CVE-2026-5590: TCP Stack Race Condition Vulnerability

CVE-2026-5590 Overview

A race condition vulnerability exists in the Zephyr RTOS TCP stack during connection teardown operations. When a TCP connection is being released while simultaneously processing incoming SYN packets, the tcp_recv() function may operate on a connection that has already been freed. If tcp_conn_search() returns NULL during this race window, stale context data results in a NULL pointer being passed to tcp_backlog_is_full(), which then dereferences the invalid pointer without validation, causing a system crash.

Critical Impact

Attackers can exploit this race condition to cause denial of service through system crashes, potentially disrupting embedded systems and IoT devices running the Zephyr RTOS.

Affected Products

• Zephyr RTOS (TCP networking stack)
• Systems utilizing Zephyr's TCP/IP implementation
• IoT and embedded devices running affected Zephyr versions

Discovery Timeline

• 2026-04-05 - CVE-2026-5590 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-5590

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference) and represents a race condition in the TCP stack's connection management logic. The flaw occurs during the critical window between TCP connection teardown and the processing of new incoming SYN packets.

The root cause lies in insufficient synchronization between the connection release mechanism and the packet processing path. When a connection is being torn down, the system may still have pending operations that reference the connection structure. If a SYN packet arrives during this transition period, the tcp_conn_search() function may fail to find the connection (returning NULL), but the code path continues to use stale context data that points to the now-released connection.

The network-accessible nature of this vulnerability allows remote attackers to trigger the race condition by carefully timing TCP connection attempts during connection teardown events. While the attack requires low privileges and no user interaction, the high complexity required to reliably exploit the race condition moderates the overall risk.

Root Cause

The vulnerability stems from a Time-of-Check Time-of-Use (TOCTOU) race condition in the TCP connection handling code. The tcp_recv() function fails to properly validate that the connection context remains valid throughout the entire packet processing operation. When tcp_conn_search() returns NULL, the code should halt processing entirely, but instead it continues with stale pointers that were captured before the NULL check.

The lack of proper memory barriers and locking mechanisms around the connection state transitions allows the race window to be exploited. Additionally, the tcp_backlog_is_full() function does not perform its own validation of the connection pointer before dereferencing it, relying entirely on the caller to provide a valid pointer.

Attack Vector

The attack exploits the network-accessible TCP stack by establishing and rapidly terminating connections while simultaneously sending new SYN packets. The attacker attempts to create a scenario where:

1. A legitimate TCP connection exists and is being processed
2. The connection enters the teardown phase
3. A new SYN packet arrives before the teardown completes
4. The packet processing code accesses the freed connection structure

The vulnerability can be triggered remotely over the network without requiring authentication. An attacker with network access to a vulnerable Zephyr device can send specially crafted sequences of TCP packets designed to maximize the probability of hitting the race condition window. While the timing requirements make exploitation non-trivial, persistent attackers can achieve reliable crashes through repeated attempts.

For detailed technical information about the vulnerability mechanism, see the GitHub Security Advisory.

Detection Methods for CVE-2026-5590

Indicators of Compromise

• Unexpected system crashes or reboots in Zephyr-based devices, particularly those with active network connections
• Kernel panic logs indicating NULL pointer dereference in TCP-related functions such as tcp_recv() or tcp_backlog_is_full()
• Abnormal patterns of TCP connection establishment followed by immediate termination from external sources
• Memory corruption indicators in crash dumps pointing to freed TCP connection structures

Detection Strategies

• Implement network intrusion detection rules to identify anomalous TCP SYN flood patterns combined with rapid connection teardowns
• Monitor system logs for crash patterns involving the TCP stack, particularly those referencing NULL pointer exceptions
• Deploy runtime memory protection tools that can detect use-after-free conditions in the networking subsystem
• Utilize SentinelOne's behavioral analysis capabilities to detect exploitation attempts targeting embedded systems

Monitoring Recommendations

• Enable detailed logging of TCP connection state transitions to identify potential race condition triggers
• Configure alerting for abnormal rates of TCP connection failures or incomplete handshakes
• Monitor device health metrics for unexpected restart patterns that could indicate successful exploitation
• Implement network flow analysis to detect reconnaissance activities probing for vulnerable Zephyr devices

How to Mitigate CVE-2026-5590

Immediate Actions Required

• Review and apply patches from the Zephyr project as detailed in the GitHub Security Advisory
• Implement network segmentation to limit exposure of vulnerable IoT and embedded devices
• Deploy network-based intrusion prevention systems to filter malicious TCP traffic patterns
• Consider temporarily disabling unnecessary network services on critical Zephyr-based devices until patches can be applied

Patch Information

The Zephyr project has addressed this vulnerability through security fixes to the TCP stack. Administrators should consult the GitHub Security Advisory for specific patch commits and updated Zephyr versions that remediate the race condition.

The fix involves adding proper synchronization primitives around TCP connection state transitions and ensuring that tcp_backlog_is_full() performs NULL pointer validation before dereferencing connection structures.

Workarounds

• Restrict network access to vulnerable devices using firewall rules that limit which systems can establish TCP connections
• Implement rate limiting on incoming TCP connections to reduce the likelihood of triggering the race condition
• Consider disabling TCP functionality if only UDP-based communication is required for the application
• Deploy network monitoring to quickly detect and respond to denial of service conditions

# Example firewall configuration to restrict TCP access
# Limit incoming connections to trusted hosts only
iptables -A INPUT -p tcp -s 192.168.1.0/24 --dport 0:65535 -j ACCEPT
iptables -A INPUT -p tcp --dport 0:65535 -j DROP

# Rate limit new TCP connections to mitigate race condition triggers
iptables -A INPUT -p tcp --syn -m limit --limit 10/s --limit-burst 20 -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROP