The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.
View on MITREThis 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.
Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.
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.
When the consequence is arbitrary code execution, this can often be used to subvert any other security service.
Choose a language that is not susceptible to these issues.
All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
No detection method information available for this CWE.
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.
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.
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.
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.
buffer underwrite in firmware verification routine allows code execution via a crafted firmware image
View DetailsBuffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
View DetailsBuffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
View DetailsBuffer underflow resultant from encoded data that triggers an integer overflow.
View DetailsProduct sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
View DetailsNegative value is used in a memcpy() operation, leading to buffer underflow.
View DetailsNo relationship information available for this CWE.
CWE-124: Buffer Underwrite ('Buffer Underflow') is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer. 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.
If exploited, CWE-124 (Buffer Underwrite ('Buffer Underflow')) it can compromise Integrity, Availability, Confidentiality, Access Control and Other, leading to outcomes such as Modify Memory, DoS: Crash, Exit, or Restart, Execute Unauthorized Code or Commands, Bypass Protection Mechanism and Other.
Recommended mitigations for CWE-124 include: Choose a language that is not susceptible to these issues. All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
CWE-124 commonly affects C and C++. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.
MITRE documents real CVEs mapped to CWE-124, including CVE-2021-24018, CVE-2002-2227, CVE-2007-4580, CVE-2007-1584 and CVE-2007-0886. 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-124 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.