A critical security vulnerability has been discovered in FreeRTOS-Plus-TCP, Amazon Web Services’ open-source TCP/IP stack implementation for real-time operating systems.
Designated as CVE-2025-5688, this out-of-bounds write vulnerability could potentially allow attackers to execute arbitrary code or crash affected systems through specially crafted LLMNR or mDNS queries containing excessively long DNS names.
The vulnerability specifically impacts systems utilizing Buffer Allocation Scheme 1 with LLMNR or mDNS protocols enabled, affecting multiple versions of the widely-deployed networking stack used in embedded and IoT devices worldwide.
CVE-2025-5688 represents a buffer overflow condition that occurs during the processing of Link-Local Multicast Name Resolution (LLMNR) or Multicast Domain Name System (mDNS) queries when attackers submit DNS names exceeding expected length boundaries.
The vulnerability exploits a weakness in FreeRTOS-Plus-TCP’s buffer management system, specifically when Buffer Allocation Scheme 1 is implemented.
This allocation scheme relies on pre-defined pools of fixed-size buffers rather than dynamic heap allocation, creating a constraint that the vulnerable code fails to properly validate.
When processing incoming LLMNR or mDNS queries, the affected versions fail to implement adequate bounds checking on DNS name lengths, allowing data to be written beyond allocated buffer boundaries.
This out-of-bounds write condition can corrupt adjacent memory regions, potentially leading to arbitrary code execution if attackers can precisely control the overflow data.
The vulnerability’s exploitation requires network access to send malformed packets to target devices, making it particularly concerning for internet-connected embedded systems and IoT infrastructure.
The technical severity is amplified by FreeRTOS-Plus-TCP’s widespread adoption in critical embedded applications, including industrial control systems, automotive components, and smart home devices.
The stack’s comprehensive protocol support, including IPv6, ARP, DHCP, DNS, LLMNR, mDNS, NBNS, Router Advertisement, Neighbor Discovery, ICMP, and ICMPv6, makes it an attractive target for attackers seeking to compromise embedded systems through network-based attacks.
FreeRTOS-Plus-TCP Vulnerability
The vulnerability demonstrates a staggered impact timeline across different protocol implementations within FreeRTOS-Plus-TCP.
Systems utilizing LLMNR functionality are affected across a broader version range, from v2.3.4 through v4.3.1, representing several years of vulnerable deployments.
Meanwhile, mDNS-related vulnerabilities affect versions v4.0.0 through v4.3.1, indicating the issue was introduced with enhanced mDNS support in the 4.0 release series.
Importantly, systems implementing Buffer Allocation Scheme 2, which dynamically allocates buffers from heap memory, remain unaffected by this particular vulnerability.
This architectural difference provides natural protection against the specific overflow condition, as dynamic allocation can accommodate variable-length DNS names without fixed buffer constraints.
However, organizations must verify their buffer allocation configuration to determine vulnerability status, as many embedded implementations default to Scheme 1 for predictable memory usage patterns.
The vulnerability’s network-accessible nature significantly increases its risk profile, as remote attackers can potentially exploit affected devices without requiring physical access or prior system compromise.
Organizations should prioritize immediate upgrades to this patched version, particularly for internet-connected devices and critical infrastructure components
Security Recommendations
Amazon Web Services has released FreeRTOS-Plus-TCP version 4.3.2 to address CVE-2025-5688, implementing proper bounds checking and input validation for DNS name processing in both LLMNR and mDNS handlers.
This characteristic makes CVE-2025-5688 particularly dangerous for internet-facing embedded devices and systems deployed in hostile network environments.
The absence of available workarounds emphasizes the necessity of applying the official patch rather than implementing temporary mitigations.
Beyond immediate patching, organizations should conduct comprehensive audits of their FreeRTOS-Plus-TCP deployments to identify affected systems and verify buffer allocation scheme configurations.
Security teams should also review network segmentation and access controls for embedded devices, implementing additional layers of protection against network-based attacks targeting similar vulnerabilities in the future.
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