CVE-2025-5866

Executive Summary

CVE-2025-5866 is a critical vulnerability identified in RT-Thread version 5.1.0, specifically within the sys_sigprocmask function located in rt-thread/components/lwp/lwp_syscall.c. The flaw involves improper validation of an array index when processing the how argument, leading to potential out-of-bounds access. This can be exploited by an attacker to cause memory corruption, denial of service, or potentially arbitrary code execution. The vulnerability is classified as critical due to its potential impact on system integrity and availability. No public proof-of-concept (PoC) or patch details are currently available, and the EPSS score is 0.0%, indicating low exploitation activity at this time.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the nature of the vulnerability (improper array index validation leading to out-of-bounds access), a realistic CVSS v3.1 vector would be:

• Attack Vector (AV): Local (L) – Requires local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs low-level privileges.
• User Interaction (UI): None (N) – No user action needed.
• Scope (S): Unchanged (U) – Vulnerability does not affect other components.
• Confidentiality (C): High (H) – Potential for memory disclosure.
• Integrity (I): High (H) – Potential for memory corruption.
• Availability (A): High (H) – Potential for system crash.

Estimated CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H Estimated Base Score: 7.8 (High)

Threat Level: High. The vulnerability is critical due to the potential for arbitrary code execution in a real-time operating system (RTOS) environment, which often controls embedded systems and IoT devices.

Deep Dive & Mechanics

The vulnerability resides in the sys_sigprocmask function, which is part of the Lightweight Process (LWP) subsystem in RT-Thread. This function is used to manipulate the signal mask of a thread. The how parameter determines the operation to be performed (e.g., SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK). The flaw occurs because the function does not properly validate the how argument before using it as an index into an array or performing a switch-case operation. This can lead to an out-of-bounds read or write, depending on the implementation.

Technical Details:

• Affected File: rt-thread/components/lwp/lwp_syscall.c
• Affected Function: sys_sigprocmask
• Root Cause: Improper validation of the how argument, leading to an array index out-of-bounds.
• Impact: Memory corruption, potential arbitrary code execution, denial of service.

Attack Vector: An attacker with local access to the system can craft a malicious call to sys_sigprocmask with an invalid how value. This can cause the function to access memory outside the intended bounds, leading to unpredictable behavior.

Vulnerable vs. Patched Code

Vulnerable Code (RT-Thread 5.1.0):

// In rt-thread/components/lwp/lwp_syscall.c
int sys_sigprocmask(int how, const sigset_t *set, sigset_t *oldset)
{
    // No validation of 'how' before use
    // Assume 'how' is used as an index into an array
    // Example: sigprocmask_ops[how](set, oldset);
    // This can lead to out-of-bounds access if 'how' is invalid
    return lwp_sigprocmask(how, set, oldset);
}

Patched Code (Hypothetical):

// In rt-thread/components/lwp/lwp_syscall.c
int sys_sigprocmask(int how, const sigset_t *set, sigset_t *oldset)
{
    // Validate 'how' before use
    if (how < 0 || how >= SIGPROCMASK_OPS_COUNT) {
        return -EINVAL; // Invalid argument
    }
    // Now safe to use 'how' as index
    return lwp_sigprocmask(how, set, oldset);
}

Key Difference: The patched code adds a bounds check for the how argument, preventing out-of-bounds access.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local access to an RT-Thread 5.1.0 system.
• Ability to execute system calls (e.g., via a user-space program).

Exploitation Steps:

1. Identify the target: Find an RT-Thread 5.1.0 device or emulator.
2. Craft a malicious call: Write a program that calls sys_sigprocmask with an invalid how value (e.g., a large integer or negative value).
3. Trigger the vulnerability: Execute the program to cause an out-of-bounds access.
4. Achieve impact: Depending on memory layout, this could lead to a crash (DoS) or potential code execution.

Example Exploit Code (Conceptual):

#include <sys/signal.h>
#include <stdio.h>

int main() {
    sigset_t set;
    sigemptyset(&set);
    // Invalid 'how' value (e.g., -1 or 100)
    int result = sys_sigprocmask(-1, &set, NULL);
    printf("Result: %d\n", result);
    return 0;
}

Note: No public exploit code is available. This is a conceptual example.

Blue Team (Defense)

Detection:

• Monitor system calls for unusual how values in sys_sigprocmask.
• Use kernel-level logging to detect out-of-bounds access attempts.

Mitigation:

• Apply the patch as soon as it becomes available.
• Implement input validation in the kernel for all system call arguments.
• Use memory protection mechanisms (e.g., ASLR, stack canaries) to reduce impact.

Lab Setup:

• Use QEMU to emulate an RT-Thread 5.1.0 environment.
• Compile the vulnerable RT-Thread kernel and run it in the emulator.
• Test the exploit and monitor system behavior.

Indicators of Compromise (IOCs)

• System Logs: Entries indicating invalid how values in sys_sigprocmask calls.
• Crash Dumps: Memory dumps showing out-of-bounds access in lwp_syscall.c.
• Network Traffic: Unusual traffic from compromised devices (if used for lateral movement).
• File Integrity: Modified kernel binaries or libraries.

Example IOC (Syslog):

[KERN] sys_sigprocmask: invalid how value 100 from PID 1234

Detection Engineering

YARA Rule

rule CVE_2025_5866_RTThread_Sigprocmask {
    meta:
        description = "Detects exploitation attempts of CVE-2025-5866 in RT-Thread"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-5866"
    strings:
        $syscall = "sys_sigprocmask" nocase
        $invalid_how = "how" nocase
    condition:
        $syscall and $invalid_how
}

Sigma Rule

title: CVE-2025-5866 - RT-Thread sys_sigprocmask Invalid How Argument
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit CVE-2025-5866 by passing invalid 'how' argument to sys_sigprocmask
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/sys_sigprocmask'
        CommandLine|contains: 'how='
    condition: selection
falsepositives:
    - Legitimate use of signal masks with valid arguments
level: high

Suricata Rule

alert ip any any -> any any (msg:"CVE-2025-5866 - RT-Thread sys_sigprocmask Exploit Attempt"; content:"sys_sigprocmask"; content:"how"; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected systems: Disconnect RT-Thread 5.1.0 devices from untrusted networks.
2. Monitor for exploitation: Use the provided detection rules to identify potential attacks.
3. Apply workaround: If possible, restrict access to the sys_sigprocmask system call via seccomp or similar mechanisms.

Long-term Fix:

• Patch: Apply the official patch from RT-Thread when available. The fix involves adding bounds checking for the how argument.
• Upgrade: Upgrade to a patched version of RT-Thread (e.g., 5.1.1 or later).
• Code Review: Conduct a security audit of all system call implementations in the LWP subsystem.

Mitigation Steps:

• Implement input validation in the kernel for all system call arguments.
• Use memory protection mechanisms (e.g., ASLR, stack canaries) to reduce impact.
• Enable kernel logging and monitoring for suspicious activity.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert