CVE-2026-77236

Executive Summary

CVE-2026-77236 is a missing minimum size validation flaw in secure context allocation within FreeRTOS-Kernel versions prior to 11.3.1. The vulnerability allows a local user to trigger an out-of-bounds write by supplying an undersized stack size parameter, potentially corrupting secure-world heap metadata. With a CVSS score of 7.3 (High), this issue carries a high availability impact and a scope change from local to secure-world context. The vulnerability is confirmed as unpatched in the wild, though a fixed version (11.3.1) has been released.

Technical Analysis

FreeRTOS-Kernel implements a dual-world model (secure and non-secure) on ARM TrustZone-enabled systems. Secure context allocation is performed when creating tasks, queues, or other kernel objects that require isolation between normal-world and secure-world memory. The vulnerability stems from the absence of a minimum size validation check during the allocation of secure contexts. When an undersized stack size parameter is passed to the allocation routine, the system proceeds without verifying that the requested size meets the minimum threshold required for safe operation. This leads to a buffer that is too small to hold the secure context data structure, resulting in an out-of-bounds write (CWE-787) when the kernel initializes and writes metadata into the undersized buffer. The overflow corrupts adjacent heap metadata in the secure world, which can lead to memory corruption, system instability, or potential privilege escalation within the secure environment.

The attack surface is limited to local access — the attacker must be able to invoke the vulnerable API path, which typically requires existing code execution in the normal world at a user or kernel level. The scope change (S:C) in the CVSS vector indicates that the compromised system transitions from a local context to a broader security domain (the secure world), meaning the impact extends beyond the immediately compromised process. The confidentiality impact is rated low (C:N), suggesting no direct data disclosure, but the integrity (I:L) and availability (A:H) impacts reflect potential metadata corruption and system denial-of-service, respectively. Because patch confidence is 0/4, there is no evidence of an official vendor advisory, patch artifact, or public fix other than the version bump to 11.3.1.

Exploitation Scenarios

The attack chain for CVE-2026-77236 is inferred from the vulnerability class (missing size validation leading to out-of-bounds write) and the local access requirement.

1. Reconnaissance / Target Identification: An attacker with local access — either a malicious user on the device or a process that has compromised the non-secure operating system — first identifies the FreeRTOS-Kernel version running on the target. This can be done by querying system version strings, inspecting firmware images, or probing kernel configuration. Target devices include but are not limited to IoT devices, embedded controllers, medical equipment, industrial control systems, and any other hardware running FreeRTOS-Kernel before 11.3.1.

2. Triggering the Flaw: The attacker invokes the secure context allocation API with an undersized stack size parameter. Because the kernel fails to validate that the provided size meets the minimum required threshold, the allocation proceeds with insufficient memory. This is a direct parameter manipulation attack — no other vulnerability is needed to reach the vulnerable code path, provided the attacker has the necessary permissions to call the allocation routine.

3. Payload / Execution Method: After the undersized buffer is allocated, the kernel writes secure context metadata into it. The write operation exceeds the buffer bounds and corrupts adjacent heap metadata in the secure world. The attacker controls the content of the metadata being written partially (e.g., by influencing task attributes), which allows shaping the corruption. At the vulnerability-class level, this is a classic heap overflow — the overflow can be leveraged to overwrite pointers, headers, or other heap control structures. No public PoC is available, so the specific overwrite targets cannot be enumerated.

4. Post-Exploitation Impact: The primary impact is heap metadata corruption in the secure world, leading to a denial-of-service (A:H). Depending on the heap layout and the attacker's ability to influence the overflow content, more advanced outcomes such as secure-world code execution may be possible, though this is speculative given the lack of public exploit data. The scope change implies the attacker gains a foothold in a separate security domain, which could facilitate further attacks against TrustZone-protected assets such as cryptographic keys, secure storage, or other sensitive data.

Impact Assessment

The business impact of CVE-2026-77236 is significant for organizations deploying FreeRTOS-based devices. The CVSS score of 7.3 (High) reflects the following subscores:

• Attack Vector (L): Local — requires an attacker with local access, lowering the bar for exploitation in shared-device environments.
• Attack Complexity (L): Low — no special conditions are needed; a simple undersized parameter triggers the flaw.
• Privileges Required (L): Low — a low-privileged user or process can attempt exploitation.
• User Interaction (N): None — no user action is needed.
• Scope (C): Changed — exploitation in the normal world impacts the secure world, isolating the fallout to a more trusted domain.
• Confidentiality (N): None — no direct data disclosure.
• Integrity (I:L): Low — heap metadata corruption could allow limited data manipulation.
• Availability (A:H): High — the most likely outcome is a crash or system hang, rendering the device unusable.

Affected systems include any device running FreeRTOS-Kernel before 11.3.1, particularly those with ARM TrustZone or similar security partitions. The blast radius is broad across IoT, automotive, industrial, medical, and smart-home sectors. For critical infrastructure devices, a denial-of-service can translate directly into operational downtime, safety risks, or financial losses. Since the attack requires local access, remote attackers must first compromise the normal world environment (e.g., via a separate network-facing vulnerability) to chain this flaw effectively.

Detection Guidance

Detection of CVE-2026-77236 is challenging because the vulnerability manifests as a local out-of-bounds write in a memory-constrained embedded environment. The following detection rules focus on identifying anomalous stack size parameters, heap corruption indicators, or unusual kernel behavior.

Sigma Rule

title: FreeRTOS-Kernel Undersized Secure Stack Allocation
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to allocate secure contexts with undersized stack parameters in FreeRTOS-Kernel
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-77236
author: Valtersit Threat Research
date: 2026/01/15
logsource:
    product: linux
    category: process_creation
detection:
    selection:
        CommandLine|contains|all:
            - 'stack'
            - 'ctx_allocation'
    condition: selection
level: high

YARA Rule

rule FreeRTOS_CVE_2026_77236_stack_allocation
{
    meta:
        author = "Valtersit Threat Research"
        date = "2026-01-15"
        description = "Detects string patterns associated with undersized stack allocation in FreeRTOS-Kernel"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77236"
        severity = "high"
    strings:
        $a1 = "secure_context" ascii wide
        $a2 = "stack_size" ascii wide
        $a3 = "AllocateSecureContext" ascii wide
        $a4 = "pxNewTCB" ascii wide
    condition:
        any of them
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"FreeRTOS-CVE-2026-77236 Secure Context Stack Overflow Attempt"; content:"stack_size"; content:"secure_context"; reference:cve,2026-77236; classtype:attempted-admin; sid:123456; rev:1;)

Note: Because the vulnerability is triggered locally (not over the network), network detection is unlikely in practice. This rule is a placeholder for network signatures if the vulnerable API is exposed via a TCP-based service (e.g., debug interfaces or management protocols).

Elastic Detection

process where process.name == "FreeRTOS" and process.command_line contains "stack" and process.command_line contains "size" and process.args <= 1024

Note: This EQL rule targets host-based process logs where stack allocation routines are invoked with suspiciously small parameters. It assumes exported kernel symbols or logging of allocation calls.

Splunk SPL Query

index=os_logs sourcetype=linux:syslog
| where match(_raw, "(?i)secure_context.*stack_size")
| where _time > now() - 3600
| stats count by host, _raw
| search count > 0

Wazuh OSSEC Rule

<rule id="127001" level="10">
  <decoded>
    <pattern>secure_context</pattern>
    <pattern>stack_size</pattern>
  </decoded>
  <description>Potential exploitation of CVE-2026-77236: undersized stack allocation in FreeRTOS-Kernel</description>
  <group>free_rtos_vuln, cve_2026_77236,</group>
  <mitre>
    <id>T1203</id>
  </mitre>
  <referenced_cve>CVE-2026-77236</referenced_cve>
</rule>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED (no patch evidence from tracked sources)

The vendor-recommended remediation is to upgrade FreeRTOS-Kernel to version 11.3.1 or later.

1. Immediate Mitigations:

   • Enforce a policy that validates all stack size parameters passed to secure context allocation routines at the application level. Implement a wrapper or shim that rejects any request below the documented minimum.
   • Review and restrict local user access to devices running vulnerable versions. Since the attack requires local privileges, limiting local account access and hardening the normal world environment reduces the attack surface.
   • Monitor heap allocations and watch for abnormal patterns (e.g., undersized buffer requests) in kernel logs.

2. Patch/Upgrade Steps:

   • Upgrade to FreeRTOS-Kernel 11.3.1 or later, which includes the minimum size validation fix.
   • After upgrade, perform regression testing on all secure context allocations to ensure no legitimate code paths relied on undersized buffers.
   • Maintain an inventory of all embedded devices running FreeRTOS to track upgrade status and identify any remaining vulnerable instances.

3. Long-Term Hardening:

   • Enable memory protection features such as stack canaries, heap guard pages, and hardened allocators in the secure world to detect or prevent out-of-bounds writes.
   • Adopt secure coding standards for RTOS development that mandate input validation for all size and length parameters.
   • Conduct regular security audits of kernel configuration and allocation routines to identify similar missing-validation flaws.

References

1. NVD Entry for CVE-2026-77236
2. FreeRTOS-Kernel Releases (for version 11.3.1 patch reference)
3. CWE-787 — Out-of-bounds Write
4. ARM TrustZone Architecture Overview (secure/non-secure world context)

No public PoC, exploit code, or evidence of active exploitation in the wild is available as of the date of this report. Patch confidence remains at 0/4, and the fix is only referenced by the version bump in the NVD description.