CVE-2024-31029: FreeCoAP DoS Vulnerability Explained

CVE-2024-31029 Overview

CVE-2024-31029 is an out-of-bounds read vulnerability [CWE-125] in the server_handle_regular function of test_coap_server.c within the FreeCoAP project. The flaw allows remote attackers to trigger a Denial of Service (DoS) by sending specially crafted Constrained Application Protocol (CoAP) packets to a vulnerable server. The issue affects keith-cullen FreeCoAP version 0.7. Because CoAP servers typically operate as long-running network services in Internet of Things (IoT) and embedded deployments, an unauthenticated remote attacker can disrupt service availability without user interaction.

Critical Impact

Remote unauthenticated attackers can crash FreeCoAP servers by sending malformed CoAP packets, resulting in service disruption for dependent IoT clients.

Affected Products

• keith-cullen FreeCoAP version 0.7
• test_coap_server reference server component
• Downstream projects embedding the vulnerable server_handle_regular routine

Discovery Timeline

• 2024-10-22 - CVE-2024-31029 published to the National Vulnerability Database (NVD)
• 2025-06-24 - Last updated in NVD database

Technical Details for CVE-2024-31029

Vulnerability Analysis

The vulnerability resides in the server_handle_regular function inside test_coap_server.c. This function processes incoming CoAP request messages on the server side. An attacker can craft a malformed CoAP packet that triggers an out-of-bounds read during request parsing. The read access violates buffer boundaries within the server process. The result is an abnormal program termination or undefined behavior, producing a Denial of Service.

FreeCoAP implements the CoAP protocol defined in RFC 7252 for constrained network environments. CoAP runs over User Datagram Protocol (UDP) by default, so the attacker does not need to complete a connection handshake. A single datagram is sufficient to reach the vulnerable parsing path.

Root Cause

The root cause is missing or insufficient bounds checking on attacker-controlled fields parsed by server_handle_regular. The function reads past the end of an input buffer when option lengths or payload markers in the inbound CoAP message are inconsistent with the actual datagram size. CWE-125 (Out-of-bounds Read) captures this class of memory safety defect.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker sends a single malformed CoAP datagram to the server's UDP listener. The server invokes server_handle_regular to parse the message, the out-of-bounds read occurs, and the process terminates. The vulnerability is described in the FreeCoAP GitHub Issue Discussion and a GitHub Gist Code Snippet demonstrating the crafted packet.

The vulnerability manifests during CoAP option parsing. No verified exploitation code is reproduced here. See the referenced advisory for technical reproduction details.

Detection Methods for CVE-2024-31029

Indicators of Compromise

• Unexpected termination or repeated restarts of coap_server or test_coap_server processes
• Core dump files generated by the FreeCoAP server process referencing server_handle_regular
• Inbound UDP traffic to CoAP port 5683 from unexpected sources immediately preceding a crash
• Gaps in CoAP service availability logs correlated with malformed packet arrival

Detection Strategies

• Inspect UDP/5683 traffic for malformed CoAP headers, inconsistent option lengths, or truncated payload markers
• Monitor host process tables for abnormal exit codes or segmentation faults in FreeCoAP binaries
• Apply network intrusion detection signatures that validate CoAP option length fields against datagram size
• Correlate crash telemetry with source IP addresses to identify scanning or targeted DoS activity

Monitoring Recommendations

• Enable verbose logging in FreeCoAP server deployments to capture parser errors before termination
• Forward host and network telemetry to a centralized analytics platform for cross-source correlation
• Set alerting thresholds for repeated CoAP server restarts within short time windows
• Track UDP packet rates targeting CoAP endpoints to identify amplification or flooding attempts

How to Mitigate CVE-2024-31029

Immediate Actions Required

• Inventory all systems running keith-cullen FreeCoAP 0.7, including embedded and IoT deployments
• Restrict network access to CoAP UDP port 5683 using firewall rules limited to trusted clients
• Place vulnerable CoAP servers behind a CoAP-aware proxy or gateway that validates message structure
• Monitor the upstream GitHub issue for a fixed release

Patch Information

At the time of NVD publication, no official patched release of FreeCoAP addressing CVE-2024-31029 is referenced in the advisory. Track the keith-cullen/FreeCoAP issue tracker for fix availability. Organizations maintaining custom forks should add bounds checks to server_handle_regular before parsing CoAP option and payload fields.

Workarounds

• Disable the test_coap_server component in production if it is not required
• Apply network segmentation so only authenticated, trusted hosts can reach the CoAP listener
• Rate-limit inbound UDP traffic to CoAP ports to reduce the impact of repeated crash attempts
• Deploy a watchdog or supervisor process to automatically restart the CoAP server after a crash while a fix is unavailable

# Example iptables rule restricting CoAP access to a trusted subnet
iptables -A INPUT -p udp --dport 5683 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p udp --dport 5683 -j DROP