The vulnerability CVE-2026-27642 affects the Unified Data Management (UDM) component of free5gc, an open-source 5G core network project. 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 %00 (null byte). This improper input validation triggers internal URL parsing errors within the net/url package, which does not accept control characters. These parsing errors cause the system to expose detailed error messages that reveal internal implementation details. Although this does not directly lead to code execution or data manipulation, the exposure of system-level error information can be leveraged by attackers for service fingerprinting and reconnaissance, aiding in crafting further attacks. The vulnerability requires no authentication or user interaction, making it remotely exploitable over the network. The issue is tracked under CWE-20 (Improper Input Validation). The free5gc project has addressed this vulnerability in pull request 75 for the udm component, but no direct workaround is available at the application level, emphasizing the need for patching.

The primary impact of CVE-2026-27642 is information disclosure through detailed error messages caused by improper input validation. Attackers can remotely exploit this vulnerability without authentication to gain insights into the internal workings of the free5gc UDM service, facilitating service fingerprinting and reconnaissance. This information can be used to identify specific versions, configurations, or weaknesses in the 5G core network infrastructure, potentially aiding in more targeted and sophisticated attacks. While it does not directly compromise confidentiality, integrity, or availability, the vulnerability lowers the attacker's effort to map the network and identify exploitable components. Given that free5gc is used in 5G core networks, which are critical infrastructure, this vulnerability could indirectly increase the risk of more severe attacks against telecommunications providers and their customers worldwide.

Organizations using free5gc UDM should immediately apply the official patch provided in free5gc/udm pull request 75 to remediate the vulnerability. Since no direct application-level workaround exists, patching is the primary mitigation strategy. Additionally, network-level protections such as Web Application Firewalls (WAFs) or Intrusion Prevention Systems (IPS) can be configured to detect and block requests containing suspicious control characters or malformed supi parameters to reduce exposure. Monitoring logs for unusual URL parsing errors or repeated malformed requests can help detect exploitation attempts. It is also advisable to restrict access to the Nudm_UEAU service to trusted networks or through secure VPNs to limit exposure. Regularly updating free5gc components and following secure coding practices for input validation in custom deployments will help prevent similar issues.