CVE-2026-1684: Free5GC SMF Denial of Service Vulnerability

CVE-2026-1684 Overview

A denial of service vulnerability has been discovered in Free5GC SMF (Session Management Function) up to version 4.1.0. The vulnerability exists in the HandleReports function within the file /internal/context/pfcp_reports.go of the PFCP (Packet Forwarding Control Protocol) UDP Endpoint component. This flaw allows remote attackers to cause a denial of service condition by sending specially crafted network requests to the affected component.

Critical Impact

Remote attackers can disrupt 5G core network operations by causing denial of service in the SMF component, potentially affecting user connectivity and network services.

Affected Products

• Free5GC SMF up to version 4.1.0
• Free5GC deployments utilizing the affected PFCP UDP Endpoint component
• 5G core network implementations based on vulnerable Free5GC versions

Discovery Timeline

• 2026-01-30 - CVE-2026-1684 published to NVD
• 2026-02-04 - Last updated in NVD database

Technical Details for CVE-2026-1684

Vulnerability Analysis

This vulnerability is classified as CWE-404 (Improper Resource Shutdown or Release), which indicates that the affected component fails to properly release resources when handling certain conditions. In the context of the Free5GC SMF component, the HandleReports function within the PFCP reports handling logic does not adequately manage resources, leading to resource exhaustion or improper state handling that results in service disruption.

The PFCP protocol is critical in 5G networks for communication between the control plane and user plane functions. When the SMF component receives malformed or specifically crafted PFCP messages through the UDP endpoint, the improper resource handling can cause the service to become unresponsive or crash, effectively disrupting 5G core network operations.

Root Cause

The root cause of this vulnerability lies in the improper resource management within the HandleReports function located in /internal/context/pfcp_reports.go. The function fails to properly release or clean up resources under certain conditions, which can be triggered remotely via network requests. This improper resource shutdown or release (CWE-404) allows attackers to exhaust available resources or corrupt internal state, leading to denial of service.

Attack Vector

The attack can be executed remotely over the network without requiring any authentication or user interaction. An attacker with network access to the PFCP UDP endpoint can send specially crafted packets to trigger the vulnerability in the HandleReports function. The attack complexity is low, making it accessible to attackers with basic networking knowledge and tools capable of generating custom UDP packets.

The vulnerability exists in the network-facing PFCP UDP endpoint, which is designed to handle Packet Forwarding Control Protocol messages. By targeting this endpoint with malicious input, attackers can exploit the improper resource handling behavior to cause service disruption.

Detection Methods for CVE-2026-1684

Indicators of Compromise

• Unusual volume of PFCP UDP packets targeting the SMF component on the standard PFCP port (typically port 8805)
• Unexpected SMF service crashes or restarts without apparent cause
• Resource exhaustion patterns in SMF container or process metrics (memory, file descriptors)
• Anomalous network traffic patterns from untrusted sources directed at 5G core infrastructure

Detection Strategies

• Monitor SMF service health and implement alerting for unexpected service restarts or unresponsive states
• Deploy network-level monitoring to detect unusual PFCP traffic patterns or traffic from unauthorized sources
• Implement intrusion detection rules to identify malformed PFCP packets or high-volume UDP traffic targeting SMF endpoints
• Review SMF application logs for error patterns related to the HandleReports function or PFCP processing failures

Monitoring Recommendations

• Enable verbose logging for PFCP message processing within the SMF component to capture potential exploit attempts
• Implement rate limiting on PFCP UDP endpoints to mitigate volumetric attack attempts
• Deploy network segmentation to restrict access to 5G core components from untrusted network zones
• Use container or process monitoring to track resource utilization patterns that may indicate exploitation attempts

How to Mitigate CVE-2026-1684

Immediate Actions Required

• Review the GitHub Pull Request #188 which contains the patch for this vulnerability
• Restrict network access to the PFCP UDP endpoint to only authorized network elements
• Implement network-level filtering to block untrusted traffic from reaching 5G core components
• Monitor affected systems for signs of exploitation while preparing for patch deployment

Patch Information

A patch addressing this vulnerability is available through GitHub Pull Request #188 in the Free5GC SMF repository. Organizations running affected versions should review the patch and apply it according to their change management procedures. Additional technical details and community discussion can be found in GitHub Issue #806.

For tracking and threat intelligence information, refer to the VulDB entry #343477.

Workarounds

• Implement strict network access controls to limit PFCP UDP endpoint exposure to only trusted network elements and management interfaces
• Deploy a firewall or network filter in front of the SMF component to drop PFCP traffic from unauthorized sources
• Consider deploying the SMF component behind a load balancer or proxy capable of rate limiting and basic protocol validation
• If feasible, temporarily disable or isolate the affected SMF instance while applying the patch, and route traffic through unaffected instances

# Example: Restrict PFCP UDP port access using iptables
# Allow PFCP (port 8805) only from trusted UPF addresses
iptables -A INPUT -p udp --dport 8805 -s <trusted_upf_ip> -j ACCEPT
iptables -A INPUT -p udp --dport 8805 -j DROP