Buffer Underwrite ('Buffer Underflow')
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
This vulnerability occurs when a program miscalculates a write position, placing it outside the allocated buffer's bounds in the 'backward' direction. Common causes include decrementing a pointer or index too many times, performing incorrect pointer arithmetic that results in a negative offset, or directly using a negative index to access an array. The result is a write operation to an unintended, and often critical, memory location. Writing to memory before a buffer can corrupt other program data, function pointers, or critical control structures. This can lead to system crashes, unpredictable behavior, or, if exploited, allow an attacker to hijack the program's execution flow. Developers should rigorously validate all pointer arithmetic and array indices, ensuring they remain within the valid, zero-based range of the allocated buffer.
Impact: Modify MemoryDoS: Crash, Exit, or Restart
Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.
Impact: Execute Unauthorized Code or CommandsModify MemoryBypass Protection MechanismOther
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.
Impact: Bypass Protection MechanismOther
When the consequence is arbitrary code execution, this can often be used to subvert any other security service.
ID : DX-87
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.Code Example:
c// 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';
c// return string without trailing whitespace* retMessage = message; return retMessage;}
ID : DX-88
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.Code Example:
cMedium
buffer underrun
Some prominent vendors and researchers use the term "buffer underrun". "Buffer underflow" is more commonly used, although both terms are also sometimes used to describe a buffer under-read (Buffer Under-read).- Memory Management
- Memory
- PLOVER
- CLASP
- Software Fault Patterns