CVE-2025-6653

Executive Summary

CVE-2025-6653 is an information disclosure vulnerability in PDF-XChange Editor, specifically within the parsing of PRC (Product Representation Compact) files. The flaw is an out-of-bounds read (CWE-125) that occurs due to insufficient validation of user-supplied data, allowing an attacker to read past the end of an allocated buffer. This can lead to the disclosure of sensitive memory contents. While the CVSS score is low (3.3), the vulnerability can be chained with other flaws to achieve arbitrary code execution. User interaction is required, as the target must open a malicious PRC file or visit a malicious page. No public exploit code or proof-of-concept is currently available, and the vendor has not released a patch.

Supply Chain & Package Security

This vulnerability does not affect any open-source package registries (NPM, PyPI, Go, Ruby). It is a proprietary desktop application (PDF-XChange Editor) and is not part of a software supply chain ecosystem.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
• Base Score: 3.3 (Low)
• Attack Vector: Local (AV:L) – The attacker must deliver a malicious file to the target system.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: Required (UI:R) – The victim must open the malicious file.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: Low (C:L) – Limited information disclosure.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: None (A:N) – No denial of service.

Threat Level: Low. However, the potential for chaining with other vulnerabilities to achieve code execution elevates the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability resides in the PRC file parsing module of PDF-XChange Editor. PRC files are 3D model files used in PDF documents. When the application processes a crafted PRC file, it fails to validate the length of user-supplied data before reading from an allocated buffer. This results in an out-of-bounds read, where the application reads memory beyond the intended buffer boundary.

Root Cause: Lack of proper bounds checking on a user-controlled size field within the PRC file structure. The application trusts the size value without verifying it against the actual buffer length.

Impact: An attacker can read sensitive data from the process memory, such as:

• Heap metadata
• Other file contents loaded in memory
• Potentially credentials or encryption keys

Chaining Potential: The ZDI advisory notes that this can be leveraged with other vulnerabilities to execute arbitrary code. For example, an out-of-bounds read could be used to leak a memory address, which then enables a write-what-where condition in a separate vulnerability.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison cannot be provided. However, the vulnerable code pattern is:

// Vulnerable pseudo-code
void parse_prc_file(char *buffer, size_t buffer_size) {
    uint32_t data_size = *(uint32_t*)buffer; // Attacker-controlled
    char *data = buffer + 4;
    // No check: data_size > buffer_size - 4
    for (uint32_t i = 0; i < data_size; i++) {
        process_byte(data[i]); // Out-of-bounds read
    }
}

A patched version would include a bounds check:

// Patched pseudo-code
void parse_prc_file(char *buffer, size_t buffer_size) {
    uint32_t data_size = *(uint32_t*)buffer;
    if (data_size > buffer_size - 4) {
        // Handle error
        return;
    }
    char *data = buffer + 4;
    for (uint32_t i = 0; i < data_size; i++) {
        process_byte(data[i]);
    }
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• PDF-XChange Editor (any version prior to patch)
• A malicious PRC file crafted to trigger the out-of-bounds read

Steps:

1. Craft a PRC file with a manipulated size field (e.g., set to a large value like 0xFFFFFFFF).
2. Embed the PRC file in a PDF or deliver it directly.
3. Convince the target to open the file (e.g., via phishing email).
4. The application will read beyond the buffer, leaking memory contents into logs or error messages.

Note: No public exploit code is available. The following is a conceptual example of a malicious PRC file structure:

# Conceptual exploit generator (not functional)
import struct

def create_malicious_prc():
    # Minimal PRC header
    header = b'\x50\x52\x43\x00'  # "PRC\0"
    # Crafted size field (large value)
    size = struct.pack('<I', 0xFFFFFFFF)
    # Payload (can be empty)
    payload = b'A' * 100
    return header + size + payload

with open('malicious.prc', 'wb') as f:
    f.write(create_malicious_prc())

Blue Team

Detection:

• Monitor for unusual PRC file downloads or attachments.
• Use endpoint detection and response (EDR) to detect out-of-bounds read crashes.
• Enable application crash dumps for analysis.

Hunting:

• Search for event logs indicating PDF-XChange Editor crashes (Event ID 1000 on Windows).
• Look for PRC files with abnormally large size fields in network traffic.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): None available (no known samples).
• Network Indicators: None specific.
• Behavioral:
  • PDF-XChange Editor process (PDFXEdit.exe) crash with access violation reading memory.
  • Unexpected memory access patterns in the PRC parsing module.
• Registry: None.
• YARA Rule: See Detection Engineering.

Detection Engineering

YARA Rule

rule CVE_2025_6653_PRC_OOB_Read {
    meta:
        description = "Detects malicious PRC files exploiting CVE-2025-6653"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-6653"
    strings:
        $prc_header = "PRC" ascii wide
        $large_size = { FF FF FF FF } // 0xFFFFFFFF
    condition:
        $prc_header at 0 and $large_size at 4
}

Sigma Rule

title: CVE-2025-6653 - PDF-XChange Editor PRC Parsing Out-of-Bounds Read
id: 8a9b3c2d-1e5f-4a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects crashes or access violations in PDF-XChange Editor related to PRC file parsing.
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\PDFXEdit.exe'
        CommandLine|contains: '.prc'
    condition: selection
falsepositives:
    - Legitimate PRC file processing
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-6653 - Potential PRC file download"; flow:to_server,established; content:"PRC"; offset:0; depth:3; content:"|FF FF FF FF|"; offset:4; depth:4; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable PRC file parsing if not required (check application settings).
2. Use application whitelisting to block unknown PRC files.
3. Educate users not to open PRC files from untrusted sources.

Long-term:

• Monitor vendor for patches and apply immediately.
• Consider using a sandbox or virtualized environment for PDF-XChange Editor.
• Implement endpoint protection with exploit prevention capabilities.

Workaround:

• Convert PRC files to a safer format (e.g., STEP) before opening.

Valters IT
DevSecOps & Cyber Threat Intelligence