CVE-2021-34858
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on TeamViewer installations by tricking users into opening malicious TVS files. The flaw exists in improper validation during TVS file parsing, leading to out-of-bounds memory reads that can be leveraged for code execution. All TeamViewer users who open untrusted TVS files are affected.
💻 Affected Systems
- TeamViewer
📦 What is this software?
Teamviewer by Teamviewer
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control of the victim's computer, enabling data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Attacker executes malicious code with the same privileges as the TeamViewer process, potentially leading to credential theft, surveillance, or installation of persistent malware.
If Mitigated
If proper controls prevent execution of untrusted TVS files, impact is limited to denial of service or application crashes.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious TVS file) but the vulnerability itself is straightforward to trigger once the file is opened.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 15.28.3 and later
Vendor Advisory: https://community.teamviewer.com/English/discussion/117794/august-updates-security-patches/p1
Restart Required: Yes
Instructions:
1. Open TeamViewer. 2. Go to Help > Check for new version. 3. Follow prompts to update to version 15.28.3 or later. 4. Restart TeamViewer and computer if prompted.
🔧 Temporary Workarounds
Disable TVS file association
windowsRemove TeamViewer as default handler for .tvs files to prevent automatic execution
Windows: Control Panel > Default Programs > Associate a file type > Select .tvs > Change program
Block TVS files at perimeter
allPrevent download of .tvs files from untrusted sources
🧯 If You Can't Patch
- Implement application whitelisting to prevent execution of unauthorized code
- Educate users to never open TVS files from untrusted sources
🔍 How to Verify
Check if Vulnerable:
Check TeamViewer version in Help > About. If version is below 15.28.3, system is vulnerable.Check Version:
Windows: "C:\Program Files (x86)\TeamViewer\TeamViewer.exe" --versionVerify Fix Applied:
Confirm version is 15.28.3 or higher in Help > About dialog.📡 Detection & Monitoring
Log Indicators:
- TeamViewer crash logs with memory access violations
- Unexpected TVS file processing events
Network Indicators:
- Downloads of .tvs files from suspicious sources
- Outbound connections after TVS file processing
SIEM Query:
process_name="TeamViewer.exe" AND (event_id=1000 OR file_extension=".tvs")🔗 References
- https://community.teamviewer.com/English/discussion/117794/august-updates-security-patches/p1
- https://www.zerodayinitiative.com/advisories/ZDI-21-1001/
- https://community.teamviewer.com/English/discussion/117794/august-updates-security-patches/p1
- https://www.zerodayinitiative.com/advisories/ZDI-21-1001/
