CVE-2025-6656

Executive Summary

CVE-2025-6656 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 when the application fails to properly validate user-supplied data, allowing an attacker to read past the end of an allocated object. This can lead to the disclosure of sensitive memory contents. The vulnerability requires user interaction (opening a malicious file or visiting a malicious page) and has a low CVSS score of 3.3 due to its limited impact (confidentiality only) and the need for local access. While the direct impact is information disclosure, the ZDI advisory notes that an attacker could leverage this in conjunction with other vulnerabilities to achieve arbitrary code execution. No public exploit code or PoC is currently available, and no patch has been released by the vendor.

CVSS Breakdown & Threat Level

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

Threat Level: Low. The vulnerability is difficult to weaponize alone, but combined with other flaws (e.g., memory corruption) it could lead to code execution. The lack of a patch and the low EPSS score (0.0%) suggest limited active exploitation.

Deep Dive & Mechanics

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

Root Cause: The parser does not check the size of the input data against the allocated buffer size. An attacker can craft a PRC file with a manipulated header or data field that specifies a larger read size than the actual buffer, causing the parser to read adjacent memory.

Impact: The out-of-bounds read can leak sensitive information from the process heap, such as pointers, encryption keys, or other data. While this alone does not allow code execution, it can be used to bypass ASLR or leak credentials, enabling further exploitation.

Attack Vector: The attacker sends a malicious PRC file via email, download, or web page. The user must open the file in PDF-XChange Editor. The vulnerability is triggered during parsing, and the leaked data is either displayed or stored in a way that the attacker can retrieve (e.g., via error messages or memory dumps).

Vulnerable vs. Patched Code

Since no patch is available, we analyze the vulnerable code pattern. The vulnerable code likely resembles:

// Vulnerable: No bounds check
void parse_prc_data(char *buffer, size_t size) {
    char *data = malloc(size);
    memcpy(data, buffer, size); // Assume size is from file header
    // Read from data without checking offset
    int value = *(int *)(data + offset); // offset can be larger than size
}

A patched version would include validation:

// Patched: Bounds check
void parse_prc_data(char *buffer, size_t size) {
    if (size > MAX_SIZE) return; // Reject oversized
    char *data = malloc(size);
    memcpy(data, buffer, size);
    if (offset + sizeof(int) > size) return; // Prevent OOB read
    int value = *(int *)(data + offset);
}

Exploitation & Lab Setup

Red Team

Objective: Leak sensitive memory from PDF-XChange Editor.

Prerequisites:

• PDF-XChange Editor (any version)
• A crafted PRC file

Steps:

1. Create a malicious PRC file with a manipulated header that specifies a large data size (e.g., 0xFFFFFFFF) but provides only a small buffer.
2. Embed the file in a phishing email or host it on a website.
3. Convince the target to open the file.
4. The parser reads beyond the buffer, leaking heap data.
5. Capture the leaked data via error messages or memory dumps.

Note: No public exploit code is available. The above is a conceptual approach.

Blue Team

Detection:

• Monitor for unusual PRC file downloads or email attachments.
• Use file integrity monitoring to detect modified PRC files.
• Enable application whitelisting to block unknown executables.

Mitigation:

• Restrict execution of PDF-XChange Editor in sandboxed environments.
• Use endpoint detection and response (EDR) to monitor for out-of-bounds read patterns.
• Apply virtual patching via web application firewalls (WAF) if the file is served via web.

Indicators of Compromise (IOCs)

• File Hashes: (None available; generic PRC file with anomalous size)
• Network Indicators: Connections to domains hosting malicious PRC files.
• Behavioral: PDF-XChange Editor crashes or produces unusual error messages when opening PRC files.
• Registry/File System: Temporary files created during parsing may contain leaked data.

Detection Engineering

YARA Rule

rule CVE_2025_6656_PRC_OOB_Read {
    meta:
        description = "Detects malicious PRC files exploiting CVE-2025-6656"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-6656"
    strings:
        $prc_header = { 50 52 43 00 } // "PRC\0" header
        $large_size = { FF FF FF 7F } // Suspicious large size field
    condition:
        $prc_header at 0 and $large_size
}

Sigma Rule

title: CVE-2025-6656 PRC File Parsing OOB Read
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects attempts to exploit CVE-2025-6656 via malicious PRC files
author: Valters IT
logsource:
    category: file_event
    product: windows
detection:
    selection:
        Image|endswith: '\PDFXEdit.exe'
        TargetFilename|endswith: '.prc'
    condition: selection
falsepositives:
    - Legitimate PRC files from trusted sources
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-6656 PRC File Download"; flow:to_server,established; content:"|50 52 43|"; http_client_body; depth:3; content:"|FF FF FF 7F|"; distance:0; within:4; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

• Disable automatic opening of PRC files in PDF-XChange Editor.
• Use a different PDF viewer that does not parse PRC files.
• Implement application sandboxing to limit the impact of memory leaks.

Long-Term:

• Monitor vendor for patches and apply when available.
• Educate users not to open PRC files from untrusted sources.
• Deploy endpoint protection with behavioral analysis.

Workaround: No official patch exists. Consider using alternative software for viewing PRC files.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert