CVE-2026-4391: TeamSpeak 3 Server Buffer Overflow Flaw

CVE-2026-4391 Overview

CVE-2026-4391 is a heap-based buffer overflow vulnerability in TeamSpeak 3 Server versions up to and including 3.13.7. The flaw resides in the Elliptic Curve Cryptography (ECC) Key Parser component and is triggered by manipulating input processed during key parsing operations. An unauthenticated remote attacker can reach the vulnerable code path over the network without user interaction. TeamSpeak released version 3.13.8 to address the issue, as documented in the TeamSpeak Security Advisory TS-SA-2026-001 and the ModZero Advisory MZ-26-01. The weakness is classified under CWE-119, improper restriction of operations within the bounds of a memory buffer.

Critical Impact

Remote, unauthenticated attackers can corrupt heap memory in the TeamSpeak 3 Server process by sending crafted ECC key data, potentially impacting service availability.

Affected Products

• TeamSpeak 3 Server versions up to and including 3.13.7
• ECC Key Parser component within TeamSpeak 3 Server
• All deployments running TeamSpeak 3 Server prior to 3.13.8

Discovery Timeline

• 2026-05-27 - CVE-2026-4391 published to NVD
• 2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-4391

Vulnerability Analysis

The vulnerability exists in the ECC Key Parser component of TeamSpeak 3 Server. When the server parses an ECC key supplied by a remote source, it fails to properly restrict write operations to the bounds of an allocated heap buffer. This results in a heap-based buffer overflow, classified under CWE-119.

The attack surface is reachable over the network without authentication or user interaction. According to the ModZero Advisory MZ-26-01, the parser mishandles malformed key structures during early connection handling. Successful exploitation primarily affects availability of the server process by corrupting adjacent heap memory.

Root Cause

The root cause is improper boundary validation in the ECC Key Parser. The parser writes attacker-controlled data into a heap-allocated buffer without sufficient length checks against the destination size. Crafted ECC key payloads cause the parser to exceed the buffer bounds and corrupt adjacent heap metadata or objects.

Attack Vector

An attacker delivers a crafted ECC key payload to a network-exposed TeamSpeak 3 Server endpoint. No credentials and no user interaction are required. The malformed input drives the ECC Key Parser into an out-of-bounds heap write, which typically destabilizes the server process and disrupts voice services for connected clients. Specific exploitation details are described in the TeamSpeak Security Advisory TS-SA-2026-001 and the VulDB entry #366315.

No public proof-of-concept exploit code is currently associated with this CVE. The vulnerability is described in prose by the vendor and the reporting researcher; refer to the linked advisories for authoritative technical detail.

Detection Methods for CVE-2026-4391

Indicators of Compromise

• Unexpected crashes or restarts of the ts3server process, particularly during client connection handshakes.
• Core dumps or segmentation faults referencing the ECC key parsing code path in TeamSpeak 3 Server logs.
• Inbound traffic to TeamSpeak server ports (default UDP 9987, TCP 10011, TCP 30033) from unknown sources immediately preceding service instability.

Detection Strategies

• Inventory all TeamSpeak 3 Server instances and compare installed versions against 3.13.8 to identify vulnerable hosts.
• Monitor server logs for repeated parser errors, abnormal disconnections, or process termination events tied to client key exchange.
• Inspect network telemetry for malformed or oversized ECC key payloads directed at TeamSpeak server endpoints.

Monitoring Recommendations

• Enable host-level process monitoring on systems running TeamSpeak 3 Server to alert on unexpected ts3server exits.
• Forward TeamSpeak server logs to a centralized log platform and create alerts for crash signatures and repeated handshake failures.
• Restrict exposure of TeamSpeak server ports to known client networks where possible and log all inbound connections.

How to Mitigate CVE-2026-4391

Immediate Actions Required

• Upgrade all TeamSpeak 3 Server installations to version 3.13.8 or later, available from the TeamSpeak Server Download Page.
• Identify any internet-exposed TeamSpeak 3 Server instances and prioritize them for immediate patching.
• Review server logs for prior crashes that may indicate exploitation attempts before the upgrade.

Patch Information

TeamSpeak addressed CVE-2026-4391 in TeamSpeak 3 Server version 3.13.8. The fix corrects boundary handling within the ECC Key Parser component. Upgrade details and verified binaries are published in the TeamSpeak Security Advisory TS-SA-2026-001.

Workarounds

• Restrict network access to TeamSpeak 3 Server ports using firewall rules, allowing only trusted client IP ranges.
• Place TeamSpeak 3 Server behind a VPN so that only authenticated network users can reach the service.
• Temporarily disable public TeamSpeak 3 Server instances that cannot be upgraded promptly to version 3.13.8.

# Example firewall configuration limiting TeamSpeak 3 Server exposure (Linux iptables)
# Replace 203.0.113.0/24 with your trusted client network
iptables -A INPUT -p udp --dport 9987 -s 203.0.113.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 10011 -s 203.0.113.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 30033 -s 203.0.113.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 9987 -j DROP
iptables -A INPUT -p tcp --dport 10011 -j DROP
iptables -A INPUT -p tcp --dport 30033 -j DROP