CVE-2026-21046 Overview
CVE-2026-21046 is a time-of-check time-of-use (TOCTOU) race condition in the fabricKeymaster trustlet on Samsung mobile devices. The flaw exists in versions prior to the Samsung Mobile Release (SMR) Jul-2026 Release 1. A local attacker with high privileges can win the race window between validation and use of a resource. Successful exploitation results in arbitrary code execution inside the trustlet context. The trustlet runs in the TrustZone Trusted Execution Environment (TEE), so code execution there compromises key material and cryptographic operations handled by Keymaster.
Critical Impact
Local privileged attackers can execute arbitrary code inside the fabricKeymaster TrustZone trustlet, breaching the boundary between the Rich Execution Environment and the Trusted Execution Environment.
Affected Products
- Samsung mobile devices running firmware prior to SMR Jul-2026 Release 1
- Devices using the fabricKeymaster trustlet for key management
- Samsung Android builds addressed by the July 2026 Samsung Mobile Security Update
Discovery Timeline
- 2026-07-10 - CVE-2026-21046 published to NVD
- 2026-07-11 - Last updated in NVD database
Technical Details for CVE-2026-21046
Vulnerability Analysis
The vulnerability is a time-of-check time-of-use race condition inside the fabricKeymaster trustlet. The trustlet validates the state of a resource, such as a buffer, pointer, or handle passed from the Rich Execution Environment. Between the validation step and the use step, an attacker races to modify the resource. The trustlet then operates on attacker-controlled data it believed was safe. This class of flaw is categorized under [CWE-367]. Because the trustlet executes in the ARM TrustZone Secure World, corruption of its execution flow bypasses OS-level isolation. Attackers gain the ability to run code with the trustlet's privileges, which typically include access to hardware-protected keys and cryptographic primitives.
Root Cause
The root cause is a missing atomicity guarantee between the check and the use of a shared resource in the trustlet. The trustlet accepts input from the Normal World through a shared memory region or IPC channel. It validates parameters once, then dereferences or acts on them without a second confirmation or without holding a lock. A concurrent thread controlled by the attacker mutates the resource after validation completes.
Attack Vector
Exploitation requires local access with high privileges on the device. The attacker must already control a component that can invoke the trustlet, such as a compromised system service or a process with Keymaster access. The attacker triggers a legitimate trustlet request while a second thread rewrites the shared input in the narrow window between validation and use. Winning the race redirects trustlet execution to attacker-supplied data.
No public exploit code or proof-of-concept has been published for CVE-2026-21046 at the time of writing. Refer to the Samsung Mobile Security Update for vendor-provided technical details.
Detection Methods for CVE-2026-21046
Indicators of Compromise
- Unexpected crashes or resets of the fabricKeymaster trustlet visible in kernel or TEE logs
- High-frequency Keymaster IPC calls from a single system-level process, indicating race-window probing
- Anomalous key operations that succeed without corresponding user authentication events
Detection Strategies
- Monitor Android system logs for repeated Keymaster errors, TEE communication failures, or trustlet reload events
- Alert on privileged processes issuing bursts of concurrent calls to Keymaster interfaces from multiple threads
- Correlate unusual cryptographic key usage patterns with process privilege changes on the device
Monitoring Recommendations
- Ingest mobile device telemetry into a centralized analytics platform to baseline Keymaster and TEE activity
- Track the installed Samsung security patch level across the mobile fleet and flag devices below SMR Jul-2026 Release 1
- Review mobile device management (MDM) attestation results for devices reporting failed or degraded TEE integrity
How to Mitigate CVE-2026-21046
Immediate Actions Required
- Apply the Samsung SMR Jul-2026 Release 1 update to all affected mobile devices as soon as the carrier makes it available
- Enforce minimum patch level policies through MDM so non-compliant devices lose access to sensitive corporate resources
- Audit installed applications and system services that hold elevated privileges and remove any that are unnecessary
Patch Information
Samsung addressed CVE-2026-21046 in SMR Jul-2026 Release 1. The fix is delivered through the standard monthly Samsung Mobile Security Update channel. See the Samsung Mobile Security Update advisory for the full list of affected models and patched builds.
Workarounds
- No vendor-supplied workaround exists; patching to SMR Jul-2026 Release 1 or later is the only supported remediation
- Reduce exposure by limiting the number of privileged applications that can invoke Keymaster on managed devices
- Require attestation-backed device posture checks before granting access to high-value applications and data
# Verify Samsung security patch level on a managed device
adb shell getprop ro.build.version.security_patch
# Expected value on remediated devices: 2026-07-01 or later
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
