CVE-2026-4390: TeamSpeak 3 Server Use After Free Flaw

CVE-2026-4390 Overview

CVE-2026-4390 is a use-after-free vulnerability in TeamSpeak 3 Server versions up to and including 3.13.7. The flaw resides in the process_resend_queue function within the Connection State Management component. An authenticated remote attacker can trigger the condition over the network to cause memory corruption affecting integrity and availability of the server process. TeamSpeak addressed the issue in version 3.13.8 through Security Advisory TS-SA-2026-001. The vulnerability was reported by modzero and tracked publicly through VulDB entry #366314.

Critical Impact

A remote, low-privileged attacker can manipulate the connection resend queue to dereference freed memory, potentially leading to denial of service or memory corruption on the TeamSpeak server.

Affected Products

• TeamSpeak 3 Server versions up to and including 3.13.7
• TeamSpeak 3 Server Connection State Management component
• All deployments running the process_resend_queue code path prior to 3.13.8

Discovery Timeline

• 2026-05-27 - CVE-2026-4390 published to NVD
• 2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-4390

Vulnerability Analysis

The vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer) and manifests specifically as a use-after-free condition. The defect is located in the process_resend_queue function, which handles retransmission logic for the TeamSpeak server's voice and control protocol. When the server processes queued packets pending resend, the function references memory associated with a connection state object that has already been released. This dangling reference allows an attacker who can interact with the server to influence the contents or layout of the freed region. The result is undefined behavior ranging from a server crash to potential corruption of adjacent allocator metadata.

Root Cause

The root cause is a lifetime management defect in the Connection State Management subsystem. The resend queue retains pointers to connection-state structures without ensuring those structures remain allocated when the queue is later processed. When a connection transitions or terminates, the associated memory is freed while the resend queue still holds references to it. Subsequent invocation of process_resend_queue operates on the stale pointer.

Attack Vector

Exploitation requires network access to the TeamSpeak server and a low level of privilege, consistent with an authenticated or connected client session. The attacker manipulates connection state transitions in a sequence that causes the server to free a connection object while items remain queued for resend. No user interaction is required. Public technical details are available in the modzero Advisory MZ-26-01 and the TeamSpeak Security Advisory TS-SA-2026-001.

No verified proof-of-concept code has been published. Refer to the vendor and researcher advisories for technical details on the trigger sequence.

Detection Methods for CVE-2026-4390

Indicators of Compromise

• Unexpected crashes or restarts of the ts3server process, particularly under load or during high client churn
• Core dumps referencing the process_resend_queue function or freed connection state structures
• Abnormal patterns of rapid client connect, disconnect, and reconnect sequences from a single source address

Detection Strategies

• Monitor TeamSpeak server logs for repeated abnormal terminations and correlate with client session events from the same source IP
• Deploy memory error detectors such as AddressSanitizer in non-production builds to validate exposure to the process_resend_queue path
• Track the running version of ts3server across hosts and flag any instance below 3.13.8

Monitoring Recommendations

• Enable verbose logging on TeamSpeak servers and forward logs to a centralized analytics platform for crash pattern analysis
• Alert on process restarts of ts3server exceeding a defined baseline within a short interval
• Capture network telemetry for UDP voice traffic to identify clients generating anomalous connection state transitions

How to Mitigate CVE-2026-4390

Immediate Actions Required

• Upgrade all TeamSpeak 3 Server instances to version 3.13.8 or later using the official TeamSpeak Server Download
• Inventory all internet-exposed TeamSpeak servers and prioritize patching of public-facing deployments first
• Review server access controls to limit which clients can establish authenticated sessions

Patch Information

TeamSpeak released version 3.13.8 to remediate this issue, as documented in TeamSpeak Security Advisory TS-SA-2026-001. The patch corrects the lifetime handling of connection state objects referenced by the resend queue. Administrators should verify the deployed version after upgrade and restart the service to ensure the new binary is loaded.

Workarounds

• Restrict inbound access to the TeamSpeak server UDP port using firewall rules that permit only known client networks
• Place the server behind a VPN or access-controlled network segment until the upgrade to 3.13.8 is completed
• Monitor and rate-limit connection attempts from single sources to reduce opportunities for state manipulation

# Verify the running TeamSpeak 3 Server version after upgrade
./ts3server_minimal_runscript.sh inifile=ts3server.ini | head -n 5

# Example firewall rule restricting TeamSpeak default ports to a trusted network
iptables -A INPUT -p udp --dport 9987 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 9987 -j DROP