The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.

Primary

buffer underrun
Some prominent vendors and researchers use the term "buffer underrun". "Buffer underflow" is more commonly used, although both terms are also sometimes used to describe a buffer under-read (CWE-127).

Implementation
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.

Medium

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
Confidentiality
Availability
Access Control
Other
Execute Unauthorized Code or Commands
Modify Memory
Bypass Protection Mechanism
Other
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. The consequences would only be limited by how the affected data is used, such as an adjacent memory location that is used to specify whether the user has special privileges.

Access Control
Other
Bypass Protection Mechanism
Other
When the consequence is arbitrary code execution, this can often be used to subvert any other security service.

Automated Static Analysis
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 searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
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.

Requirements
Choose a language that is not susceptible to these issues.

Implementation
All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.

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 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-2021-24018
buffer underwrite in firmware verification routine allows code execution via a crafted firmware image
https://www.cve.org/CVERecord?id=CVE-2021-24018

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

UNDER - Boundary beginning violation ('buffer underflow'?)

Buffer underwrite

SFP8
Faulty Buffer Access

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

This could be resultant from several errors, including a bad offset or an array index that decrements before the beginning of the buffer (see CWE-129).

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Description, Relationships, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities

CWE Content Team
MITRE
2009-01-12
updated Common_Consequences

CWE Content Team
MITRE
2009-10-29
updated Description, Name, Relationships

CWE Content Team
MITRE
2011-03-29
updated Demonstrative_Examples, Relationships

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

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

CWE Content Team
MITRE
2014-07-30
updated Relationships, Taxonomy_Mappings

CWE Content Team
MITRE
2017-11-08
updated Causal_Nature, Demonstrative_Examples, References

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2021-03-15
updated Potential_Mitigations

CWE Content Team
MITRE
2021-10-28
updated Observed_Examples

CWE Content Team
MITRE
2022-04-28
updated Research_Gaps

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated References, Relationships, Time_of_Introduction

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
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, Description, Detection_Factors, Modes_of_Introduction, References

Muchen Xu
Naive Systems
2023-02-06
Pointed out that the demonstrative example #2 was incorrect and instead demonstrated a Buffer Under-read.

Boundary Beginning Violation ('Buffer Underwrite')