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Home > CWE List > CWE-786: Access of Memory Location Before Start of Buffer (4.18)
ID
CWE Glossary Definition

CWE-786: Access of Memory Location Before Start of Buffer

Weakness ID: 786
Vulnerability Mapping: DISCOURAGED This CWE ID should not 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 reads or 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.
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

Read Memory

Scope: Confidentiality

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.

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.

Modify Memory; Execute Unauthorized Code or Commands

Scope: Integrity

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.
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 Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 119 Improper Restriction of Operations within the Bounds of a Memory Buffer
ParentOf 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. 124 Buffer Underwrite ('Buffer Underflow')
ParentOf Variant Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource. 127 Buffer Under-read
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
Relevant to the view "CISQ Quality Measures (2020)" (View-1305)
Nature Type ID Name
ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 119 Improper Restriction of Operations within the Bounds of a Memory Buffer
Relevant to the view "CISQ Data Protection Measures" (View-1340)
Nature Type ID Name
ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 119 Improper Restriction of Operations within the Bounds of a Memory Buffer
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 (Often Prevalent)

C++ (Often Prevalent)

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

(bad code)
Example Language: C

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).



Example 3


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
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
Detection Methods
Method Details

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.

Effectiveness: High
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 ViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries). 884 CWE Cross-section
MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1160 SEI CERT C Coding Standard - Guidelines 06. Arrays (ARR)
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 DISCOURAGED
(this CWE ID should not be used to map to real-world vulnerabilities)
Reasons Potential Deprecation, Frequent Misuse

Rationale

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.

Comments

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.
Taxonomy Mappings
Mapped Taxonomy Name Node ID Fit Mapped Node Name
CERT C Secure Coding ARR30-C CWE More Specific Do not form or use out-of-bounds pointers or array subscripts
Content History
Submissions
Submission Date Submitter Organization
2009年10月21日
(CWE 1.6, 2009年10月29日) 		CWE Content Team MITRE
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
2025年04月03日
(CWE 4.17, 2025年04月03日) 		CWE Content Team MITRE
updated Applicable_Platforms
2024年07月16日
(CWE 4.15, 2024年07月16日) 		CWE Content Team MITRE
updated Common_Consequences
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 Detection_Factors, Relationships
2023年01月31日 CWE Content Team MITRE
updated Description
2020年12月10日 CWE Content Team MITRE
updated Relationships
2020年08月20日 CWE Content Team MITRE
updated Relationships
2020年02月24日 CWE Content Team MITRE
updated Relationships
2019年01月03日 CWE Content Team MITRE
updated Relationships
2017年11月08日 CWE Content Team MITRE
updated Common_Consequences, Demonstrative_Examples, Taxonomy_Mappings
2012年05月11日 CWE Content Team MITRE
updated Common_Consequences, Demonstrative_Examples, Observed_Examples, Relationships
2011年06月01日 CWE Content Team MITRE
updated Common_Consequences
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Page Last Updated: September 09, 2025

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