Your code checks permissions before it reads protected data. You wrote the check, you tested it, and it works. Then the processor, trying to be fast, runs ahead of the check and touches the data anyway.
CWE-1037 is the weakness behind that scenario. The developer builds a security critical protection mechanism into the software, but the processor optimizes the execution of the program so that the mechanism is removed or modified. MITRE lists it as a Base weakness on processor hardware, not specific to any language.
Most application developers will never write a line of hardware code. This weakness still matters to them, because mitigation lands in operating systems, compilers, runtimes, cloud platforms and sometimes in application code.
How it happens
Modern processors do not execute instructions strictly in the order you wrote them. They predict branches, execute instructions speculatively, and keep data in caches to avoid waiting on memory. When the guess is wrong, the processor discards the results. When the guess is right, the program runs faster.
The trouble is that discarded results can leave traces. MITRE says a successful exploitation will change the order of an application's execution, and will likely be used to bypass specific protection mechanisms. That bypass can then be used to potentially read data that would otherwise be inaccessible.
The observed examples on the MITRE page are the best known processor flaws:
- CVE-2017-5715 and CVE-2017-5753: Intel, ARM and AMD processor optimizations related to speculative execution and branch prediction cause access control checks to be bypassed when placing data into the cache. MITRE notes this is often known as Spectre.
- CVE-2017-5754: Intel processor optimizations related to speculative execution cause access control checks to be bypassed when placing data into the cache. Often known as Meltdown.
A related idea sits in the parent class, CWE-1038, insecure automated optimizations, which covers any mechanism that optimizes code in a way that violates a security assumption.
A conceptual picture of the bounds check pattern at the heart of one Spectre variant, written in C for illustration only:
// The check is correct in the source. Under speculation, the processor may
// execute the body before the comparison resolves, leaving traces in the cache.
if (index < array_length) {
value = array[index];
leak_probe = probe_table[value * 64];
}
The source code is not wrong in the usual sense. The weakness lives in the gap between what the language promises and what the hardware does.
What an attacker gains
MITRE's consequence entry is short and serious: integrity impact, bypass of a protection mechanism, and a high likelihood of exploit when the conditions are met.
In practical terms, a bypassed access control check can let code that is not allowed to read certain memory infer what is in it. That matters most where different parties share a processor, such as separate processes, virtual machines or tenants on shared hosts. The CVE descriptions themselves speak of access control checks being bypassed when data is placed in the cache.
How to find it
This is one of the hardest weaknesses in the catalog to detect. MITRE says that in theory it can be detected with white box testing, using specially crafted test cases together with debuggers to verify the order of statements and instructions as they actually execute.
That is expert work, and it points to a realistic division of labor:
- Processor and platform vendors find and fix the underlying flaw, often through microcode and firmware.
- Operating system and runtime teams ship software mitigations.
- You keep the stack patched and know which of your systems are exposed.
What you can check yourself:
- Inventory the processors, firmware and operating systems in use, including cloud instance types.
- Check whether your operating system reports mitigation status for known speculative execution issues.
- Review where untrusted code runs on the same hardware as sensitive workloads.
- Track the vendor advisories for the CVEs listed above, and the ones that followed.
How to fix it
The MITRE record has no formal mitigation list for this entry. The practical response follows from how the issues were handled and from the structure of the weakness.
- Apply platform updates. Install operating system, hypervisor, firmware and microcode updates as vendors release them, and track that they actually reached every host.
- Update compilers and runtimes where advised. Software mitigations often live in toolchains. Rebuilding with an updated toolchain may be part of the fix.
- Reduce sharing. Keep highly sensitive workloads on separate hosts or tenants from untrusted code where your risk assessment calls for it.
- Limit what untrusted code can do. Fewer places to run attacker influenced code means fewer ways to train the predictor.
- Keep an asset and dependency inventory. You cannot patch what you cannot list. The same inventory that answers a library CVE question answers a platform question.
- Assume the check is not the only layer. Do not keep secrets in memory longer than needed, and avoid placing very sensitive data in the same address space as untrusted code.
None of these steps remove the hardware behavior. They narrow who can exploit it.
Try it yourself
Platform and library vulnerabilities share one hard question: which of my systems are affected? This demo shows how an inventory of dependencies is built so you can answer it quickly.
Click through it at your own pace, or open it full screen.
For your own projects, run the free in-browser dependency scan at gold.safeguard.sh/scan. No login, your file stays on your machine, up to 100 dependencies.
Quick check
What makes CWE-1037 different from most weaknesses?
The flaw is in how the processor optimizes execution, not in the code's logic. A correct check in source can still be bypassed in hardware.
Which well known issues does MITRE list as examples?
Spectre, through CVE-2017-5715 and CVE-2017-5753, and Meltdown, through CVE-2017-5754.
How does MITRE say it can be detected?
In theory through white box testing, with crafted test cases and debuggers that verify the order of statements and instructions in execution.
