CVE-2020-35844
📋 TL;DR
CVE-2020-35844 is an out-of-bounds write vulnerability in FastStone Image Viewer 7.5 that allows remote code execution when a user opens a specially crafted image file. This affects all users of FastStone Image Viewer 7.5 who open untrusted image files. The vulnerability can lead to complete system compromise.
💻 Affected Systems
- FastStone Image Viewer
📦 What is this software?
Image Viewer by Faststone
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control of the affected system, potentially leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Remote code execution with the privileges of the current user, allowing installation of malware, data exfiltration, or system disruption.
If Mitigated
Limited impact if proper application sandboxing, least privilege principles, and network segmentation are implemented, potentially containing the exploit to the user's session.
🎯 Exploit Status
Exploitation requires user interaction to open a malicious image file. The vulnerability is well-documented with technical details available in public repositories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 7.6 or later
Vendor Advisory: https://www.faststone.org/FSViewerDetail.htm
Restart Required: Yes
Instructions:
1. Download the latest version of FastStone Image Viewer from the official website. 2. Uninstall the current version. 3. Install the new version. 4. Restart the system if prompted.
🔧 Temporary Workarounds
Disable image file associations
windowsRemove FastStone Image Viewer as the default application for image file types to prevent automatic exploitation
Control Panel > Default Programs > Set Default Programs > Choose another program
Application control policy
windowsUse Windows AppLocker or similar to block execution of FastStone Image Viewer 7.5
🧯 If You Can't Patch
- Implement strict network segmentation to limit lateral movement if exploitation occurs
- Run FastStone Image Viewer with minimal user privileges and in sandboxed environments
🔍 How to Verify
Check if Vulnerable:
Check the version in Help > About FastStone Image Viewer. If version is 7.5, the system is vulnerable.Check Version:
Not applicable - check via GUI in Help > AboutVerify Fix Applied:
Verify the version is 7.6 or higher in Help > About FastStone Image Viewer.📡 Detection & Monitoring
Log Indicators:
- Process creation events for FSViewer.exe with suspicious parent processes
- Crash reports for FSViewer.exe
Network Indicators:
- Unusual outbound connections from systems running FastStone Image Viewer
- Downloads of image files from untrusted sources
SIEM Query:
Process Name="FSViewer.exe" AND (Parent Process contains "cmd" OR Parent Process contains "powershell" OR Parent Process contains "wscript")