CVE-2021-25394: Samsung Android Use After Free Vulnerability

CVE-2021-25394 Overview

CVE-2021-25394 is a use-after-free vulnerability triggered by a race condition in the MFC (Multi-Function Charger) charger driver found in Samsung Android devices. This kernel-level vulnerability allows an attacker with compromised radio privileges to achieve arbitrary write capabilities, potentially leading to privilege escalation and device compromise.

Critical Impact

This vulnerability is actively exploited in the wild and is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Organizations using affected Samsung devices should prioritize immediate patching.

Affected Products

• Samsung Android 8.1
• Samsung Android 9.0 (SMR releases prior to MAY-2021 Release 1)
• Samsung Android 10.0 (SMR releases prior to MAY-2021 Release 1)
• Samsung Android 11.0 (SMR releases prior to MAY-2021 Release 1)

Discovery Timeline

• 2021-06-11 - CVE-2021-25394 published to NVD
• 2025-10-30 - Last updated in NVD database

Technical Details for CVE-2021-25394

Vulnerability Analysis

This vulnerability represents a classic use-after-free condition combined with a race condition in the Samsung MFC charger driver. The MFC charger driver is a kernel-level component responsible for managing the device's charging functionality, including communication with the charging hardware and power management.

The race condition occurs when multiple threads or processes attempt to access shared memory resources within the charger driver simultaneously without proper synchronization. When a memory object is freed by one execution path while another path still holds a reference to it, subsequent operations on that stale reference result in a use-after-free condition.

What makes this vulnerability particularly concerning is that successful exploitation allows arbitrary write operations. An attacker who has already compromised radio privileges can leverage this flaw to write to arbitrary memory locations within the kernel space, potentially modifying critical data structures, disabling security mechanisms, or achieving code execution with elevated privileges.

Root Cause

The root cause stems from inadequate synchronization mechanisms in the MFC charger driver code. The driver fails to properly protect shared resources during concurrent access scenarios, leading to a Time-of-Check Time-of-Use (TOCTOU) race condition. When memory is deallocated in one thread while another thread continues to reference that memory region, the subsequent access leads to undefined behavior that attackers can exploit.

The combination of CWE-416 (Use After Free) and CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization - Race Condition) indicates that the vulnerability requires precise timing to exploit but provides powerful primitive capabilities once triggered.

Attack Vector

The attack requires local access to the device and compromised radio privileges as a prerequisite. An attacker must first gain access to the radio subsystem, potentially through a separate vulnerability in the baseband or modem components. Once radio privileges are obtained, the attacker can trigger the race condition in the MFC charger driver by carefully timing operations to:

1. Initiate a charger driver operation that allocates memory
2. Race to free that memory while another operation still references it
3. Reallocate the freed memory with attacker-controlled content
4. Trigger the use-after-free to write arbitrary values

The vulnerability mechanism involves manipulating the timing of charger driver operations to create a window where memory state becomes inconsistent. While the local attack vector and high privilege requirements increase the difficulty of exploitation, the confirmed in-the-wild exploitation demonstrates that determined adversaries can successfully leverage this flaw. See the Samsung Mobile Security Update for additional technical details.

Detection Methods for CVE-2021-25394

Indicators of Compromise

• Unusual kernel crashes or panics related to the MFC charger driver modules
• Unexpected privilege escalation attempts from processes with radio permissions
• Abnormal memory allocation patterns in charger-related kernel components
• System logs showing driver errors or memory corruption in charging subsystems

Detection Strategies

• Monitor for kernel oops or panic messages referencing MFC charger driver functions
• Implement behavioral analysis to detect unusual process activity from radio subsystem components
• Deploy endpoint detection solutions capable of monitoring kernel-level memory operations
• Check device firmware versions against the SMR MAY-2021 Release 1 baseline

Monitoring Recommendations

• Enable comprehensive kernel logging to capture driver-related anomalies
• Implement mobile device management (MDM) solutions to track firmware versions across the fleet
• Monitor for indicators of baseband or modem compromise that could precede exploitation
• Review security logs for signs of privilege escalation from radio-privileged processes

How to Mitigate CVE-2021-25394

Immediate Actions Required

• Update all affected Samsung devices to SMR MAY-2021 Release 1 or later immediately
• Prioritize patching due to active exploitation and CISA KEV listing
• Inventory all Samsung Android devices to identify those running vulnerable firmware versions
• Implement network segmentation to limit exposure of unpatched devices

Patch Information

Samsung has addressed this vulnerability in the SMR MAY-2021 Release 1 security maintenance release. The fix implements proper synchronization mechanisms in the MFC charger driver to prevent the race condition from occurring. Organizations should apply the update through standard Samsung device update channels or enterprise MDM solutions.

For detailed patch information, refer to the Samsung Mobile Security Update for May 2021.

This vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, which mandates federal agencies to remediate within prescribed timeframes.

Workarounds

• No direct workarounds are available; patching is the only complete remediation
• Restrict physical access to affected devices until patches can be applied
• Monitor devices for signs of compromise and isolate suspicious systems
• Consider temporarily removing affected devices from sensitive network segments until updates are deployed

# Verify Samsung device firmware version via ADB
adb shell getprop ro.build.PDA
adb shell getprop ro.build.version.security_patch
# Compare output against SMR MAY-2021 Release 1 baseline
# Security patch level should be 2021-05-01 or later