CWE

Common Weakness Enumeration

A community-developed list of SW & HW weaknesses that can become vulnerabilities

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ID

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

Weakness ID: 124
Vulnerability Mapping: ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities
Abstraction: BaseBase - 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).
+ Relationships
Section HelpThis 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" (CWE-1000)
NatureTypeIDName
ChildOfBaseBase - 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.787Out-of-bounds Write
ChildOfBaseBase - 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.786Access of Memory Location Before Start of Buffer
CanFollowBaseBase - 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.839Numeric Range Comparison Without Minimum Check
Section HelpThis 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 "Software Development" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1218Memory 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.
PhaseNote
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)

+ 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.
ScopeImpactLikelihood
Integrity
Availability

Technical Impact: 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

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
Other

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
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:
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.

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:
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.

+ Observed Examples
ReferenceDescription
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
+ Potential Mitigations

Phase: Requirements

Choose a language that is not susceptible to these issues.

Phase: 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.
+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ 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.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.970SFP Secondary Cluster: Faulty Buffer Access
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1399Comprehensive Categorization: Memory Safety
+ Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID could 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 NameNode IDFitMapped Node Name
PLOVERUNDER - Boundary beginning violation ('buffer underflow'?)
CLASPBuffer underwrite
Software Fault PatternsSFP8Faulty Buffer Access
+ References
[REF-90] "Buffer UNDERFLOWS: What do you know about it?". Vuln-Dev Mailing List. 2004-01-10. <https://seclists.org/vuln-dev/2004/Jan/22>. URL validated: 2023-04-07.
[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 DateSubmitterOrganization
2006-07-19
(CWE Draft 3, 2006-07-19)
PLOVER
+ Contributions
Contribution DateContributorOrganization
2023-02-06Muchen XuNaive Systems
Pointed out that the demonstrative example #2 was incorrect and instead demonstrated a Buffer Under-read.
+ Modifications
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Description, Relationships, Relationship_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-01-12CWE Content TeamMITRE
updated Common_Consequences
2009-10-29CWE Content TeamMITRE
updated Description, Name, Relationships
2011-03-29CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, References, Relationships
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Causal_Nature, Demonstrative_Examples, References
2020-02-24CWE Content TeamMITRE
updated Relationships
2021-03-15CWE Content TeamMITRE
updated Potential_Mitigations
2021-10-28CWE Content TeamMITRE
updated Observed_Examples
2022-04-28CWE Content TeamMITRE
updated Research_Gaps
2023-01-31CWE Content TeamMITRE
updated Description
2023-04-27CWE Content TeamMITRE
updated References, Relationships, Time_of_Introduction
2023-06-29CWE Content TeamMITRE
updated Mapping_Notes
2024-02-29
(CWE 4.14, 2024-02-29)
CWE Content TeamMITRE
updated Demonstrative_Examples
+ Previous Entry Names
Change DatePrevious Entry Name
2009-10-29Boundary Beginning Violation ('Buffer Underwrite')
Page Last Updated: February 29, 2024