The vulnerability identified as CVE-2026-27642 affects the Unified Data Management (UDM) component of free5gc, an open-source 5G core network implementation. Specifically, in versions up to and including 1.4.1, the UDM's Nudm_UEAU service improperly validates input in the supi parameter, allowing remote attackers to inject control characters such as the null byte (%00). This improper input validation (classified under CWE-20) leads to internal URL parsing errors within the Go net/url package, which does not accept control characters. When such malformed input is processed, the system returns detailed error messages exposing internal parsing logic and error states. Although this does not directly lead to remote code execution or denial of service, the exposure of internal error details can facilitate service fingerprinting by attackers, aiding in reconnaissance and potentially enabling more targeted attacks. The vulnerability is remotely exploitable without authentication or user interaction, increasing its risk profile. The free5gc project has addressed this issue in a pull request (PR 75), but no direct application-level workarounds exist, making patching the primary mitigation strategy. The vulnerability has a CVSS v4.0 base score of 6.6, reflecting medium severity, with network attack vector, low attack complexity, and no privileges or user interaction required.

The primary impact of CVE-2026-27642 is information disclosure through detailed error messages, which can aid attackers in fingerprinting the free5gc UDM service and understanding its internal workings. This reconnaissance capability can be leveraged to identify vulnerable deployments and plan subsequent attacks, potentially targeting 5G core network infrastructure. While the vulnerability itself does not directly compromise confidentiality, integrity, or availability, the information gained can increase the risk of further exploitation. For organizations deploying free5gc as part of their 5G core network, this could lead to increased exposure to targeted attacks, potentially affecting network reliability and security. Given the critical role of UDM in subscriber data management within 5G networks, any compromise or attack facilitated by reconnaissance could have downstream effects on subscriber privacy and network operations. The lack of authentication requirement and ease of remote exploitation increase the threat surface, especially for publicly accessible UDM services.

Organizations should prioritize applying the official patch provided by the free5gc project (pull request 75) to remediate this vulnerability. Since no direct application-level workarounds exist, patching is the most effective mitigation. Additionally, network-level protections should be implemented, such as restricting access to the Nudm_UEAU service to trusted management networks or using firewall rules to limit exposure. Deploying Web Application Firewalls (WAFs) or intrusion detection/prevention systems (IDS/IPS) capable of detecting and blocking control character injection attempts can provide an additional layer of defense. Monitoring logs for unusual URL parsing errors or malformed requests targeting the supi parameter can help detect exploitation attempts. Finally, organizations should ensure that their free5gc deployments are regularly updated and incorporate security best practices for 5G core network components, including segmentation and strict access controls.