The product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations prior to the targeted buffer.

Primary

Implementation
This typically occurs when the 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. This may result in exposure of sensitive information or possibly a crash.

Confidentiality
Read Memory

Confidentiality
Bypass Protection Mechanism
By reading out-of-bounds memory, an attacker might be able to get secret values, such as memory addresses, which can bypass protection mechanisms such as ASLR in order to improve the reliability and likelihood of exploiting a separate weakness to achieve code execution instead of just denial of 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.

In the following code, the method retrieves a value from an array at a specific array index location that is given as an input parameter to the method

int getValueFromArray(int *array, int len, int index) {

int value;

// check that the array index is less than the maximum

// length of the array
if (index < len) {

// get the value at the specified index of the array
value = array[index];
}
// if array index is invalid then output error message

// and return value indicating error
else {printf("Value is: %d\n", array[index]);value = -1;}
return value;
}

However, this method only verifies that the given array index is less than the maximum length of the array but does not check for the minimum value (CWE-839). This will allow a negative value to be accepted as the input array index, which will result in reading data before the beginning of the buffer (CWE-127) and may allow access to sensitive memory. The input array index should be checked to verify that is within the maximum and minimum range required for the array (CWE-129). In this example the if statement should be modified to include a minimum range check, as shown below.

...

// check that the array index is within the correct

// range of values for the array
if (index >= 0 && index < len) {
...

CVE-2021-40985
HTML conversion package has a buffer under-read, allowing a crash
https://www.cve.org/CVERecord?id=CVE-2021-40985

Memory Management

Memory

Buffer under-read

SFP8
Faulty Buffer Access

Allowed
This CWE entry is at the Variant 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.

Under-studied.

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

CWE Content Team
MITRE
2008-09-08
updated Applicable_Platforms, Relationships, Taxonomy_Mappings, Weakness_Ordinalities

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

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Relationships

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

CWE Content Team
MITRE
2017-11-08
updated Causal_Nature

CWE Content Team
MITRE
2019-09-19
updated Common_Consequences, References

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2023-01-31
updated Description

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

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2023-10-26
updated Observed_Examples

CWE Content Team
MITRE
2025-09-09
4.18
2025-09-09
updated Affected_Resources, Common_Consequences, Demonstrative_Examples, Functional_Areas

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Applicable_Platforms, Description, Detection_Factors, Modes_of_Introduction, References