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Home > CWE List > CWE-124: Buffer Underwrite ('Buffer Underflow') (4.18)
ID
CWE Glossary Definition

CWE-124: Buffer Underwrite ('Buffer Underflow')

Weakness ID: 124
Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The product writes to a buffer using an index or pointer that references a memory location prior to the beginning 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 beginning of the valid memory location, or when a negative index is used.
Alternate Terms
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).
Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Impact Details

Modify Memory; DoS: Crash, Exit, or Restart

Scope: Integrity, Availability

Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.

Execute Unauthorized Code or Commands; Modify Memory; Bypass Protection Mechanism; Other

Scope: Integrity, Confidentiality, Availability, Access Control, 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.

Bypass Protection Mechanism; Other

Scope: Access Control, Other

When the consequence is arbitrary code execution, this can often be used to subvert any other security service.
Potential Mitigations
Phase(s) Mitigation

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.
Relationships
Section Help This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (View-1000)
Nature Type ID Name
ChildOf Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 786 Access of Memory Location Before Start of Buffer
ChildOf Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 787 Out-of-bounds Write
CanFollow Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. 839 Numeric Range Comparison Without Minimum Check
Relevant to the view "Software Development" (View-699)
Nature Type ID Name
MemberOf Category Category - a CWE entry that contains a set of other entries that share a common characteristic. 1218 Memory Buffer Errors
Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase Note
Implementation
Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages

C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

Likelihood Of Exploit
Medium
Demonstrative Examples

Example 1


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.

(bad code)
Example Language: C
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円';

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



Example 2


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.

(bad code)
Example Language: C
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.



Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

Reference Description
buffer underwrite in firmware verification routine allows code execution via a crafted firmware image
Unchecked length of SSLv2 challenge value leads to buffer underflow.
Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
Buffer underflow resultant from encoded data that triggers an integer overflow.
Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
Negative value is used in a memcpy() operation, leading to buffer underflow.
Buffer underflow due to mishandled special characters
Weakness Ordinalities
Ordinality Description
Primary
(where the weakness exists independent of other weaknesses)
Functional Areas
  • Memory Management
Affected Resources
  • Memory
Memberships
Section HelpThis MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature Type ID Name
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 970 SFP Secondary Cluster: Faulty Buffer Access
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1399 Comprehensive Categorization: Memory Safety
Vulnerability Mapping Notes
Usage ALLOWED
(this CWE ID may be used to map to real-world vulnerabilities)
Reason Acceptable-Use

Rationale

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.

Comments

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.
Notes

Relationship

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).
Taxonomy Mappings
Mapped Taxonomy Name Node ID Fit Mapped Node Name
PLOVER UNDER - Boundary beginning violation ('buffer underflow'?)
CLASP Buffer underwrite
Software Fault Patterns SFP8 Faulty Buffer Access
References
[REF-90] "Buffer UNDERFLOWS: What do you know about it?". Vuln-Dev Mailing List. 2004年01月10日.
<https://seclists.org/vuln-dev/2004/Jan/22>. (URL validated: 2023年04月07日)
[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.
Content History
Submissions
Submission Date Submitter Organization
2006年07月19日
(CWE Draft 3, 2006年07月19日) 		PLOVER
Contributions
Contribution Date Contributor Organization
2023年02月06日 Muchen Xu Naive Systems
Pointed out that the demonstrative example #2 was incorrect and instead demonstrated a Buffer Under-read.
Modifications
Modification Date Modifier Organization
2025年09月09日
(CWE 4.18, 2025年09月09日) 		CWE Content Team MITRE
updated Affected_Resources, Functional_Areas
2024年02月29日
(CWE 4.14, 2024年02月29日) 		CWE Content Team MITRE
updated Demonstrative_Examples
2023年06月29日 CWE Content Team MITRE
updated Mapping_Notes
2023年04月27日 CWE Content Team MITRE
updated References, Relationships, Time_of_Introduction
2023年01月31日 CWE Content Team MITRE
updated Description
2022年04月28日 CWE Content Team MITRE
updated Research_Gaps
2021年10月28日 CWE Content Team MITRE
updated Observed_Examples
2021年03月15日 CWE Content Team MITRE
updated Potential_Mitigations
2020年02月24日 CWE Content Team MITRE
updated Relationships
2017年11月08日 CWE Content Team MITRE
updated Causal_Nature, Demonstrative_Examples, References
2014年07月30日 CWE Content Team MITRE
updated Relationships, Taxonomy_Mappings
2012年05月11日 CWE Content Team MITRE
updated Demonstrative_Examples, References, Relationships
2011年06月01日 CWE Content Team MITRE
updated Common_Consequences
2011年03月29日 CWE Content Team MITRE
updated Demonstrative_Examples, Relationships
2009年10月29日 CWE Content Team MITRE
updated Description, Name, Relationships
2009年01月12日 CWE Content Team MITRE
updated Common_Consequences
2008年09月08日 CWE Content Team MITRE
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Description, Relationships, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2008年07月01日 Eric Dalci Cigital
updated Time_of_Introduction
Previous Entry Names
Change Date Previous Entry Name
2009年10月29日 Boundary Beginning Violation ('Buffer Underwrite')
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Page Last Updated: September 09, 2025

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