CVE-2026-77235 Overview
CVE-2026-77235 is a use-after-free vulnerability [CWE-416] in the FreeRTOS-Kernel secure context cleanup handler. The flaw stems from missing privilege verification in the Supervisor Call (SVC) handler responsible for secure context deallocation. Local users on affected devices can trigger the handler to free secure-world memory that remains referenced, producing a use-after-free condition inside the secure world. FreeRTOS is widely deployed across embedded and Internet of Things (IoT) devices, so exploitation can compromise the integrity and availability of trusted execution environments. Amazon Web Services (AWS) addressed the issue in FreeRTOS-Kernel version 11.3.1.
Critical Impact
Local attackers can corrupt secure-world memory in FreeRTOS-based devices, undermining the isolation guarantees of the trusted execution environment and potentially disrupting or compromising secure services.
Affected Products
- Amazon FreeRTOS-Kernel versions prior to 11.3.1
- Embedded and IoT devices using FreeRTOS with TrustZone-based secure context support
- Downstream products bundling vulnerable FreeRTOS-Kernel builds
Discovery Timeline
- 2026-08-21 - CVE-2026-77235 published to the National Vulnerability Database (NVD)
- 2026-08-27 - Last updated in NVD database
Technical Details for CVE-2026-77235
Vulnerability Analysis
The vulnerability resides in the FreeRTOS-Kernel handler that services SVC instructions for secure context cleanup. The handler deallocates secure-world memory associated with a task's secure context but fails to verify the caller's privilege level or ownership of the target context. A local, unprivileged task can issue the SVC and cause the kernel to free a secure context that is still in use, leaving dangling references inside the secure world. Subsequent operations that dereference those references produce a use-after-free condition, which can corrupt secure memory, crash secure services, or enable further exploitation of the trusted execution environment.
Root Cause
The root cause is missing authorization enforcement in the SVC handler for secure context deallocation. The handler trusts the caller without validating that the requesting task owns the secure context being released. This design gap violates the isolation contract between the non-secure and secure worlds on Arm TrustZone-enabled platforms.
Attack Vector
Exploitation requires local access on the device and low privileges. An attacker executing code as an ordinary task on the non-secure side invokes the vulnerable SVC to free a secure context that another task depends on. The attack does not require user interaction. Because embedded devices often run untrusted or third-party workloads alongside secure services, this local vector is realistic in multi-tenant firmware, updateable IoT gateways, and devices exposing scripting or plugin surfaces.
No public proof-of-concept exploit is available for CVE-2026-77235. See the GitHub Security Advisory GHSA-55pf-q87x-c58c for authoritative technical details.
Detection Methods for CVE-2026-77235
Indicators of Compromise
- Unexpected resets, hard faults, or MemManage exceptions originating from the secure world on FreeRTOS devices
- Anomalous invocations of the secure context cleanup SVC from tasks that do not own a secure context
- Firmware images reporting FreeRTOS-Kernel versions earlier than 11.3.1
Detection Strategies
- Inventory firmware and Software Bill of Materials (SBOM) data to identify FreeRTOS-Kernel builds below 11.3.1
- Instrument secure-side fault handlers to log SVC numbers, calling task identifiers, and context ownership at deallocation time
- Review kernel port code for calls into the secure context cleanup path and add assertions on caller identity
Monitoring Recommendations
- Aggregate device telemetry and crash reports into a centralized security data lake to identify clusters of secure-world faults
- Monitor for repeated task terminations correlated with secure context operations, which may indicate exploitation attempts
- Track vendor advisories from AWS and the FreeRTOS project for updated guidance and patch releases
How to Mitigate CVE-2026-77235
Immediate Actions Required
- Upgrade FreeRTOS-Kernel to version 11.3.1 or later across all affected firmware images
- Rebuild and redeploy downstream products that statically link the FreeRTOS-Kernel
- Restrict which tasks can execute the secure context cleanup SVC where kernel configuration permits
- Audit third-party or field-loadable code that runs on affected devices for use of secure context APIs
Patch Information
Amazon and the FreeRTOS project fixed CVE-2026-77235 in the FreeRTOS-Kernel V11.3.1 release. The patch adds privilege and ownership verification in the secure context cleanup handler. See the AWS Security Bulletin 2026-086 for vendor guidance and remediation details.
Workarounds
- Disable secure-side context support in kernel configuration on devices that do not require TrustZone isolation
- Enforce Memory Protection Unit (MPU) policies that prevent unprivileged tasks from issuing sensitive SVCs
- Limit deployment of untrusted or third-party tasks on devices that cannot be immediately upgraded
# Verify the FreeRTOS-Kernel version in your build tree
grep -R "tskKERNEL_VERSION_NUMBER" ./FreeRTOS-Kernel/include/task.h
# Update to the patched release
cd FreeRTOS-Kernel
git fetch --tags
git checkout V11.3.1
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
