MITRE’s description of CWE-120: The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
Common consequences MITRE lists
- Integrity, Confidentiality, Availability: Modify Memory; Execute Unauthorized Code or Commands
- Availability: Modify Memory; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)
Copying input into a fixed-size buffer without checking its length overwrites adjacent memory, the classic path to crashes and code execution in C and C++.
- Replace unbounded copies such as
strcpy,strcat,sprintfandgetswith size-aware alternatives, and check the source length first. - Prefer memory-safe languages or containers that track their own size.
- Enable compiler hardening and fuzz the code paths that copy external input.
Guides on this site
Find, fix and contain memory-safety bugs in C and C++
Recognize out-of-bounds access, use after free, NULL dereference and integer overflow in C and C++, fix them with checked lengths and single ownership, harden release builds, catch the rest with sanitizers and fuzzing, and plan new code in a memory-safe language.
Fuzz a parser and turn crashes into lasting regressions
Build a narrow harness, observe memory and resource failures, minimize the input, and test the repaired parser.
Practice
- Buffer over-read lab: Stop a heartbeat over-read · Browser lab
- Parse bounded JSON and XML invoices safely · Local project
- Repair a native parser using sanitizer evidence · Local project
References
- CWE-120 in the MITRE CWE list (version 4.20)
- 2025 CWE Top 25 Most Dangerous Software Weaknesses (MITRE CWE Program)
List data from MITRE; the name, definition and consequences are from MITRE’s CWE 4.20 catalogue. A guide is listed as the main subject when the weakness is what the guide is about, and as covering it when a dedicated section or worked example addresses it.
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