Skynet just published an article: CWE-120: Buffer Copy Without Checking Size of Input (“Classic Buffer Overflow”) – 7312.us and here’s my review of it.
Overall Assessment
The article is a solid, accurate primer on classic buffer overflows. It covers the right concepts at the right depth for an introductory piece, and most of its technical claims hold up. However, there are several areas where it oversimplifies, omits important nuance, or could mislead developers into a false sense of security.
What the Author Got Right
The fundamental mechanics are correctly described. The definition of CWE-120 as copying input into a fixed-size buffer without verifying the input fits is accurate, and the strcpy(buf, userInput) example is the canonical illustration. The list of memory regions that can be corrupted (adjacent variables, saved frame pointers, return addresses, function pointers) is correct and appropriately ordered by historical significance.
The relationship between CWE-120 and CWE-787 is described correctly. CWE-120 is indeed a more specific weakness focused on unchecked copy operations, while CWE-787 is the broader “out-of-bounds write” category. This is a distinction many writeups botch.
The exploitation techniques section is technically accurate. ROP as a bypass for NX/DEP, heap overflow chaining, partial overwrites, and data-only attacks are all real, current techniques. Calling out data-only exploitation is particularly good — it’s frequently overlooked in introductory material despite being increasingly important as control-flow protections improve.
The mitigation taxonomy is reasonable. Stack canaries, ASLR, DEP/NX, and CFI are correctly categorized as mitigations that reduce exploitability rather than fix the root cause. The recommendation to use sanitizers (ASan, UBSan, MSan) in CI is sound advice that many teams still neglect.
The warning that “safer” APIs like strncpy are routinely misused is correct and important. strncpy famously does not guarantee null termination if the source equals or exceeds the destination size, and pairing it with sizeof on a pointer rather than an array is a classic bug.
What the Author Got Wrong or Oversimplified
The snprintf example is presented as “safer” without caveats, but it has its own pitfalls. snprintf returns the number of bytes that would have been written, not the number actually written, and it can still produce truncated output that the program then treats as complete. Truncation bugs have caused real CVEs. The article should note that the return value must be checked against the buffer size to detect truncation.
The C++ example std::string dest = src; as “better” is misleading without context. If src is a const char* from untrusted input that isn’t null-terminated, this constructor will read past the intended bounds — itself a CWE-126 (out-of-bounds read) or worse. std::string is safer for managing storage, but it doesn’t magically validate input. The example needs a length parameter or a guarantee that src is properly terminated.
The list of “memory-safe languages” lumps Go, Java, and C# with Rust uncritically. All four prevent classic stack/heap buffer overflows in safe code, but they differ enormously in other respects. Go has data races that can cause memory corruption. Java and C# have JNI/P-Invoke escape hatches that reintroduce the entire problem. Rust’s unsafe blocks do the same. A more honest framing would say these languages eliminate the default path to buffer overflows but don’t eliminate them entirely.
The ROP description is dated. Modern exploitation more commonly uses JOP (jump-oriented programming), COOP (counterfeit object-oriented programming), or data-only attacks because CFI deployments (Intel CET, ARM BTI, Clang CFI) have made traditional ROP harder on hardened targets. The article mentions CFI as a mitigation but doesn’t connect it back to why exploitation has shifted.
The article omits CWE-120’s most important modern context: it’s a deprecated/discouraged CWE in many practices. MITRE’s own guidance increasingly steers analysts toward CWE-787 (out-of-bounds write) or CWE-121/122 (stack/heap-based buffer overflow) for more precise classification. CWE-120 is broad enough that it’s often used as a catch-all when finer classification would be better. Developers reading this article might not realize that.
No mention of integer overflow as a precursor. A huge fraction of real-world buffer overflows start with an integer overflow in a size calculation (e.g., malloc(count * size) where the multiplication wraps). Any serious treatment of CWE-120 should mention CWE-190 as a frequent root cause.
The “validate length before copy” advice is correct but underspecified. It doesn’t mention TOCTOU (time-of-check-to-time-of-use) issues, where the length is validated and then the source mutates before the copy. This matters in multithreaded code and in shared-memory scenarios.
No mention of compiler-level fortification. _FORTIFY_SOURCE (glibc) and equivalent features in MSVC catch many buffer overflow patterns at compile or runtime with essentially zero developer effort. Its omission is a notable gap given the article’s focus on practical mitigations.
Recommendations for Developers
For developers working with C and C++, the honest recommendation is to treat every fixed-size buffer as a potential CWE-120 site and write code that makes the bounds explicit at every boundary. Use snprintf and always check its return value against the buffer size to catch truncation. Prefer strlcpy/strlcat (BSD, available via libbsd on Linux) over strncpy because they guarantee null termination and report truncation. In C++, use std::string, std::string_view, and std::span (C++20) to carry length information alongside pointers, and avoid raw char* interfaces at module boundaries.
Enable every cheap defensive measure your toolchain offers. Compile with -D_FORTIFY_SOURCE=3 (or =2 on older glibc), -fstack-protector-strong, -fstack-clash-protection, -fcf-protection=full on x86, and the equivalent on other architectures. Link with -Wl,-z,relro,-z,now. On Windows, enable /GS, /guard:cf, and /CETCOMPAT. None of these fix root causes, but they raise the cost of exploitation substantially and catch some bugs outright.
Run AddressSanitizer and UndefinedBehaviorSanitizer in CI on every pull request, not just nightly. ASan catches buffer overflows that fuzzers and unit tests would otherwise miss. Pair it with libFuzzer or AFL++ for any code that parses untrusted input — fuzzing is genuinely the most effective way to find CWE-120 bugs in existing C/C++ code.
Validate sizes using checked arithmetic. Use __builtin_mul_overflow (GCC/Clang) or ckd_mul (C23) for size calculations involving multiplication, and reject inputs that would overflow before allocating. This closes the integer-overflow-to-buffer-overflow pipeline that the article doesn’t mention.
For new code, take the memory-safety question seriously. If the component handles untrusted input and doesn’t have hard performance constraints that rule it out, write it in Rust or another memory-safe language. CISA, the NSA, and the White House ONCD have all publicly recommended this direction, and it’s the only intervention that actually eliminates the bug class rather than mitigating its consequences.
For legacy C/C++ code you can’t rewrite, isolate it. Run parsers in separate processes with seccomp-bpf (Linux) or AppContainer/sandbox (Windows), drop privileges, and treat any crash as a security event. The article’s “isolate high-risk parsers” advice is right; the practical mechanisms deserve to be named.
Finally, when classifying findings in your own SDLC, prefer CWE-787, CWE-121, or CWE-122 over CWE-120 when you have enough information to be specific. It makes triage and metrics more useful over time.
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