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.
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.
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.
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.
No mitigation information available for this CWE.
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 example asks a user for an offset into an array to select an item.
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).
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 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-786: Access of Memory Location Before Start of Buffer is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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. 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-786 (Access of Memory Location Before Start of Buffer) it can compromise Confidentiality, Integrity and Availability, leading to outcomes such as Read Memory, Modify Memory, DoS: Crash, Exit, or Restart and Execute Unauthorized Code or Commands.
CWE-786 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-786, including CVE-2002-2227, CVE-2007-4580, CVE-2007-1584, CVE-2007-0886 and CVE-2006-6171. 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-786 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.