CVE-2023-31889

Executive Summary

CVE-2023-31889 is a denial-of-service (DoS) vulnerability discovered in the httpd component of the ASUS RT-AC51U router. The flaw allows a local attacker to crash the web server by sending a crafted GET request. The vulnerability affects firmware versions up to and including 3.0.0.4.380.8591. No patch has been confirmed by the vendor, and no public proof-of-concept (PoC) code is available. The EPSS score is 0.0%, indicating very low exploitation activity in the wild. This report provides a comprehensive technical analysis, detection guidance, and mitigation strategies.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned)
• Attack Vector: Local (AV:L)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H)
• Threat Level: Medium (Local DoS, no patch available)

Deep Dive & Mechanics

The vulnerability resides in the httpd web server of the ASUS RT-AC51U router. The httpd process fails to properly validate or sanitize incoming HTTP GET requests. By sending a specially crafted GET request, an attacker can trigger a buffer overflow, null pointer dereference, or infinite loop, causing the httpd process to crash. Since httpd is a critical service for the router's web management interface, its crash results in a denial of service for legitimate administrative access.

The attack is local, meaning the attacker must already have network access to the router's LAN interface (e.g., connected to the same Wi-Fi or Ethernet). No authentication is required to send the malicious request, but the attacker must be able to reach the router's IP address.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code is in the httpd binary of firmware version 3.0.0.4.380.8591. The exact vulnerable function is unknown, but it likely involves improper handling of long or malformed URI strings in the GET request.

Vulnerable code (hypothetical):

void handle_get_request(char *uri) {
    char buffer[256];
    strcpy(buffer, uri);  // No bounds checking
    // Process request...
}

Patched code (hypothetical):

void handle_get_request(char *uri) {
    char buffer[256];
    strncpy(buffer, uri, sizeof(buffer) - 1);  // Bounds checking
    buffer[sizeof(buffer) - 1] = '\0';
    // Process request...
}

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Obtain an ASUS RT-AC51U router with firmware 3.0.0.4.380.8591.
2. Connect to the router's LAN (e.g., via Ethernet or Wi-Fi).
3. Identify the router's IP address (default: 192.168.1.1).

Exploitation Steps:

1. Craft a malicious GET request with an overly long URI or special characters.
2. Send the request using a tool like curl or netcat:

   curl -v "http://192.168.1.1/$(python3 -c 'print("A"*10000)')"
   

3. Observe the httpd process crash. The web interface becomes unresponsive.
4. The router may reboot or require manual power cycle to restore service.

Note: No public exploit code is available. The above is a theoretical approach based on the vulnerability description.

Blue Team

Detection:

• Monitor for unusually long HTTP GET requests to the router's web interface.
• Use network intrusion detection systems (NIDS) to flag requests with URI lengths exceeding 2048 bytes.
• Enable logging on the router (if supported) to capture failed HTTP requests.

Response:

• Isolate the affected router from the network.
• Reboot the router to restore httpd service.
• Apply any future firmware patches from ASUS.
• Consider replacing the router if no patch is released.

Indicators of Compromise (IOCs)

• Network:
  • HTTP GET requests with URI length > 2048 bytes.
  • Repeated connection attempts to port 80/TCP on the router.
• System:
  • httpd process crash logs in the router's syslog.
  • Router web interface unresponsive.
• File:
  • No file-based IOCs due to the nature of the vulnerability.

Detection Engineering

YARA Rule

rule CVE_2023_31889_ASUS_RT_AC51U_DoS {
    meta:
        description = "Detects malicious HTTP GET requests targeting ASUS RT-AC51U httpd DoS"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-31889"
    strings:
        $http_get = "GET /"
        $long_uri = /GET \/[A-Za-z0-9]{2048,} HTTP/
    condition:
        $http_get and $long_uri
}

Sigma Rule

title: ASUS RT-AC51U httpd DoS Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP GET requests with excessively long URIs targeting ASUS RT-AC51U routers
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: asus_rt_ac51u
detection:
    selection:
        cs-method: 'GET'
        cs-uri-length: '>2048'
    condition: selection
falsepositives:
    - Legitimate long URLs from web applications
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET 80 (msg:"CVE-2023-31889 ASUS RT-AC51U httpd DoS Attempt"; flow:to_server; content:"GET"; http_method; content:"/"; http_uri; pcre:"/^\/[A-Za-z0-9]{2048,}/"; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Restrict access to the router's web interface to trusted IPs only. Disable remote management if not needed.
• Short-term: Monitor for firmware updates from ASUS. If no patch is released, consider using a third-party firmware like DD-WRT or OpenWrt (if available for RT-AC51U).
• Long-term: Replace the router with a supported model that receives regular security updates.
• Workaround: Use SSH or telnet for router management instead of the web interface, if available.

Valters IT
DevSecOps & Cyber Threat Intelligence