CVE-2021-26341: AMD Athlon X4 940 Information Disclosure

CVE-2021-26341 Overview

CVE-2021-26341 is a speculative execution vulnerability affecting a wide range of AMD processors, including consumer-grade Ryzen and Athlon series as well as enterprise EPYC server processors. The vulnerability allows affected AMD CPUs to transiently execute instructions beyond unconditional direct branches, potentially resulting in sensitive data leakage through side-channel analysis.

This class of vulnerability, often associated with Spectre-like attacks, exploits the speculative execution behavior of modern processors. When the CPU speculatively executes instructions that should not be reached (past unconditional branches), it may temporarily access and cache sensitive data. Even though the speculative results are eventually discarded, the timing characteristics of cache accesses can be measured by an attacker to infer the leaked data.

Critical Impact

Local attackers with code execution privileges can potentially extract sensitive information from privileged memory regions by exploiting speculative execution past unconditional branches, compromising data confidentiality across security boundaries.

Affected Products

• AMD Ryzen Desktop Processors (2000, 3000, 4000, 5000 series)
• AMD Ryzen Mobile Processors (2000, 3000, 4000, 5000 series)
• AMD Ryzen Threadripper and Threadripper Pro Processors
• AMD EPYC Server Processors (1st, 2nd, and 3rd generation)
• AMD Athlon X4 and Athlon Gold/Silver/Pro Processors
• AMD A-Series APUs (A4, A6, A9, A10, A12 series)

Discovery Timeline

• March 11, 2022 - CVE-2021-26341 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-26341

Vulnerability Analysis

This vulnerability is rooted in the speculative execution capabilities of AMD processors. Modern CPUs use speculative execution to improve performance by predicting the outcome of branch instructions and executing instructions ahead of time. However, in affected AMD processors, the branch predictor may incorrectly continue speculative execution past unconditional direct branches (such as JMP instructions).

When this occurs, the processor may speculatively access memory locations that should never be reached during normal program execution. Although these speculative accesses are architecturally discarded when the misprediction is detected, they leave observable traces in the processor's cache hierarchy. An attacker can exploit these timing differences using cache side-channel techniques to extract sensitive information.

The impact is primarily focused on confidentiality, where an attacker with local access could potentially read sensitive data from kernel memory, other processes, or across virtualization boundaries. This makes the vulnerability particularly concerning for cloud computing environments where multiple tenants share the same physical hardware.

Root Cause

The vulnerability stems from the AMD branch predictor's handling of unconditional direct branches. The processor's speculative execution engine fails to properly recognize that certain instruction sequences should never be reached, allowing transient execution of code paths past unconditional jumps. This architectural behavior in the branch prediction unit creates a window where sensitive data can be loaded into cache during speculative execution, enabling subsequent side-channel extraction.

The fundamental issue is classified under CWE-212 (Improper Removal of Sensitive Information Before Storage or Transfer), as the speculative execution leaves traces of sensitive data in microarchitectural state that can be observed by an attacker.

Attack Vector

Exploitation of CVE-2021-26341 requires local access to the target system with the ability to execute code. The attacker must craft specific instruction sequences that trigger misprediction past unconditional branches, combined with carefully timed cache probing techniques to extract the leaked data.

A typical attack scenario involves:

1. The attacker executes a carefully crafted code sequence containing unconditional branches followed by gadgets that speculatively access sensitive memory
2. The CPU speculatively executes past the unconditional branch, accessing protected data
3. The sensitive data is loaded into the cache during speculative execution
4. The attacker uses timing-based side-channel techniques (such as Flush+Reload or Prime+Probe) to determine which cache lines were accessed
5. By repeating this process, the attacker can reconstruct the sensitive data byte-by-byte

This attack does not require any user interaction and operates entirely through the timing analysis of CPU cache behavior. In virtualized environments, an attacker in one VM could potentially extract data from the hypervisor or other VMs sharing the same physical processor.

Detection Methods for CVE-2021-26341

Indicators of Compromise

• Unusual patterns of precise timing measurements or high-resolution timer usage in processes
• Processes repeatedly executing code sequences with unconditional branches followed by memory access operations
• Abnormal CPU cache activity patterns that may indicate cache probing attempts
• Elevated use of performance monitoring counters related to branch mispredictions

Detection Strategies

• Monitor for processes utilizing high-resolution timers or performance counters that could enable timing attacks
• Deploy hardware performance counter monitoring to detect abnormal branch misprediction rates and cache miss patterns
• Utilize kernel integrity monitoring to detect attempts to access privileged memory regions
• Implement behavioral analysis for code patterns consistent with Spectre-variant attacks

Monitoring Recommendations

• Enable enhanced auditing on systems with sensitive workloads to track process behavior anomalies
• Deploy endpoint detection solutions capable of identifying side-channel attack patterns
• Monitor virtualization environments for cross-VM information leakage indicators
• Regularly review system logs for signs of exploitation attempts or unusual process behavior

How to Mitigate CVE-2021-26341

Immediate Actions Required

• Apply the latest BIOS/UEFI firmware updates from system vendors that incorporate AMD's microcode mitigations
• Update operating system kernels to versions that include software-based mitigations for this vulnerability
• Review the AMD Product Security Bulletin for processor-specific guidance
• Prioritize patching for systems processing sensitive data or hosting multi-tenant workloads

Patch Information

AMD has released microcode updates and AGESA firmware updates to address this vulnerability. Organizations should obtain the appropriate BIOS updates from their system or motherboard vendors. Additionally, operating system vendors have released kernel patches that implement software mitigations for this class of vulnerability.

For Linux systems, check for kernel updates that address AMD speculative execution vulnerabilities. Windows systems should have the latest cumulative updates applied. Consult your operating system vendor's security advisories for specific patch information.

Workarounds

• Enable Retpoline mitigations in the kernel if available, which can help reduce speculative execution attack surface
• Implement process isolation techniques to limit the impact of potential data leakage between security domains
• In virtualized environments, consider pinning VMs to specific CPU cores to reduce cross-tenant exposure
• Disable high-resolution timers for untrusted processes where possible to increase the difficulty of timing attacks

# Check current mitigation status on Linux
cat /sys/devices/system/cpu/vulnerabilities/*

# Verify AMD microcode version is updated
dmesg | grep microcode

# Check kernel for speculative execution mitigations
grep -r . /sys/devices/system/cpu/vulnerabilities/ 2>/dev/null