CVE-2024-4568

Executive Summary

CVE-2024-4568 is a denial-of-service (DoS) vulnerability in Xpdf versions 4.05 and earlier. The flaw resides in the PDF resource parsing logic, where a crafted PDF containing a circular reference (object loop) causes infinite recursion, leading to a stack overflow and application crash. With a CVSS score of 2.9, this is a low-severity issue that requires user interaction (opening a malicious PDF) and does not allow code execution or data exfiltration. No public exploit code or proof-of-concept is currently available, and the vendor has not released a patch.

CVSS Breakdown & Threat Level

• CVSS Score: 2.9 (Low)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
• Attack Vector: Local (AV:L) – requires the attacker to deliver a malicious PDF file to the victim.
• Attack Complexity: Low (AC:L) – no special conditions needed.
• Privileges Required: None (PR:N) – no authentication needed.
• User Interaction: Required (UI:R) – victim must open the file.
• Scope: Unchanged (S:U) – impact is confined to the vulnerable component.
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L) – application crash, but can be restarted.

Threat Level: Low. Exploitation is limited to causing a denial of service. No remote code execution or data breach is possible.

Deep Dive & Mechanics

The vulnerability is a classic CWE-674: Uncontrolled Recursion. In Xpdf, when parsing a PDF file, the resource dictionary can contain references to other objects. If a PDF is crafted with a circular reference (e.g., Object A references Object B, which references Object A), the parser enters an infinite recursive loop. Each recursive call consumes stack space, eventually causing a stack overflow and crashing the application.

The affected code is in the PDF resource parsing routines, specifically in the handling of XRef entries and object streams. The parser does not implement a depth limit or cycle detection for object references.

Vulnerable vs. Patched Code

Vulnerable Code (Xpdf 4.05 and earlier):

// Simplified example of recursive parsing without cycle detection
Object* parseObject(XRef* xref, int objNum) {
    // ... fetch object data ...
    if (object is a reference) {
        return parseObject(xref, reference->objNum); // No depth check
    }
    // ... parse object ...
}

Patched Code (Hypothetical fix):

// With depth limit and cycle detection
Object* parseObject(XRef* xref, int objNum, int depth, set<int>& visited) {
    if (depth > MAX_DEPTH || visited.count(objNum)) {
        return nullptr; // or error object
    }
    visited.insert(objNum);
    // ... fetch object data ...
    if (object is a reference) {
        return parseObject(xref, reference->objNum, depth+1, visited);
    }
    // ... parse object ...
}

No official patch has been released by Xpdf as of this report.

Exploitation & Lab Setup

Red Team (Attack Simulation)

1. Craft a malicious PDF with a circular reference:

   • Use a PDF manipulation library (e.g., pdf-parser.py from Didier Stevens) to create an object loop.
   • Example: Object 1 references Object 2, Object 2 references Object 1.

2. Deliver the PDF to the victim via email, download, or other means.

3. Trigger the vulnerability by opening the PDF in Xpdf 4.05.

4. Observe crash – the application will hang or crash due to stack overflow.

No public exploit code is available. The following is a conceptual Python script to generate a malicious PDF:

# generate_malicious_pdf.py
# Creates a PDF with circular object references
from pdfminer.pdfparser import PDFParser
from pdfminer.pdfdocument import PDFDocument
# ... (simplified)

Blue Team (Defense & Detection)

• Monitor for crashes of Xpdf processes (e.g., via Windows Event Log or syslog).
• Use file scanning to detect PDFs with deep or circular object references.
• Implement application whitelisting to restrict PDF viewers.
• Train users not to open PDFs from untrusted sources.

Indicators of Compromise (IOCs)

• File Hashes: No known malicious samples.
• Process Behavior: Xpdf process crashes with stack overflow error.
• Network: None (local vulnerability).
• Logs: Application crash logs (e.g., Windows Event ID 1000, Linux core dumps).

Detection Engineering

YARA Rule

rule CVE_2024_4568_Xpdf_StackOverflow {
    meta:
        description = "Detects PDF files with circular object references (CVE-2024-4568)"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-4568"
    strings:
        $xref = "xref" nocase
        $obj = "obj" nocase
        $endobj = "endobj" nocase
    condition:
        $xref and #obj > 100 and #endobj > 100
}

Sigma Rule

title: Xpdf Stack Overflow via Circular PDF Reference
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects Xpdf crashes due to CVE-2024-4568
logsource:
    product: windows
    service: application
detection:
    selection:
        EventID: 1000
        Application: "xpdf.exe"
        FaultingModule: "xpdf.exe"
    condition: selection
falsepositives:
    - Legitimate large PDFs
level: low

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-4568 Xpdf PDF Download"; flow:to_client,established; file.name:"*.pdf"; content:"xref"; within:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• No patch available. Consider using an alternative PDF viewer (e.g., MuPDF, Evince).
• Apply input validation – use a PDF pre-processor to detect circular references.
• Restrict execution – use AppArmor or SELinux to limit Xpdf's capabilities.
• Monitor for crashes and investigate suspicious PDF files.

Valters IT
Securing Digital Frontiers