CVE-2025-65288

Executive Summary

CVE-2025-65288 describes a critical buffer overflow vulnerability in the Mercury MR816v2 router (firmware version 4.8.7 Build 110427 Rel 36550n). The device fails to validate the length of hostnames received from LAN hosts, leading to unchecked memory copies into fixed-size buffers. This flaw can be exploited by a local attacker to cause a denial of service (DoS) and potentially achieve remote code execution (RCE). No public proof-of-concept (PoC) or patch is currently available, and the vendor has not issued a security advisory. The vulnerability is classified as a classic buffer overflow (CWE-120) and is accessible from the local network, making it a significant risk for internal network compromise.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating insufficient data for scoring. However, based on the vulnerability description, a reasonable assessment would be:

• Attack Vector (AV): Adjacent Network (A) – requires LAN access.
• Attack Complexity (AC): Low (L) – no special conditions.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – vulnerability affects only the device.
• Confidentiality (C): High (H) – potential for code execution.
• Integrity (I): High (H) – potential for code execution.
• Availability (A): High (H) – DoS is immediate.

Estimated CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Estimated Base Score: 8.8 (High)

Threat Level: High – due to ease of exploitation from LAN and potential for RCE.

Deep Dive & Mechanics

The vulnerability resides in the hostname processing logic of the Mercury MR816v2 router. When a LAN host sends a DHCP request or other network packet containing a hostname, the device copies this hostname into a fixed-size buffer without checking its length. The affected code likely uses unsafe functions like strcpy() or sprintf() instead of bounded alternatives like strncpy() or snprintf().

Root Cause:

• The firmware does not enforce a maximum hostname length (typically 64 bytes per RFC 1035).
• The buffer is fixed-size (e.g., 64 bytes), but the input is copied without bounds checking.
• An attacker can send a hostname longer than the buffer, overwriting adjacent memory regions.

Impact:

• Denial of Service (DoS): Overwriting critical data structures causes the router to crash or become unresponsive.
• Remote Code Execution (RCE): By carefully crafting the overflow, an attacker can overwrite a return address or function pointer to redirect execution to arbitrary shellcode.

Attack Vector:

• The attacker must be on the same LAN as the vulnerable router.
• The attack can be triggered by sending a crafted DHCP request with an excessively long hostname option (option 12).
• No authentication is required.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is hypothetical but typical for such devices:

Vulnerable Code (C-like pseudocode):

void process_dhcp_hostname(char *hostname) {
    char buffer[64];
    strcpy(buffer, hostname); // Unsafe: no length check
    // ... further processing
}

Patched Code (hypothetical):

void process_dhcp_hostname(char *hostname, size_t len) {
    char buffer[64];
    if (len >= sizeof(buffer)) {
        // Log error and reject
        return;
    }
    strncpy(buffer, hostname, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... further processing
}

The patched version uses strncpy() with a length limit and ensures null-termination.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Mercury MR816v2 router with firmware 4.8.7 Build 110427 Rel 36550n.
• Attacker: Linux machine on the same LAN (e.g., Kali Linux).
• Tools: dhcpig, scapy, or custom Python script.

Exploitation Steps:

1. Craft a malicious DHCP request with an oversized hostname option.
2. Send the packet to the router's broadcast address (or directly to its IP if known).
3. Observe crash (DoS) or attempt to inject shellcode for RCE.

Example Python script using Scapy:

from scapy.all import *

# Craft DHCP packet with long hostname
hostname = "A" * 200  # Overflow buffer
ether = Ether(dst="ff:ff:ff:ff:ff:ff")
ip = IP(src="0.0.0.0", dst="255.255.255.255")
udp = UDP(sport=68, dport=67)
bootp = BOOTP(chaddr="00:11:22:33:44:55")
dhcp = DHCP(options=[("message-type", "discover"),
                     ("hostname", hostname),
                     ("end")])
packet = ether/ip/udp/bootp/dhcp
sendp(packet, iface="eth0")

Note: No public exploit code is available; this is a theoretical approach.

Blue Team (Defensive)

Detection:

• Monitor DHCP traffic for abnormally long hostname fields ( > 64 bytes).
• Use network intrusion detection systems (NIDS) to alert on oversized DHCP options.
• Enable logging on the router (if possible) to capture crash events.

Mitigation:

• Isolate the router on a separate VLAN with strict access controls.
• Apply firewall rules to limit DHCP traffic to trusted hosts only.
• Monitor for unusual router reboots or crashes.

Indicators of Compromise (IOCs)

• Network IOCs:

  • DHCP packets with hostname option length > 64 bytes.
  • Repeated DHCP DISCOVER or REQUEST packets from a single MAC address.
  • Router logs showing "buffer overflow" or "crash" events (if available).

• Host IOCs:

  • Unusual router behavior (frequent reboots, unresponsive web interface).
  • Unexpected open ports or processes on the router (post-exploitation).

• File IOCs:

  • Firmware binary with hardcoded unsafe functions (e.g., strcpy).

Detection Engineering

YARA Rule

rule CVE_2025_65288_DHCP_Hostname_Overflow {
    meta:
        description = "Detects DHCP packets with excessively long hostname fields targeting Mercury MR816v2"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-65288"
    strings:
        $dhcp_hostname = /hostname.{0,4}([A-Za-z0-9\-\.]{65,})/ nocase
    condition:
        $dhcp_hostname
}

Sigma Rule

title: DHCP Hostname Buffer Overflow Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects DHCP packets with hostname field exceeding 64 bytes, indicating possible CVE-2025-65288 exploitation
author: Valters IT
logsource:
    category: network
    product: dhcp
detection:
    selection:
        dhcp.option.hostname|length: > 64
    condition: selection
falsepositives:
    - Legitimate long hostnames (rare)
level: high

Suricata Rule

alert udp $HOME_NET any -> $EXTERNAL_NET 67 (msg:"CVE-2025-65288 DHCP Hostname Buffer Overflow Attempt"; content:"|63 68 61 6d 70 69 6f 6e|"; offset:0; depth:8; byte_test:1,>,64,0; classtype:attempted-admin; sid:1000001; rev:1;)

Note: The Suricata rule above is a simplified example. A more accurate rule would parse the DHCP option length field.

Remediation & Mitigation

Immediate Actions:

1. Isolate the device – Place the Mercury MR816v2 on a separate VLAN with no internet access.
2. Disable DHCP server if not needed, or restrict it to known MAC addresses.
3. Monitor logs for crashes or unusual DHCP traffic.

Long-term Solutions:

• Vendor patch: Await an official firmware update from Mercury. Contact vendor support.
• Replace device: Consider replacing with a more secure router that receives regular updates.
• Network segmentation: Use a firewall to block untrusted LAN hosts from sending DHCP requests to the router.

Workaround:

• Manually configure static IP addresses on all LAN hosts to avoid DHCP altogether.

Valters IT