CVE-2022-28693 - Exploiting Unprotected Return Branch Target Prediction in Intel® CPUs

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CVE-2022-28693 is a processor vulnerability quietly affecting several Intel® CPUs. This flaw allows local attackers to potentially disclose sensitive information by abusing an unprotected alternative channel for return branch target prediction. In simple words, someone already on your computer could trick the processor into leaking secrets through a clever combination of code and processor quirks.

This post breaks it down in simple terms, shares code snippets to show how the flaw works, and points you to references for deeper dives.

What's the Problem?

Modern Intel® CPUs use *speculative execution* and *branch prediction* to maximize speed. When a function uses a ret (return) instruction, the processor gueses where it will go next using something called the Return Stack Buffer (RSB).

For security, especially after Spectre, Intel designs processors to clear or protect branch targets after context changes, like moving from kernel to user space, so secrets don't leak accidentally.

But in some CPUs, an alternative and unprotected channel for return branch prediction is used. If that channel isn't sanitized at the right time, a motivated user could create a scenario where the CPU "remembers" branch targets from privileged code (like the OS kernel), and uses that prediction when running regular user code. That might let secrets leak, like cryptographic keys or other private info.

Poison the Prediction Buffer: Influence return address predictions from user space.

2. Trigger Speculative Execution: Trick the CPU into executing sensitive code paths and speculatively load secrets.
3. Side Channel Extraction: Use cache timing attacks (like Spectre) to infer secrets brought into the cache during speculation.

Basic Exploit Flow

// This is a simplified, educational version:
void victim_function(size_t idx) {
    if (idx < array1_size) {
        // Secret-dependent access!
        uint8_t secret = array1[idx];
        uint8_t dummy = array2[secret * 512];
    }
}

void attack() {
    size_t malicious_idx = (size_t)secret_offset; // Offset to secret
    flush(&array1_size);              // Ensure array1_size is uncached
    // Train the RSB by repeatedly calling with valid index
    for (int i = ; i < 10; i++) {
        victim_function(valid_idx);
    }
    // Flip to malicious index (speculative execution will leak)
    victim_function(malicious_idx);
    // Use cache timing to infer secret byte
    for (int i = ; i < 256; i++) {
        if (flush_reload(&array2[i * 512])) {
            printf("Leaked byte: %d\n", i);
        }
    }
}

This demonstrates how an attacker trains the RSB, then executes victim_function with a malicious index, causing the speculative path to leak a secret into the cache by using an unprotected prediction buffer.

Not remote: Cannot be exploited over the network directly.

- Affects multi-user environments: Like cloud servers, where one user could potentially spy on another user's data.

Many Intel CPUs, especially older models, are affected.

- Use the Intel Security Advisory to check your processor.

Mitigation

Intel has released microcode updates for vulnerable CPUs. Install all firmware and OS patches. Some operating systems may add extra RSB or branch predictor flushing after privilege changes.

- Intel Microcode Download Center

To check on Linux

grep . /sys/devices/system/cpu/vulnerabilities/*

References & Further Reading

- Intel Security Advisory - INTEL-SA-00656 (CVE-2022-28693)
- Spectre and Meltdown FAQs
- Return Stack Buffer (RSB) Attacks
- Openwall - Spectre, Meltdown, and more

Conclusion

CVE-2022-28693 highlights how deep processor internals can create surprising security issues. Even with mitigations for Spectre, attackers keep finding subtle processor features to exploit. Keeping your system firmware and OS up to date is the best protection. As always in security, never trust user processes to behave! If you’re a system admin for servers or cloud infrastructure—patch early, patch often.


*Stay safe and keep learning. For more deep-dives, follow this blog!*

Timeline

Published on: 02/14/2025 21:15:13 UTC