CVE-2021-34859
📋 TL;DR
CVE-2021-34859 is a remote code execution vulnerability in TeamViewer 15.16.8.0 that allows attackers to execute arbitrary code by tricking users into opening malicious TVS files or visiting malicious web pages. The vulnerability stems from improper validation of user-supplied data during TVS file parsing, leading to memory corruption. All users running the affected TeamViewer version are at risk.
💻 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 are in place (patched version, limited user privileges), impact is minimal with only temporary disruption possible.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file or visiting malicious page) but the vulnerability itself is unauthenticated. The ZDI advisory suggests reliable exploitation is possible.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: TeamViewer 15.17.6 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 15.17.6 or later. 4. Restart TeamViewer after update completes.
🔧 Temporary Workarounds
Disable TVS file association
windowsRemove the association between .tvs files and TeamViewer to prevent automatic opening
On Windows: assoc .tvs=
On Windows: ftype TVSFile=
User awareness training
allEducate users not to open .tvs files from untrusted sources
🧯 If You Can't Patch
- Run TeamViewer with minimal user privileges (not as administrator)
- Implement application whitelisting to block execution of unknown binaries
🔍 How to Verify
Check if Vulnerable:
Check TeamViewer version in Help > About. If version is exactly 15.16.8.0, the system is vulnerable.Check Version:
On Windows: "C:\Program Files (x86)\TeamViewer\TeamViewer.exe" --versionVerify Fix Applied:
Verify TeamViewer version is 15.17.6 or higher in Help > About.📡 Detection & Monitoring
Log Indicators:
- Unexpected TeamViewer crashes
- Creation of suspicious .tvs files
- Unusual process execution from TeamViewer context
Network Indicators:
- TeamViewer connections to unexpected external IPs
- Unusual outbound traffic patterns following .tvs file access
SIEM Query:
source="TeamViewer" AND (event_type="crash" OR file_extension=".tvs")🔗 References
- https://community.teamviewer.com/English/discussion/117794/august-updates-security-patches/p1
- https://www.zerodayinitiative.com/advisories/ZDI-21-1003/
- https://community.teamviewer.com/English/discussion/117794/august-updates-security-patches/p1
- https://www.zerodayinitiative.com/advisories/ZDI-21-1003/
