CVE-2026-27643 is a vulnerability classified under CWE-209 (Generation of Error Message Containing Sensitive Information) found in the free5GC open-source 5G core network project, specifically in the User Data Repository (UDR) component up to version 1.4.1. The issue arises in the NEF (Network Exposure Function) component, which handles network exposure services including the Nnef_PfdManagement service. When the NEF component encounters parsing errors—such as invalid characters in JSON payloads—it returns detailed error messages to remote clients. These messages include internal parsing details like the exact invalid character and its position, which should normally be suppressed to avoid leaking implementation details. This leakage can facilitate attackers in fingerprinting the service, understanding its internal data handling, and potentially crafting more targeted attacks. The vulnerability is remotely exploitable without requiring authentication or user interaction, increasing its risk profile. The free5gc/udr repository has addressed this issue in pull request 56 by sanitizing error messages to prevent sensitive information disclosure. No direct application-level workarounds exist, so applying the patch is the recommended mitigation. The CVSS v4.0 base score is 6.6, reflecting medium severity due to the high confidentiality impact but no direct impact on integrity or availability.

The primary impact of CVE-2026-27643 is information disclosure that can aid attackers in reconnaissance and service fingerprinting. By revealing detailed parsing errors, attackers can infer the internal structure and behavior of the free5GC UDR component, which may facilitate further exploitation attempts or targeted attacks against 5G core network infrastructure. Since free5GC is an open-source 5G core network implementation, organizations deploying it in production environments—such as telecom operators, network equipment providers, and research institutions—could have their network core components exposed to reconnaissance. This could lead to increased risk of subsequent attacks like protocol manipulation, unauthorized data access, or denial of service. Although the vulnerability does not directly compromise data integrity or availability, the leakage of sensitive internal error details undermines the confidentiality of the system and can weaken overall security posture. Given the critical role of 5G core networks in telecommunications infrastructure, any vulnerability facilitating attacker reconnaissance is significant.

The definitive mitigation for CVE-2026-27643 is to apply the patch provided in free5gc/udr pull request 56, which sanitizes error messages to prevent leakage of sensitive parsing details. Organizations should upgrade free5GC UDR to a version that includes this fix or manually apply the patch if upgrading is not immediately feasible. Since no application-level workarounds exist, network-level controls can provide temporary risk reduction: implementing strict input validation and filtering at network boundaries to prevent malformed requests from reaching the NEF component can reduce exposure. Additionally, deploying web application firewalls (WAFs) configured to detect and block suspicious payloads targeting the Nnef_PfdManagement service may help. Monitoring and logging access to the NEF component for unusual error message requests can aid in early detection of reconnaissance attempts. Finally, organizations should follow secure deployment best practices for 5G core components, including network segmentation and least privilege access controls, to limit the impact of any potential exploitation.