asks a user for an offset into an array to select an item.

int main (int argc, char **argv) {char *items[] = {"boat", "car", "truck", "train"};int index = GetUntrustedOffset();printf("You selected %s\n", items[index-1]);}

The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).

The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.

int main() {
...
char *result = strstr(destBuf, "Replace Me");
int idx = result - destBuf;
strcpy(&destBuf[idx], srcBuf);
...}

In the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.

CVE-2002-2227
Unchecked length of SSLv2 challenge value leads to buffer underflow.
https://www.cve.org/CVERecord?id=CVE-2002-2227

CVE-2007-4580
Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
https://www.cve.org/CVERecord?id=CVE-2007-4580

CVE-2007-1584
Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
https://www.cve.org/CVERecord?id=CVE-2007-1584

CVE-2007-0886
Buffer underflow resultant from encoded data that triggers an integer overflow.
https://www.cve.org/CVERecord?id=CVE-2007-0886

CVE-2006-6171
Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
https://www.cve.org/CVERecord?id=CVE-2006-6171

CVE-2006-4024
Negative value is used in a memcpy() operation, leading to buffer underflow.
https://www.cve.org/CVERecord?id=CVE-2006-4024

CVE-2004-2620
Buffer underflow due to mishandled special characters
https://www.cve.org/CVERecord?id=CVE-2004-2620

Memory Management

Memory

ARR30-C
Do not form or use out-of-bounds pointers or array subscripts
CWE More Specific

Discouraged
The CWE entry might be misused when lower-level CWE entries might be available. It also overlaps existing CWE entries and might be deprecated in the future.
If the "Access" operation is known to be a read or a write, then investigate children of entries such as CWE-787: Out-of-bounds Write and CWE-125: Out-of-bounds Read.

CWE Content Team
MITRE
2009-10-21
1.6
2009-10-29

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Common_Consequences, Demonstrative_Examples, Observed_Examples, Relationships

CWE Content Team
MITRE
2017-11-08
updated Common_Consequences, Demonstrative_Examples, Taxonomy_Mappings

CWE Content Team
MITRE
2019-01-03
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-08-20
updated Relationships

CWE Content Team
MITRE
2020-12-10
updated Relationships

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated Detection_Factors, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2024-02-29
4.14
2024-02-29
updated Demonstrative_Examples

CWE Content Team
MITRE
2024-07-16
4.15
2024-07-16
updated Common_Consequences

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Applicable_Platforms

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated Affected_Resources, Functional_Areas

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Detection_Factors, References, Time_of_Introduction, Weakness_Ordinalities