CVE-2025-56424

Executive Summary

CVE-2025-56424 affects Insiders Technologies GmbH e-invoice pro, a commercial electronic invoicing solution. The vulnerability allows a remote attacker to cause a denial of service (DoS) by sending a crafted script to the application. The issue is present in versions prior to release 1 Service Pack 2. No public exploit code or proof-of-concept is currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild. The vendor has not released a public patch, but the advisory suggests upgrading to Service Pack 2.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby ecosystems. It affects a proprietary commercial software package from Insiders Technologies GmbH. Organizations using e-invoice pro should verify their deployment version and apply the vendor’s Service Pack 2 update.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which likely indicates that no official CVSS vector has been assigned by NVD. Based on the description (remote attacker, crafted script, denial of service), a reasonable estimated vector would be:

• Attack Vector (AV): Network (N) – remote exploitation possible.
• Attack Complexity (AC): Low (L) – crafted script likely requires minimal skill.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – affects only the vulnerable component.
• Confidentiality (C): None (N) – no data breach.
• Integrity (I): None (N) – no data modification.
• Availability (A): High (H) – service disruption.

Estimated CVSS v3.1 Score: 7.5 (High) – AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Threat Level: High – Remote, unauthenticated DoS can disrupt business operations.

Deep Dive & Mechanics

The vulnerability resides in the e-invoice pro application’s handling of user-supplied input. A remote attacker can send a specially crafted script (likely via HTTP request, API call, or file upload) that triggers an unhandled exception, infinite loop, or resource exhaustion, leading to a denial of service.

Potential Attack Vector:

• The application may parse XML, JSON, or other structured data for invoice processing.
• A crafted script could contain malformed data, recursive entities, or excessive nesting that overwhelms the parser.
• Alternatively, the script could exploit a buffer overflow or memory corruption in the processing logic.

Root Cause: Improper input validation (CWE-20) – the application fails to sanitize or limit the complexity of incoming data before processing.

Impact: Service unavailability, potential data loss if the crash occurs during write operations, and business disruption for organizations relying on e-invoicing.

Vulnerable vs. Patched Code

No specific code snippets are available. However, based on the vulnerability type, the patched version likely includes:

• Vulnerable: Input parsing without size limits or recursion depth checks.
• Patched: Added validation for input size, recursion depth, and safe parsing libraries.

Example (hypothetical):

# Vulnerable
def process_invoice(data):
    parsed = parse_xml(data)  # No limits
    # ... process

# Patched
def process_invoice(data):
    if len(data) > MAX_SIZE:
        raise ValueError("Input too large")
    parsed = parse_xml_safe(data, max_depth=10)
    # ... process

Exploitation & Lab Setup

Red Team

Objective: Cause denial of service on e-invoice pro server.

Prerequisites:

• Network access to the e-invoice pro service (e.g., HTTP endpoint, API).
• Knowledge of the service’s input format (e.g., XML invoice structure).

Steps:

1. Identify the target endpoint (e.g., /api/invoice/upload).
2. Craft a malicious payload – for example, an XML file with recursive entity expansion (Billion Laughs attack):

   <?xml version="1.0"?>
   <!DOCTYPE lolz [
     <!ENTITY lol "lol">
     <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
     <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
     ...
   ]>
   <root>&lol999;</root>
   

3. Send the payload using a tool like curl:

   curl -X POST -H "Content-Type: application/xml" --data-binary @malicious.xml http://target/api/invoice/upload
   

4. Monitor the target service – if it becomes unresponsive, the DoS is successful.

Alternative: Use a script that sends a large number of concurrent requests with malformed data to exhaust resources.

Blue Team

Detection:

• Monitor for unusually large or deeply nested XML/JSON payloads.
• Set up alerts for high CPU/memory usage on the e-invoice server.
• Use WAF rules to block requests with excessive entity expansions or recursion.

Mitigation:

• Immediately upgrade to e-invoice pro release 1 Service Pack 2.
• If patching is not possible, restrict network access to the service (e.g., VPN only).
• Implement input validation and rate limiting at the reverse proxy level.

Hunting:

• Search logs for requests with Content-Type: application/xml and unusually large body sizes.
• Look for repeated requests from the same IP within a short time window.

Indicators of Compromise (IOCs)

• Network:

  • HTTP POST requests with Content-Type: application/xml and body size > 10 MB.
  • Requests containing <!ENTITY or <!DOCTYPE in the payload.
  • Repeated requests to /api/invoice/upload or similar endpoints from a single IP.

• Host:

  • High CPU usage on the e-invoice server process.
  • Application crash logs or out-of-memory errors.
  • Temporary files with large sizes in the application’s temp directory.

• File:

  • Malicious XML files with recursive entity definitions (e.g., &lol; patterns).

Detection Engineering

YARA Rule

rule CVE_2025_56424_DoS_Crafted_Script {
    meta:
        description = "Detects crafted scripts targeting CVE-2025-56424 in e-invoice pro"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-56424"
    strings:
        $entity = /<!ENTITY\s+\w+\s+"[^"]*"/ nocase
        $doctype = "<!DOCTYPE" nocase
        $lol_pattern = /&lol\d*;/ nocase
    condition:
        any of ($entity) and $doctype and #lol_pattern > 5
}

Sigma Rule

title: CVE-2025-56424 - Denial of Service via Crafted Script
id: 8a3f2c1e-5d7b-4a9f-8c2e-1b3d4e5f6a7b
status: experimental
description: Detects HTTP requests with malicious XML payloads targeting e-invoice pro
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/api/invoice/upload'
        sc-status: 200
        cs-body|contains: 
            - '<!DOCTYPE'
            - '<!ENTITY'
    condition: selection
falsepositives:
    - Legitimate large XML invoices (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-56424 - e-invoice pro DoS attempt"; flow:to_server; content:"POST"; http_method; content:"/api/invoice/upload"; http_uri; content:"<!DOCTYPE"; http_client_body; content:"<!ENTITY"; http_client_body; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to e-invoice pro release 1 Service Pack 2 as per vendor advisory.
2. If upgrade is not possible, isolate the e-invoice server behind a firewall and restrict access to trusted IPs only.
3. Implement a Web Application Firewall (WAF) with rules to block XML entity expansion attacks.

Long-term Measures:

• Enforce input validation for all data processed by the application.
• Use safe parsing libraries that limit recursion depth and entity expansion.
• Conduct regular security audits and penetration testing.
• Monitor vendor advisories for future patches.

Workaround:

• Disable XML external entity (XXE) processing if the application allows it.
• Set resource limits (e.g., CPU, memory) for the e-invoice process.

Valters IT