Can you explain this vulnerability to me?

[{'type': 'paragraph', 'content': "CVE-2026-27643 is an improper error handling vulnerability in the free5GC UDR (Unified Data Repository) component, specifically affecting the NEF (Network Exposure Function) component's handling of the Nnef_PfdManagement GET request."}, {'type': 'paragraph', 'content': "When a client requests Policy Flow Description (PFD) data for a non-existent application ID, the UDR correctly returns a 404 Not Found error. However, the NEF component incorrectly tries to parse the 404 error response body as JSON, which fails due to malformed content (e.g., an unexpected 'null' string appended). This parsing failure causes NEF to respond with a misleading 500 Internal Server Error instead of propagating the correct 404 status."}, {'type': 'paragraph', 'content': 'This improper handling leaks internal parsing error details (such as "invalid character \'n\' after top-level value") to remote clients, which can aid attackers in fingerprinting the service and understanding internal logic.'}, {'type': 'paragraph', 'content': 'The root cause was missing return statements after sending error responses in several functions, allowing further unintended code execution that corrupted the response body. The issue was fixed by adding these missing return statements to properly halt execution after errors.'}] [1, 3, 4]

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How can this vulnerability impact me? :

This vulnerability can impact you by causing the NEF component to leak internal error details to remote clients, which can be exploited by attackers to fingerprint the service and gain insights into its internal workings.

Additionally, the NEF incorrectly returns a 500 Internal Server Error instead of the appropriate 404 Not Found status when requested data is missing. This misrepresentation of error states can confuse clients and complicate troubleshooting.

Such information exposure and improper error signaling can be leveraged in probing or reconnaissance attacks against your 5G core network infrastructure.

There is no direct application-level workaround; the recommended action is to apply the patch that fixes the error handling.

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How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:

I don't know

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How can this vulnerability be detected on my network or system? Can you suggest some commands?

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by issuing a GET request to the NEF endpoint for a non-existent application PFD and observing the response and logs.'}, {'type': 'paragraph', 'content': 'Specifically, if the NEF component returns a 500 Internal Server Error with an internal parsing error message such as "invalid character \'n\' after top-level value" instead of a 404 Not Found, this indicates the presence of the vulnerability.'}, {'type': 'paragraph', 'content': 'A suggested command to test this is to disable OAuth in the nrfcfg.yaml configuration, start free5GC, and then run a GET request like:'}, {'type': 'list_item', 'content': 'curl -X GET http://<NEF_IP>:<NEF_PORT>/nnrf-nnef-pfdmanagement/v1/applications/<non-existent-app-id>/pfd'}, {'type': 'paragraph', 'content': 'Check the NEF logs for APIService errors related to JSON parsing failures and verify that the UDR returns a 404 Not Found while NEF incorrectly returns a 500 error.'}] [1]

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What immediate steps should I take to mitigate this vulnerability?

The immediate mitigation step is to apply the patch provided in free5GC UDR pull request #56, which adds missing return statements after error responses to prevent improper error handling and information leakage.

There is no direct workaround at the application level other than upgrading to a fixed version of free5GC that includes this patch.

Users should upgrade to free5GC versions including the fix merged on January 12, 2026, which corrects the error handling in the NEF and UDR components.

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