CVE-2026-7423: FreeRTOS-Plus-TCP DoS Vulnerability

CVE-2026-7423 Overview

An integer underflow vulnerability has been discovered in the ICMP and ICMPv6 echo reply handlers within FreeRTOS-Plus-TCP, the popular TCP/IP stack designed for embedded systems and IoT devices. This vulnerability affects versions prior to V4.4.1 and V4.2.6, allowing an attacker on an adjacent network to cause a denial of service condition by crashing the affected device.

The vulnerability is triggered when outgoing ping support is enabled on the device. The flaw occurs because header sizes are subtracted from a packet length field without proper validation that the field contains a sufficiently large value, resulting in an integer underflow. This subsequently causes a heap out-of-bounds read of up to approximately 65KB of memory, leading to device crash and service disruption.

Critical Impact
Adjacent network attackers can crash embedded devices and IoT systems running vulnerable FreeRTOS-Plus-TCP versions by exploiting improper packet length validation in ICMP handlers, causing denial of service across connected infrastructure.

Affected Products

FreeRTOS-Plus-TCP versions prior to V4.4.1
FreeRTOS-Plus-TCP versions prior to V4.2.6
Embedded systems and IoT devices using vulnerable FreeRTOS-Plus-TCP TCP/IP stack with outgoing ping enabled

Discovery Timeline

April 29, 2026 - CVE-2026-7423 published to NVD
April 29, 2026 - Last updated in NVD database

Technical Details for CVE-2026-7423

Vulnerability Analysis

This vulnerability (CWE-191: Integer Underflow) exists in the network packet processing logic of FreeRTOS-Plus-TCP, specifically within the ICMP and ICMPv6 echo reply handler functions. When the TCP/IP stack processes incoming ICMP echo reply packets, it performs arithmetic operations to calculate payload sizes by subtracting header lengths from the total packet length field.

The flaw manifests when a maliciously crafted packet arrives with a reported length value smaller than the expected header sizes. Without proper boundary checking, the subtraction operation results in an integer underflow, causing the calculated size to wrap around to a very large unsigned value. The system then attempts to read this calculated amount of data from heap memory, accessing up to approximately 65KB beyond the allocated buffer boundaries.

This out-of-bounds read operation corrupts memory management structures and leads to device instability, ultimately resulting in a crash. The attack requires the attacker to be on an adjacent network segment, limiting exposure to local network environments rather than remote internet-based attacks.

Root Cause

The root cause of CVE-2026-7423 is insufficient input validation in the ICMP echo reply processing code. The vulnerable code path subtracts fixed header size values from a packet length field received from the network without first verifying that the packet length is large enough to accommodate this subtraction. Since packet length fields use unsigned integer types, the subtraction of a larger value from a smaller one causes the result to underflow and wrap around to a near-maximum unsigned value.

This classic integer underflow pattern (CWE-191) demonstrates the critical importance of validating arithmetic operations on untrusted network input, particularly in embedded systems where memory safety mechanisms may be limited compared to full operating system environments.

Attack Vector

The attack vector requires adjacent network access, meaning the attacker must be on the same network segment as the target device. The attack sequence involves:

The attacker identifies a device running FreeRTOS-Plus-TCP with outgoing ping functionality enabled
The attacker crafts a malicious ICMP or ICMPv6 echo reply packet with a deliberately small length field value
When the vulnerable device receives this packet, the ICMP handler attempts to calculate the payload size
The subtraction operation underflows, producing an extremely large size value
The subsequent heap read operation accesses memory beyond allocated boundaries
The out-of-bounds access causes memory corruption and crashes the device

The vulnerability mechanism involves improper validation of packet length fields before performing header size subtraction in the ICMP echo reply handlers. When a crafted packet with an insufficient length value is processed, the arithmetic underflow triggers a heap out-of-bounds read condition. For detailed technical information about the vulnerable code paths and the specific fix implemented, refer to the GitHub Security Advisory GHSA-7r59-2pgv-9v2r.

Detection Methods for CVE-2026-7423

Indicators of Compromise

Unexpected device crashes or reboots in embedded systems running FreeRTOS-Plus-TCP
Abnormal ICMP or ICMPv6 echo reply packets with unusually small length field values observed in network traffic
Multiple devices on the same network segment experiencing simultaneous denial of service conditions
Log entries indicating memory access violations or heap corruption in affected systems

Detection Strategies

Deploy network intrusion detection systems (NIDS) with rules to identify malformed ICMP packets with invalid length fields
Monitor embedded device health metrics for unexpected restart patterns or crash events
Implement packet inspection at network boundaries to filter ICMP traffic with suspicious characteristics
Conduct firmware version audits across all FreeRTOS-Plus-TCP deployments to identify vulnerable systems

Monitoring Recommendations

Enable comprehensive logging for ICMP traffic on network infrastructure devices
Configure alerts for device availability monitoring to detect denial of service conditions quickly
Monitor network segments containing IoT and embedded devices for unusual ICMP traffic patterns
Establish baseline metrics for device uptime to identify anomalous crash patterns

How to Mitigate CVE-2026-7423

Immediate Actions Required

Upgrade all FreeRTOS-Plus-TCP deployments to version V4.4.1 or V4.2.6 (depending on your version branch)
Disable outgoing ping support on devices where this functionality is not required to eliminate the attack surface
Implement network segmentation to isolate vulnerable IoT and embedded devices from untrusted network segments
Deploy network-level filtering to block malformed ICMP packets at perimeter devices

Patch Information

Security patches addressing this integer underflow vulnerability are available in FreeRTOS-Plus-TCP versions V4.4.1 and V4.2.6. The fix implements proper validation of packet length fields before performing subtraction operations, ensuring that the length value is sufficient to accommodate header sizes.

Organizations should download the patched versions from the official FreeRTOS GitHub repository:

Additional details are available in the AWS Security Bulletin 2026-021.

Workarounds

Disable outgoing ping functionality by setting ipconfigSUPPORT_OUTGOING_PINGS to 0 in the FreeRTOS-Plus-TCP configuration if ping support is not operationally required
Implement network access controls to restrict which hosts can send ICMP traffic to vulnerable devices
Deploy application-layer gateways or firewalls that can inspect and filter malformed ICMP packets before they reach vulnerable endpoints
Isolate critical embedded systems on dedicated VLANs with strict ingress filtering

bash

# FreeRTOSIPConfig.h configuration to disable outgoing ping support
# Set this value to 0 to disable outgoing ping functionality
#define ipconfigSUPPORT_OUTGOING_PINGS    0

# Network-level mitigation using iptables to rate-limit ICMP on Linux gateways
iptables -A FORWARD -p icmp --icmp-type echo-reply -m limit --limit 10/sec -j ACCEPT
iptables -A FORWARD -p icmp --icmp-type echo-reply -j DROP