Compiler Optimization Removal or Modification of Security-critical Code

Incomplete Base

Structure: Simple

Description

This vulnerability occurs when a compiler's optimization process unintentionally strips out or alters security-critical code that a developer intentionally wrote, leaving the application exposed.

Extended Description

Developers often add specific checks, like verifying a buffer size or clearing sensitive data from memory, as a deliberate security measure. However, when the code is compiled with high optimization levels (like -O2 or -O3), the compiler might analyze this security code and deem it 'unnecessary' for the program's core functionality. In an effort to make the software faster or smaller, the compiler removes or modifies these crucial safeguards, effectively creating a hidden vulnerability that wasn't present in the source code. This issue is particularly dangerous because the vulnerability is invisible in the source code review. It only manifests in the compiled binary, making it a silent failure. To mitigate this, developers must be aware of compiler-specific behaviors, use volatile qualifiers or compiler barriers (like `asm volatile("" ::: "memory")` in GCC) for critical operations, and test security-sensitive code with optimization enabled to ensure protections remain intact after compilation.

Common Consequences 1

Scope: Access ControlOther

Impact: Bypass Protection MechanismOther

Detection Methods 2

Black Box

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 whether the programmer intended to clear the memory or not, so this weakness is indistinguishable from others.

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.

Demonstrative Examples 1

ID : DX-200

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().

Code Example:

Bad

C

// 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, such as Microsoft Visual C++ .NET or GCC 3.x, then the call to memset() will be removed as a dead store because the buffer pwd is not used after its value is overwritten [18]. 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.

Observed Examples 2

CVE-2008-1685C compiler optimization, as allowed by specifications, removes code that is used to perform checks to detect integer overflows.

CVE-2019-1010006Chain: compiler optimization (Compiler Optimization Removal or Modification of Security-critical Code) removes or modifies code used to detect integer overflow (Integer Overflow or Wraparound), allowing out-of-bounds write (Out-of-bounds Write).

References 1

Writing Secure Code

Michael Howard and David LeBlanc

Microsoft Press

04-12-2002

https://www.microsoftpressstore.com/store/writing-secure-code-9780735617223

ID: REF-7

Applicable Platforms

Languages:

C : OftenC++ : OftenCompiled : Undetermined

Related Attack Patterns

Related Weaknesses

ChildOf:

Insecure Automated Optimizations (CWE-1038)