CWE-733 is an unusual weakness because the source code is correct. The developer builds a security-critical protection into the software, but the compiler optimizes the program so that the mechanism is removed or modified.
MITRE lists C, C++ and compiled languages in general. If you ship compiled code with security checks in it, this one is for you.
How it happens
Compilers may remove code that has no effect the language can observe. A check that depends on undefined behavior, or a write to memory that is never read again, can look like dead code. The optimizer deletes it.
MITRE's examples show the pattern. CVE-2008-1685 describes a C compiler optimization, allowed by the specifications, that removed code used to detect integer overflows. CVE-2019-1010006 is recorded as a chain in which an optimization removed or modified overflow detection code, allowing an out of bounds write.
// Risky: after the last use, the compiler may drop this "useless" write
char key[32];
use_key(key);
memset(key, 0, sizeof(key));
What an attacker gains
The consequence is a bypassed protection mechanism and altered execution logic. The overflow check or memory scrub you relied on is simply not in the binary.
How to find it
MITRE is blunt here. Black box testing cannot find it, because the compiler already removed the code. Examining memory might show a buffer was not scrubbed, but analyzing the executable alone will not explain why. Only white box methods work: review the source for security checks and scrubs that could be considered removable, then inspect the compiled output for them.
How to fix it
MITRE's record carries no mitigation list for this entry, so these are practical steps.
- Write overflow checks so they do not depend on undefined behavior, for example by testing before the arithmetic.
- Use a scrubbing function documented to resist optimization, such as explicit_bzero where your platform provides it.
- Test the optimized release build, not only the debug build, and confirm the protection is present in it.
Why it is easy to miss
This weakness breaks a comfortable assumption: that what you wrote is what runs. Code review sees the check. Unit tests built in debug mode see the check. Only the optimized release binary lacks it. That is why the build configuration you test matters as much as the source.
It also explains why the two records MITRE cites involve integer overflow checks. A check like "if a + b is smaller than a" depends on overflow behavior the language does not define for signed values, so the optimizer is allowed to assume it never happens and fold the check away. The out of bounds write that follows in the chain is then not a bug in the arithmetic, it is a protection that vanished.
A good habit is to treat every security check and every secret scrub as something to verify in the shipped artifact. Compile with the release flags, disassemble the function, and confirm the comparison or the write is still there. It takes a few minutes per critical routine and it is the only reliable detection, given that black box testing cannot see what the compiler removed.
Quick check
Why can a black box scan not find CWE-733?
Because the compiler removed the relevant code before the binary existed. There is nothing left in the executable to observe.
