Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-124: Buffer Underwrite ('Buffer Underflow')

Weakness ID: 124
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

The software 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).

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms




+ Common Consequences

Technical Impact: Modify memory; DoS: crash / exit / restart

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

Access Control

Technical Impact: 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

Technical Impact: Bypass protection mechanism; Other

When the consequence is arbitrary code execution, this can often be used to subvert any other security service.

+ Likelihood of Exploit


+ 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 Languages: C and 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';

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

// 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, if find() returns a negative value to indicate that ch is not found in srcBuf:

(Bad Code)
Example Language:
int main() {
strncpy(destBuf, &srcBuf[find(srcBuf, ch)], 1024);

If the index to srcBuf is somehow under user control, this is an arbitrary write-what-where condition.

+ Observed Examples
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
+ Potential Mitigations

Requirements specification: The choice could be made to use a language that is not susceptible to these issues.

Phase: Implementation

Sanity checks should be performed on all calculated values used as index or for pointer arithmetic.

+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base786Access of Memory Location Before Start of Buffer
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfWeakness BaseWeakness Base787Out-of-bounds Write
Development Concepts699
Research Concepts1000
ChildOfCategoryCategory970SFP Secondary Cluster: Faulty Buffer Access
Software Fault Pattern (SFP) Clusters (primary)888
CanFollowWeakness BaseWeakness Base839Numeric Range Comparison Without Minimum Check
Research Concepts1000
CanAlsoBeWeakness VariantWeakness Variant196Unsigned to Signed Conversion Error
Research Concepts1000
+ Relationship Notes

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).

+ Research Gaps

Much attention has been paid to buffer overflows, but "underflows" sometimes exist in products that are relatively free of overflows, so it is likely that this variant has been under-studied.

+ Causal Nature


+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERUNDER - Boundary beginning violation ('buffer underflow'?)
CLASPBuffer underwrite
Software Fault PatternsSFP8Faulty Buffer Access
+ References
"Buffer UNDERFLOWS: What do you know about it?". Vuln-Dev Mailing List. 2004-01-10. <>.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Description, Relationships, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-01-12CWE Content TeamMITREInternal
updated Common_Consequences
2009-10-29CWE Content TeamMITREInternal
updated Description, Name, Relationships
2011-03-29CWE Content TeamMITREInternal
updated Demonstrative_Examples, Relationships
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, References, Relationships
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2009-10-29Boundary Beginning Violation ('Buffer Underwrite')

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Page Last Updated: January 18, 2017