COMMON WEAKNESS ENUMERATIONIncomplete

CWE-124

Buffer Underwrite ('Buffer Underflow')

The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.

Abstraction

Base

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

The product writes to a buffer using an index or pointer that references a memory location prior to the beginning of the buffer.

Terminology

Alternate terms and usages

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 (CWE-127).

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

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.

Risk

What does successful exploitation lead to?

Modify MemoryDoS: Crash, Exit, or Restart

Affected security scopes: Integrity, Availability

Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.
Execute Unauthorized Code or CommandsModify MemoryBypass Protection MechanismOther

Affected security scopes: Integrity, Confidentiality, Availability, Access Control, Other

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.
Bypass Protection MechanismOther

Affected security scopes: Access Control, Other

When the consequence is arbitrary code execution, this can often be used to subvert any other security service.

Defence

How is it prevented and fixed?

Requirements

Security requirement

Detail
Choose a language that is not susceptible to these issues.

Implementation

Implementation-phase defence

Detail
All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.

Verification

How is it detected?

Automated Static Analysis

High
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.)

Automated Dynamic Analysis

Moderate
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].

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.