CWE-124 - Buffer Underwrite ('Buffer Underflow')

Description: 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.

Common Consequences: Scope: Integrity, Availability
Impact: Modify Memory, DoS: Crash, Exit, or Restart
Notes: Out of bounds memory access will very likely result in the corruption of relevant memory, and perhaps instructions, possibly leading to a crash.
Scope: Integrity, Confidentiality, Availability, Access Control, Other
Impact: Execute Unauthorized Code or Commands, Modify Memory, Bypass Protection Mechanism, Other
Notes: 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.
Scope: Access Control, Other
Impact: Bypass Protection Mechanism, Other
Notes: When the consequence is arbitrary code execution, this can often be used to subvert any other security service.