Skynet just published an article: CWE-122: Heap-Based Buffer Overflow — When Memory Corruption Escapes the Heap Boundary – 7312.us and here’s my review of it.
The article is a competent short-form primer (3-minute read) that covers the basics of CWE-122 accurately, but it has notable gaps, a couple of subtle technical missteps, and one example that is straight-up wrong. Here’s a breakdown.
What the Author Gets Right
The fundamentals are solid. The definition — writing beyond the bounds of a heap-allocated buffer — is correct, and the relationship between CWE-121 (stack), CWE-122 (heap), and CWE-787 (broader out-of-bounds write umbrella) is described accurately. CWE-122 is indeed a child of CWE-787 in MITRE’s hierarchy.
The list of what gets corrupted is right and well-scoped: adjacent allocations, object fields, vtable pointers, and allocator metadata (free lists, chunk headers). Calling out that heap overflows are often more powerful than stack overflows because they can corrupt long-lived state is a good framing.
The modern exploitation techniques section names the right primitives: adjacent object corruption, vtable/function pointer overwrite, allocator metadata corruption, tcache/fastbin poisoning, and chaining with UAF. These are genuinely the techniques that show up in real CVEs against glibc-based targets.
The root-cause analysis is also reasonable — manual memory management, integer miscalculation (particularly important; this underlies a huge fraction of real-world heap overflows), and attacker-controlled allocation sizes are exactly the patterns to flag.
The mitigation list hits the right notes too: ASLR, DEP/NX, CFI, AddressSanitizer, hardened allocators, and the recommendation to prefer memory-safe languages.
What the Author Gets Wrong or Misleading
The “Safer” code example is wrong. This is the most concrete error:
char *buf = malloc(32);
strcpy(buf, input); // unsafe
snprintf(buf, 32, "%s", input); // "safer"
snprintf writing into a 32-byte buffer is fine, but the example as written never shows buf being allocated in the “safer” version — and more importantly, calling this “safer” without commentary glosses over that snprintf truncates silently, which can itself become a security or correctness bug (truncated paths, partial authentication tokens, etc.). The genuinely safer pattern is to check the return value of snprintf against the buffer size and treat truncation as an error.
The C++ “Better” example is also subtly wrong. std::vector<char> buf(input.begin(), input.end()) doesn’t null-terminate, so if buf.data() is later passed to anything expecting a C string, you’ve just moved the bug. std::string would be the more honest recommendation for string data.
“Exploit mitigations increase complexity, not safety” is misleading. Mitigations like ASLR, CFI, and hardened allocators demonstrably do increase safety — they don’t eliminate bugs, but they raise exploitation cost substantially. The intended point (mitigations are not a substitute for fixing bugs) is correct, but the phrasing as written is wrong and would mislead a junior developer into deprioritizing mitigation hygiene.
Important omissions. For a 2026 article, several things should be present and aren’t:
- No mention of MTE (Memory Tagging Extension) on ARM, which is the most significant new hardware defense against heap corruption and is shipping on Pixel devices and increasingly in server silicon.
- No mention of GWP-ASan or scudo, the production-deployable sampling/hardened allocators used in Android and Chrome.
- No mention of HWASan, which is more practical than ASan for production-like environments.
- No mention of integer overflow checks specifically (
-fsanitize=integer,__builtin_mul_overflow), despite correctly identifying integer miscalculation as a root cause. - No mention of CHERI, which is now in real silicon (Morello, and ARM’s announced direction) and is arguably the most important long-term answer to this class of bug.
The hardened-allocator section conflates defenses. “Heap cookies, safe unlinking, tcache hardening, guard pages” are listed as if they’re a checklist a developer turns on, but these are properties of the allocator (glibc has some, scudo has more, hardened_malloc more still). The article should tell developers to choose an appropriate allocator, not imply they can sprinkle these on.
The “Visual” sections are non-functional — they’re just words mashed together (“Heap Chunk AHeap Chunk BOverflow Corruption…”), which suggests something failed during publishing. Not a technical error, but it hurts the article.
Recommendations for Developers
Drawing on what the article got right and supplementing where it’s thin, here’s what actually matters:
First, default to a memory-safe language for new code. Rust, Go, C#, Swift, and Java all eliminate CWE-122 at the language level for the vast majority of code. The article is right to list these — treat C and C++ as the exception, not the default, when starting greenfield work. For existing C/C++ codebases, target the highest-risk components (parsers, decoders, network input handling) for incremental rewrites; this is the strategy Microsoft, Google, and the Linux kernel are actively pursuing.
Second, validate sizes before you allocate, and check arithmetic for overflow. A huge share of real-world heap overflows trace back to malloc(n * elem_size) where n * elem_size wrapped. Use __builtin_mul_overflow (or ckd_mul in C23) and calloc instead of multiplying yourself. Treat any attacker-influenced size as suspect until proven bounded.
Third, never use strcpy, strcat, sprintf, gets, or unbounded memcpy with attacker-controlled lengths. Prefer snprintf and bounded variants, and check their return values for truncation. In C++, prefer std::string, std::vector, and std::span over raw buffers and pointer arithmetic; in C, consider memcpy_s-style bounded copies or wrap your own with explicit length checks.
Fourth, run your code under sanitizers during development and CI. AddressSanitizer (ASan) and HWASan catch heap overflows reliably during testing. UBSan with integer overflow checks catches the upstream arithmetic bugs that lead to undersized allocations. These have meaningful overhead, but they belong in your test pipeline unconditionally.
Fifth, fuzz the parts of your code that touch untrusted input. Coverage-guided fuzzers (libFuzzer, AFL++, Honggfuzz) combined with ASan are extraordinarily effective at finding heap corruption. The article mentions fuzzing in passing; it deserves more weight — for parser/decoder code, fuzzing is arguably the single highest-ROI defensive practice available.
Sixth, choose a hardened allocator where you can. scudo (the default in Android and Fuchsia), hardened_malloc (GrapheneOS), and PartitionAlloc (Chrome) provide stronger guarantees than stock glibc. For deployment on ARMv9 hardware, enable MTE — it’s the closest thing to a silver bullet currently shipping. For long-term roadmaps, watch CHERI/Morello.
Seventh, turn on all the compiler and OS mitigations. ASLR, DEP/NX, stack canaries, full RELRO, fortify source (-D_FORTIFY_SOURCE=3), CFI (-fsanitize=cfi), and shadow stacks where supported. These don’t fix bugs, but they make the difference between a crash and a remote code execution. The article’s “mitigations increase complexity, not safety” line is the wrong takeaway — defense in depth genuinely works.
Eighth, when you do find a heap overflow, fix the root cause, not the symptom. A length check at the call site is fine, but ask whether the size calculation upstream is also wrong, whether sibling call sites have the same pattern, and whether the API itself should be redesigned to make the bug unrepresentable.
The article is a reasonable five-minute orientation for a developer who’s never heard of CWE-122. It would be a better starting point with the code examples corrected, the mitigation framing rebalanced, and modern defenses like MTE and hardened allocators given their due.
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