The product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations prior to the targeted buffer.
View on MITREBy 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.
No mitigation information available for this CWE.
No detection method information available for this CWE.
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
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.
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; }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
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) { ...Additional facts reviewed against primary or authoritative security sources.
Documented detection approaches include Automated Static Analysis, and Automated Dynamic Analysis; recorded impacts include Read Memory, and Bypass Protection Mechanism; observed examples include CVE-2021-40985. Use these source-defined anchors to turn CWE-127 into implementation, review, and verification checks for the affected component.
CWE-127: Buffer Under-read — MITRE CWEUse NIST SSDF practices to make prevention of CWE-127, Buffer Under-read, an explicit development outcome. Translate the CWE definition into security requirements and design constraints, choose safer implementation patterns and toolchain checks, and define acceptance evidence before release. Feed every confirmed occurrence back into the requirements, design review, and coding rules so the same root cause is removed across the product rather than patched in one location.
NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and TechnologyUse CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-127, Buffer Under-read, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.
Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security AgencyApply MITRE's full root-cause mapping guidance when using CWE-127, Buffer Under-read. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.
CVE to CWE Root Cause Mapping Guidance — MITRE CWEApply MITRE's root-cause mapping quick tips to CWE-127, Buffer Under-read. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.
CVE to CWE Root Cause Mapping Quick Tips — MITRE CWECWE-127: Buffer Under-read is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations prior to the targeted buffer.
If exploited, CWE-127 (Buffer Under-read) it can compromise Confidentiality, leading to outcomes such as Read Memory and Bypass Protection Mechanism.
CWE-127 commonly affects C, C++ and Memory-Unsafe. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.
MITRE documents real CVEs mapped to CWE-127, including CVE-2021-40985. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.
A CWE (Common Weakness Enumeration) like CWE-127 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.