CVE-2026-1736 is a security vulnerability identified in Open5GS, an open-source 5G core network implementation widely used for research and production environments. The flaw exists in the SGWC (Serving Gateway Control) component, specifically within the function sgwc_s11_handle_create_indirect_data_forwarding_tunnel_request located in /src/sgwc/s11-handler.c. This function handles Create Indirect Data Forwarding Tunnel requests over the S11 interface, which is critical for managing user plane tunnels between the Serving Gateway and other 5G core components. The vulnerability manifests as a reachable assertion failure, meaning that crafted network messages can trigger an assertion condition that causes the process to terminate unexpectedly. Since the attack vector is network-exposed and requires no authentication or user interaction, an attacker can remotely cause a denial of service by crashing the SGWC process, potentially disrupting 5G data forwarding services. The CVSS v4.0 base score is 6.9 (medium severity), reflecting the network attack vector, low complexity, and no privileges or user interaction required, but limited impact confined to availability. The vulnerability was publicly disclosed and fixed after version 2.7.6, and no known exploits have been observed in the wild to date. The issue underscores the importance of robust input validation and error handling in telecom protocol implementations to maintain service continuity in 5G networks.

For European organizations, particularly telecom operators and infrastructure providers deploying Open5GS as part of their 5G core network, this vulnerability poses a risk of service disruption. An attacker exploiting this flaw can remotely crash the SGWC component, leading to denial of service conditions affecting user plane data forwarding. This could degrade network performance, interrupt subscriber connectivity, and impact critical services relying on 5G connectivity. Given the increasing reliance on 5G for industrial automation, smart city applications, and emergency services in Europe, such disruptions could have broader economic and societal consequences. Additionally, repeated exploitation attempts could increase operational costs due to incident response and recovery efforts. While the vulnerability does not directly compromise confidentiality or integrity, availability impacts in telecom infrastructure are critical and can cascade to affect multiple dependent services and customers.

European organizations should immediately apply the official patch or upgrade Open5GS to a version later than 2.7.6 where the vulnerability is fixed. In parallel, network segmentation and strict access controls should be enforced on the S11 interface to limit exposure to untrusted networks. Deploying intrusion detection or anomaly detection systems that monitor S11 signaling traffic can help identify suspicious or malformed tunnel creation requests indicative of exploitation attempts. Operators should also implement robust logging and alerting for SGWC process crashes or restarts to enable rapid incident response. Regular security audits and fuzz testing of telecom protocol handlers are recommended to proactively identify similar vulnerabilities. Finally, organizations should maintain up-to-date threat intelligence feeds and collaborate with industry groups to share information about emerging threats targeting 5G core components.