CVE-2024-53863

Executive Summary

CVE-2024-53863 is a security vulnerability in Synapse, an open-source Matrix homeserver, affecting versions prior to 1.120.1. The vulnerability arises from the dynamic_thumbnails option, which, when enabled, could trigger the decoding and thumbnail generation of uncommon image formats. This process could invoke external tools like Ghostscript, significantly expanding the attack surface in a historically vulnerable area. The risk is heightened because these uncommon formats are rarely used on the open web or within the Matrix ecosystem, making the feature's benefit negligible compared to the potential for exploitation. The vulnerability is fixed in Synapse 1.120.1 by restricting thumbnail generation to widely used formats: PNG, JPEG, GIF, and WebP.

Supply Chain & Package Security

Synapse is a Python-based project distributed via PyPI. The vulnerability is not a direct supply chain compromise but rather a configuration and feature risk. However, the reliance on external tools like Ghostscript introduces a dependency risk. Users should ensure that Ghostscript and other image processing libraries are kept up to date and that the dynamic_thumbnails option is disabled unless absolutely necessary.

CVSS Breakdown & Threat Level

The CVSS score is not provided in the NVD data, but based on the description, the vulnerability can be assessed as follows:

• Attack Vector (AV): Network (N) – The vulnerability can be triggered via network requests.
• Attack Complexity (AC): Low (L) – No special conditions are required beyond sending a crafted request.
• Privileges Required (PR): None (N) – No authentication is needed.
• User Interaction (UI): None (N) – No user action is required.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): High (H) – Potential for arbitrary code execution leading to data exposure.
• Integrity (I): High (H) – Potential for system modification.
• Availability (A): High (H) – Potential for denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Estimated Score: 9.8 (Critical)

Deep Dive & Mechanics

The vulnerability is rooted in Synapse's thumbnail generation functionality. When dynamic_thumbnails is enabled, Synapse attempts to generate thumbnails for uploaded images. The issue is that Synapse would attempt to decode and generate thumbnails for uncommon image formats (e.g., TIFF, EPS, PDF) by invoking external tools like Ghostscript. Ghostscript has a history of critical vulnerabilities (e.g., CVE-2023-36664, CVE-2023-43115) that allow arbitrary code execution via specially crafted files. By sending a crafted image file in one of these uncommon formats, an attacker could trigger Ghostscript to execute arbitrary commands on the server.

The attack surface is expanded because:

1. The dynamic_thumbnails option is enabled by default in some configurations.
2. External tools like Ghostscript are invoked without proper sandboxing.
3. The uncommon formats are rarely used, making the feature unnecessary for most deployments.

Vulnerable vs. Patched Code

Vulnerable Code (pre-1.120.1): The vulnerable code likely resides in the thumbnail generation module, where a list of allowed image formats was not restricted. For example:

# Vulnerable: Allows all formats supported by PIL and external tools
ALLOWED_FORMATS = ['PNG', 'JPEG', 'GIF', 'WebP', 'TIFF', 'EPS', 'PDF']

Patched Code (1.120.1): The patch restricts the allowed formats to only safe, widely used ones:

# Patched: Only safe formats
ALLOWED_FORMATS = ['PNG', 'JPEG', 'GIF', 'WebP']

Additionally, the patch likely disables the invocation of external tools like Ghostscript for thumbnail generation.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A vulnerable Synapse instance (version < 1.120.1) with dynamic_thumbnails enabled.
• Access to the Matrix server's media upload endpoint.

Exploitation Steps:

1. Craft a malicious file: Create a specially crafted EPS or PDF file that exploits a known Ghostscript vulnerability (e.g., CVE-2023-36664). This file should contain a payload that executes arbitrary commands.
2. Upload the file: Use the Matrix client API to upload the malicious file to the server.
3. Trigger thumbnail generation: The server will automatically attempt to generate a thumbnail for the uploaded file, invoking Ghostscript.
4. Execute payload: The Ghostscript vulnerability will execute the payload, giving the attacker remote code execution on the server.

Example Payload (EPS file):

%!PS-Adobe-3.0 EPSF-3.0
%%BoundingBox: 0 0 100 100
%%EndComments
%%BeginProlog
%%EndProlog
%%BeginSetup
%%EndSetup
%%Page: 1 1
%%BeginPageSetup
%%EndPageSetup
(currentdevice /OutputFile (%pipe%id) (w) .outputdevice) stopped pop
%%EndPage
%%EOF

Blue Team

Detection:

• Monitor for unusual image file uploads, especially EPS, PDF, TIFF, and other uncommon formats.
• Monitor for Ghostscript process execution triggered by Synapse.
• Look for anomalous network connections or command execution from the Synapse process.

Mitigation:

• Immediately upgrade to Synapse 1.120.1 or later.
• Disable dynamic_thumbnails if not required.
• Restrict outbound network access from the Synapse server.
• Implement file upload validation to block uncommon image formats.

Indicators of Compromise (IOCs)

• File Hashes: (Example – actual hashes depend on the exploit)
  • sha256: a1b2c3d4e5f6... (malicious EPS file)
• Network Indicators:
  • Outbound connections to known malicious IPs from the Synapse process.
  • Unusual DNS queries from the server.
• Process Indicators:
  • Ghostscript (gs) process spawned by Synapse.
  • Unexpected child processes (e.g., shell, curl, wget) spawned by Ghostscript.
• File System Indicators:
  • Unusual files in /tmp/ or other writable directories.
  • Modified system binaries or configuration files.

Detection Engineering

YARA Rule

rule CVE_2024_53863_Exploit_EPS {
    meta:
        description = "Detects malicious EPS files targeting CVE-2024-53863"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-53863"
    strings:
        $s1 = "%!PS-Adobe-3.0 EPSF-3.0"
        $s2 = "%pipe%" nocase
        $s3 = "outputdevice" nocase
        $s4 = "currentdevice" nocase
    condition:
        all of ($s*) and filesize < 10KB
}

Sigma Rule

title: CVE-2024-53863 Ghostscript Invocation via Synapse
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects Ghostscript process spawned by Synapse for thumbnail generation
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        ParentImage|endswith: '/synapse'
        Image|endswith: '/gs'
    condition: selection
falsepositives:
    - Legitimate use of Ghostscript for other purposes
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-53863 - Malicious EPS Upload to Synapse"; flow:to_server,established; content:"POST"; http_method; content:"/upload"; http_uri; content:"application/postscript"; http_client_body; pcre:"/%!PS-Adobe-3.0 EPSF-3.0/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Synapse to version 1.120.1 or later.
2. Disable dynamic_thumbnails in the configuration file (homeserver.yaml):

   dynamic_thumbnails: false
   

3. Restrict file upload types at the reverse proxy level (e.g., Nginx) to block uncommon image formats.

Long-term Measures:

• Implement a Web Application Firewall (WAF) to filter malicious uploads.
• Regularly update Ghostscript and other image processing libraries.
• Conduct security audits of Synapse configurations.
• Monitor for new CVEs related to Ghostscript and other dependencies.

Valters IT Signature: This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.