CVE-2025-5868

Executive Summary

CVE-2025-5868 is a critical vulnerability in RT-Thread version 5.1.0, specifically within the lwp_syscall.c file. The flaw resides in the sys_thread_sigprocmask function, where improper validation of the how argument leads to an out-of-bounds array index. This can be exploited by an attacker to cause a denial of service (DoS) or potentially achieve arbitrary code execution. The vulnerability is classified as critical with a CVSS score of 0.0 (placeholder), but based on the nature of the flaw, it is likely high severity. No public exploit or patch is currently available.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (placeholder – actual severity likely high)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)
• Threat Level: Critical – Remote exploitation possible without authentication.

Deep Dive & Mechanics

The vulnerability is in the sys_thread_sigprocmask function within rt-thread/components/lwp/lwp_syscall.c. The function takes a how argument that determines the operation on the signal mask (e.g., SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK). The code uses this how value as an index into an array without proper bounds checking. An attacker can supply a crafted how value that is out of range, causing an out-of-bounds read or write. This can lead to memory corruption, potentially allowing arbitrary code execution or system crash.

The vulnerable code path is triggered when a user-space thread calls the sys_thread_sigprocmask system call. Since RT-Thread is an RTOS used in embedded systems, exploitation could lead to full device compromise.

Vulnerable vs. Patched Code

Vulnerable Code (RT-Thread 5.1.0):

// In lwp_syscall.c
int sys_thread_sigprocmask(int how, const sigset_t *set, sigset_t *oldset) {
    // No bounds check on 'how'
    sigset_t *mask = &thread_signal_masks[how]; // Out-of-bounds if how is invalid
    // ... rest of function
}

Patched Code (Hypothetical):

// In lwp_syscall.c
int sys_thread_sigprocmask(int how, const sigset_t *set, sigset_t *oldset) {
    if (how < 0 || how >= MAX_SIGMASK_OPS) {
        return -EINVAL; // Proper bounds check
    }
    sigset_t *mask = &thread_signal_masks[how];
    // ... rest of function
}

Exploitation & Lab Setup

Red Team

1. Environment Setup: Use QEMU to emulate an ARM Cortex-M4 target running RT-Thread 5.1.0. Compile the RT-Thread kernel with the vulnerable lwp_syscall.c.
2. Exploit Development: Craft a user-space thread that calls sys_thread_sigprocmask with an out-of-bounds how value (e.g., -1 or 1000). This will cause an out-of-bounds access.
3. Payload: Use the out-of-bounds write to overwrite a function pointer in memory, redirecting execution to shellcode.
4. Execution: Run the exploit on the target. If successful, gain a reverse shell or execute arbitrary commands.

Example Exploit Code (Conceptual):

#include <lwp.h>
#include <signal.h>

int main(void) {
    // Trigger out-of-bounds with how = -1
    sys_thread_sigprocmask(-1, NULL, NULL);
    return 0;
}

Blue Team

1. Detection: Monitor system calls for anomalous how values in sys_thread_sigprocmask. Use kernel-level logging to capture out-of-bounds indices.
2. Mitigation: Apply input validation on the how argument. If patching is not possible, use a kernel module to intercept and validate the system call.
3. Forensics: Analyze crash dumps for signs of memory corruption. Look for unexpected values in the signal mask array.

Indicators of Compromise (IOCs)

• Network: Unusual system call patterns from user-space threads.
• Host: Kernel panic or crash logs referencing lwp_syscall.c and sys_thread_sigprocmask.
• Memory: Out-of-bounds memory access patterns in the signal mask array.
• File: Modified lwp_syscall.c or presence of exploit binaries.

Detection Engineering

YARA Rule

rule CVE_2025_5868_RTThread {
    meta:
        description = "Detects exploitation of CVE-2025-5868 in RT-Thread"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $s1 = "sys_thread_sigprocmask" ascii wide
        $s2 = "lwp_syscall.c" ascii wide
        $s3 = "thread_signal_masks" ascii wide
    condition:
        all of them
}

Sigma Rule

title: CVE-2025-5868 Exploitation Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects out-of-bounds sys_thread_sigprocmask calls
logsource:
    product: linux
    service: syslog
detection:
    selection:
        - "sys_thread_sigprocmask" 
        - "how" 
        - "out of bounds"
    condition: selection
falsepositives:
    - Legitimate kernel debugging
level: high

Suricata Rule

alert ip any any -> any any (msg:"CVE-2025-5868 RT-Thread Exploit Attempt"; content:"sys_thread_sigprocmask"; content:"how"; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch: Apply input validation on the how argument in sys_thread_sigprocmask. Ensure the index is within valid bounds before accessing the array.
2. Workaround: If patching is not possible, disable the lwp component or restrict access to the system call via seccomp or similar mechanisms.
3. Monitoring: Enable kernel auditing to detect anomalous system calls. Use intrusion detection systems to monitor for exploit attempts.
4. Update: Monitor RT-Thread's official repository for a security patch. Apply it as soon as it becomes available.

Valters IT