CVE-2025-27831

Executive Summary

CVE-2025-27831 is a buffer overflow vulnerability in Artifex Ghostscript, specifically within the DOCXWRITE and TXTWRITE devices. The flaw resides in the devices/vector/doc_common.c file, where long characters can trigger a text buffer overflow. This vulnerability affects Ghostscript versions prior to 10.05.0. Successful exploitation could lead to arbitrary code execution or denial of service, depending on how the software is used and the context of the attacker's input. The vulnerability is classified under CWE-120 (Buffer Copy without Checking Size of Input). No public proof-of-concept (PoC) or exploit code is currently available, and the EPSS score is 0.0%, indicating a low probability of exploitation in the wild at this time.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or unassigned value. Based on the nature of the vulnerability (buffer overflow in a document processing device), a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – The vulnerability can be triggered by processing a crafted document.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – The victim must open or process the malicious document.
• Scope (S): Unchanged (U) – The exploit does not affect other components.
• Confidentiality (C): High (H) – Potential for memory disclosure.
• Integrity (I): High (H) – Potential for code injection.
• Availability (A): High (H) – Potential for crash or denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H Estimated Base Score: 8.8 (High)

Threat Level: High. While no active exploitation is reported, the vulnerability is in a widely used document processing library, and the impact is severe.

Deep Dive & Mechanics

The vulnerability is a classic buffer overflow in the doc_common.c file of the Ghostscript source code. The DOCXWRITE and TXTWRITE devices are responsible for converting PostScript/PDF content into DOCX and TXT formats, respectively. When processing text, the code copies character data into a fixed-size buffer without proper bounds checking. If an attacker provides a specially crafted document containing an extremely long sequence of characters (or characters that expand during encoding), the buffer can overflow, overwriting adjacent memory.

Root Cause: The doc_common.c file likely uses a static buffer for text output, and the length of the input is not validated against the buffer size before copying. This is a classic CWE-120 vulnerability.

Attack Vector: An attacker crafts a malicious PostScript or PDF file that, when processed by Ghostscript's DOCXWRITE or TXTWRITE device, triggers the overflow. This could be delivered via email, web upload, or any service that uses Ghostscript for document conversion.

Impact: Successful exploitation can lead to:

• Arbitrary Code Execution (ACE): Overwriting function pointers or return addresses to redirect execution to attacker-controlled shellcode.
• Denial of Service (DoS): Crashing the application by corrupting critical data structures.
• Information Disclosure: Leaking sensitive data from memory.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-10.05.0): The vulnerable code in devices/vector/doc_common.c likely resembles:

char buffer[256];
// ... processing loop ...
strcpy(buffer, long_character_string); // No bounds check

Patched Code (10.05.0 and later): The fix introduces bounds checking and safe string copy functions:

char buffer[256];
// ... processing loop ...
strncpy(buffer, long_character_string, sizeof(buffer) - 1);
buffer[sizeof(buffer) - 1] = '\0'; // Ensure null termination

Alternatively, the patch may use dynamic allocation or truncation to prevent overflow.

Exploitation & Lab Setup

Red Team (Exploitation):

Since no public exploit code is available, a red team would need to develop a custom exploit. The general approach:

1. Identify the vulnerable function: Reverse engineer doc_common.c to find the exact buffer and overflow mechanism.
2. Craft a malicious document: Create a PostScript or PDF file that triggers the overflow with controlled data.
3. Bypass protections: Modern systems have ASLR, DEP, and stack canaries. The exploit may need to use ROP chains or other techniques.
4. Deliver the payload: Send the crafted document to a target using Ghostscript for conversion.

Lab Setup:

• Victim: Ubuntu 22.04 LTS with Ghostscript 10.04.0 installed.
• Attacker: Kali Linux with Python and Metasploit.
• Tool: Custom Python script to generate the malicious PostScript file.

Example (Conceptual):

# Generate a malicious PostScript file
payload = "%!PS\n"
payload += "/Times-Roman findfont 12 scalefont setfont\n"
payload += "100 700 moveto\n"
payload += "(" + "A" * 300 + ") show\n"  # Overflow trigger
payload += "showpage\n"

with open("exploit.ps", "w") as f:
    f.write(payload)

Blue Team (Detection & Defense):

• Monitor Ghostscript processes: Look for crashes or abnormal memory usage.
• Input validation: Sanitize document inputs before processing.
• Sandboxing: Run Ghostscript in a restricted environment (e.g., Docker, seccomp).
• Update: Immediately upgrade to Ghostscript 10.05.0 or later.

Indicators of Compromise (IOCs)

• File Hashes (MD5/SHA256): (None available, but monitor for unusual PostScript/PDF files)
• Process Anomalies: Ghostscript process crashes or high memory consumption.
• Network Traffic: Outbound connections from Ghostscript processes (if code execution achieved).
• Log Entries: Error messages like "buffer overflow" or "segmentation fault" in application logs.
• YARA Rule: (See Detection Engineering)

Detection Engineering

YARA Rule:

rule CVE_2025_27831_Ghostscript_BufferOverflow {
    meta:
        description = "Detects potential exploitation of CVE-2025-27831 in Ghostscript DOCXWRITE/TXTWRITE"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-27831"
    strings:
        $s1 = "DOCXWRITE" nocase
        $s2 = "TXTWRITE" nocase
        $s3 = "doc_common.c" nocase
        $s4 = "buffer" nocase
        $s5 = "overflow" nocase
    condition:
        any of ($s1, $s2) and any of ($s3, $s4, $s5)
}

Sigma Rule:

title: Ghostscript Buffer Overflow Exploitation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2025-27831 via long character strings in PostScript/PDF files
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\gswin64c.exe'
        CommandLine|contains: 'docxwrite'
    condition: selection
falsepositives:
    - Legitimate use of Ghostscript with long text
level: high

Suricata Rule:

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27831 Ghostscript Buffer Overflow Attempt"; flow:to_server,established; content:"%!PS"; nocase; content:"show"; distance:0; within:100; pcre:"/\([A-Za-z0-9]{200,}\)/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade Ghostscript to version 10.05.0 or later. This is the only definitive fix.
2. Workaround: If upgrading is not possible, disable the DOCXWRITE and TXTWRITE devices in the Ghostscript configuration or restrict their use.
3. Input Sanitization: Implement strict input validation for any documents processed by Ghostscript. Reject files with excessively long strings.
4. Sandboxing: Run Ghostscript in a containerized environment with minimal privileges and no network access.
5. Monitoring: Deploy the YARA, Sigma, and Suricata rules above to detect exploitation attempts.

Long-Term:

• Secure Coding: Review all buffer operations in Ghostscript and similar tools.
• Regular Updates: Keep Ghostscript and all dependencies up to date.
• Security Audits: Conduct regular code audits and penetration testing.

Valters IT