CWE-121 is the Common Weakness Enumeration entry for stack-based buffer overflows. It describes a memory-safety flaw where a program writes more data to a stack buffer than it can hold, overwriting nearby stack data such as saved frame information or return addresses. Because the stack is used for function calls and local variables, this kind of bug can destabilize a process, corrupt execution flow, or allow code execution if an attacker can shape the overwritten data.
In cyber security, CWE-121 matters because it often appears in network-facing services, parsers, and authentication code where attacker-controlled input reaches unsafe memory operations. Defenders look for it with secure coding reviews, fuzzing, and compiler and platform protections such as stack canaries, ASLR, and non-executable memory. The best fix is usually to remove the overflow condition itself by using bounds-checked APIs, validating input lengths, and applying patches to vulnerable software.