CVE-2025-62526

Executive Summary

CVE-2025-62526 is a critical heap buffer overflow vulnerability in the ubusd daemon of OpenWrt, a Linux operating system for embedded devices. The flaw resides in the event registration parsing code, allowing an unauthenticated attacker to corrupt heap memory and potentially execute arbitrary code in the context of the ubusd daemon. Critically, the vulnerable code executes before ACL checks, meaning any ubus client can trigger the overflow. Additionally, a crafted subscription can bypass the listen ACL. The vulnerability affects all versions prior to OpenWrt 24.10.4. No public proof-of-concept (PoC) or exploit code is currently available, and the EPSS score is 0.0%, indicating low exploitation activity at this time. However, due to the high CVSS score (7.9) and the potential for remote code execution, immediate patching is strongly recommended.

CVSS Breakdown & Threat Level

• CVSS Score: 7.9 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required for exploitation.
• Privileges Required (PR): None (N) – No authentication is needed.
• User Interaction (UI): None (N) – No user action is required.
• Scope (S): Unchanged (U) – The exploit cannot affect other components.
• Confidentiality (C): High (H) – Full disclosure of sensitive data is possible.
• Integrity (I): High (H) – Complete compromise of system integrity.
• Availability (A): High (H) – Full denial of service or system crash is possible.

Threat Level: Critical. The vulnerability allows remote, unauthenticated code execution with high impact on confidentiality, integrity, and availability. The lack of ACL checks before the vulnerable code makes it especially dangerous.

Deep Dive & Mechanics

The ubusd daemon is the core message bus for OpenWrt's ubus inter-process communication system. It handles event subscriptions, which allow clients to register for specific events. The vulnerability exists in the event registration parsing code.

Root Cause: A heap buffer overflow occurs when parsing crafted event subscription messages. The ubusd daemon does not properly validate the size of the data being written to a heap buffer during event registration. An attacker can send a specially crafted message that causes the daemon to write beyond the allocated buffer, corrupting adjacent heap memory.

Attack Flow:

1. An attacker sends a malicious ubus message to the ubusd daemon (typically on TCP port 1883 or Unix socket).
2. The message is processed by the event registration parsing code before any ACL checks are performed.
3. The parsing code writes data beyond the bounds of a heap buffer, corrupting the heap.
4. The attacker can control the overflow data to overwrite critical heap metadata or function pointers.
5. This can lead to arbitrary code execution in the context of the ubusd daemon.

ACL Bypass: The crafted subscription also bypasses the listen ACL, meaning the attacker can subscribe to events they would normally be restricted from.

Vulnerable vs. Patched Code

The exact code changes are not publicly available, but the fix likely involves adding proper bounds checking in the event registration parsing routine.

Vulnerable Code (Conceptual):

// In ubusd_event.c or similar
void handle_event_subscription(struct ubus_client *cl, struct ubus_msg *msg) {
    char *buffer = malloc(256);
    // No size check on msg->data
    memcpy(buffer, msg->data, msg->data_len); // Heap buffer overflow
    // ... process subscription
}

Patched Code (Conceptual):

void handle_event_subscription(struct ubus_client *cl, struct ubus_msg *msg) {
    if (msg->data_len > 256) {
        // Reject oversized message
        return;
    }
    char *buffer = malloc(256);
    memcpy(buffer, msg->data, msg->data_len); // Safe
    // ... process subscription
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• OpenWrt device or emulator running a vulnerable version (< 24.10.4).
• Network access to the ubusd daemon (default port 1883 or Unix socket).
• Ability to send raw ubus messages.

Exploitation Steps:

1. Identify Target: Scan for OpenWrt devices with ubusd exposed (e.g., Shodan dork: OpenWrt ubusd).
2. Craft Malicious Message: Create a ubus message with an oversized data_len field in the event subscription section.
3. Send Message: Use a tool like ubus or a custom Python script to send the crafted message to the target.
4. Trigger Overflow: The heap buffer overflow corrupts memory, potentially allowing code execution.
5. Gain Shell: If successful, execute arbitrary commands as the ubusd daemon (often root).

Example Python Script (Conceptual):

import socket

def exploit(target_ip, target_port=1883):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, target_port))
    
    # Craft a malicious ubus message with oversized data
    # This is a simplified example; actual ubus protocol is more complex
    payload = b"\x00" * 1000  # Oversized data to trigger overflow
    sock.send(payload)
    sock.close()

if __name__ == "__main__":
    exploit("192.168.1.1")

Blue Team

Detection:

• Monitor for unusual ubus messages with abnormally large data lengths.
• Use network intrusion detection systems (NIDS) to flag malformed ubus packets.
• Enable logging on ubusd to capture suspicious subscription attempts.

Hunting:

• Search for processes crashing or exhibiting abnormal behavior (e.g., ubusd segfaults).
• Look for unexpected network connections to port 1883 from untrusted sources.
• Analyze heap memory dumps for signs of corruption.

Containment:

• Isolate affected devices from the network.
• Apply the patch (upgrade to OpenWrt 24.10.4) immediately.
• If patching is not possible, restrict network access to the ubusd daemon using firewall rules.

Indicators of Compromise (IOCs)

• Network IOCs:

  • Unusually large ubus messages (data length > 256 bytes) on port 1883.
  • Multiple failed subscription attempts from the same source IP.
  • Traffic to/from known malicious IPs associated with IoT botnets.

• Host IOCs:

  • ubusd process crashes or segfaults.
  • Heap corruption detected in system logs.
  • Unexpected processes spawned by ubusd.
  • Modified system binaries or configuration files.

• File IOCs:

  • Suspicious files in /tmp or /var/run created by ubusd.
  • Unusual entries in /proc/ or /sys/ indicating kernel exploitation.

Detection Engineering

YARA Rule

rule CVE_2025_62526_ubusd_heap_overflow {
    meta:
        description = "Detects heap buffer overflow in ubusd event registration"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-62526"
    strings:
        $ubus_magic = { 00 01 02 03 } // Example ubus magic bytes
        $overflow_pattern = { FF FF FF FF } // Example overflow indicator
    condition:
        $ubus_magic at 0 and #overflow_pattern > 5
}

Sigma Rule

title: CVE-2025-62526 ubusd Heap Overflow Detection
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects potential exploitation of CVE-2025-62526 via oversized ubus messages
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: suricata
detection:
    selection:
        dest_port: 1883
        payload_length: "> 256"
    condition: selection
falsepositives:
    - Legitimate large ubus messages (rare)
level: high

Suricata Rule

alert tcp any any -> any 1883 (msg:"CVE-2025-62526 ubusd Heap Overflow Attempt"; flow:to_server; content:"|00 01 02 03|"; depth:4; byte_test:4,>,256,4; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade all OpenWrt devices to version 24.10.4 or later.
2. Firewall: Restrict access to port 1883 (ubusd) to trusted networks only.
3. Monitor: Enable logging and monitoring for suspicious ubus traffic.

Long-Term Measures:

• Implement network segmentation for IoT devices.
• Use intrusion detection systems (IDS) to detect exploitation attempts.
• Regularly update firmware and apply security patches.
• Consider disabling ubusd if not required.

Workarounds:

• No official workarounds exist. The only mitigation is patching or network-level access control.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert