CWE-733: Compiler Optimization Removal or Modification of Security-critical Code

Description

The developer builds a security-critical protection mechanism into the software, but the compiler optimizes the program such that the mechanism is removed or modified.

Potential Impact

Access Control, Other

Bypass Protection Mechanism, Alter Execution Logic

Demonstrative Examples

The following code reads a password from the
              user, uses the password to connect to a back-end
              mainframe, and then attempts to scrub the password from
              memory using memset().

Bad

void GetData(char *MFAddr) {
				 
				   char pwd[64];
				   if (GetPasswordFromUser(pwd, sizeof(pwd))) {
				   
					 if (ConnectToMainframe(MFAddr, pwd)) {
					 
					   // Interaction with mainframe
					 
					 }
				   
				   }
				   memset(pwd, 0, sizeof(pwd));
                 
				 }

The code in the example will behave
               correctly if it is executed verbatim, but if the code
               is compiled using an optimizing compiler, then the call to
               memset() might be removed as a dead store, because the
               buffer pwd is not used after its value is overwritten.
               Because the buffer pwd contains a sensitive
               value, the application may be vulnerable to attack if
               the data are left memory resident. If attackers are
               able to access the correct region of memory, they may
               use the recovered password to gain control of the
               system.

It is common practice to overwrite sensitive
               data manipulated in memory, such as passwords or
               cryptographic keys, in order to prevent attackers from
               learning system secrets. However, with the advent of
               optimizing compilers, programs do not always behave as
               their source code alone would suggest. In the example,
               the compiler interprets the call to memset() as dead
               code because the memory being written to is not
               subsequently used, despite the fact that there is
               clearly a security motivation for the operation to
               occur. The problem here is that many compilers, and in
               fact many programming languages, do not take this and
               other security concerns into consideration in their
               efforts to improve efficiency.

Attackers typically exploit this type of
               vulnerability by using a core dump or runtime mechanism
               to access the memory used by a particular application
               and recover the secret information. Once an attacker
               has access to the secret information, it is relatively
               straightforward to further exploit the system and
               possibly compromise other resources with which the
               application interacts.

Detection Methods

Black Box Limited — This specific weakness is impossible to detect using black box methods. While an analyst could examine memory to see that it has not been scrubbed, an analysis of the executable would not be successful. This is because the compiler has already removed the relevant code. Only the source code shows wh
White Box — This weakness is only detectable using white box methods (see black box detection factor). Careful analysis is required to determine if the code is likely to be removed by the compiler.

Real-World CVE Examples

CVE ID	Description
CVE-2008-1685	C compiler optimization, as allowed by specifications, removes code that is used to perform checks to detect integer overflows.
CVE-2019-1010006	Chain: compiler optimization (CWE-733) removes or modifies code used to detect integer overflow (CWE-190), allowing out-of-bounds write (CWE-787).

Frequently Asked Questions

What is CWE-733?

CWE-733 (Compiler Optimization Removal or Modification of Security-critical Code) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The developer builds a security-critical protection mechanism into the software, but the compiler optimizes the program such that the mechanism is removed or modified.

How can CWE-733 be exploited?

Attackers can exploit CWE-733 (Compiler Optimization Removal or Modification of Security-critical Code) to bypass protection mechanism, alter execution logic. This weakness is typically introduced during the Build and Compilation phase of software development.

How do I prevent CWE-733?

Follow secure coding practices, conduct code reviews, and use automated security testing tools (SAST/DAST) to detect this weakness early in the development lifecycle.

What is the severity of CWE-733?

CWE-733 is classified as a Base-level weakness (Medium abstraction). It has been observed in 2 real-world CVEs.