free5GC NEF Denial-of-Service via Unreachable notifyUri

free5GC’s Network Exposure Function (NEF) is susceptible to a denial-of-service vulnerability. An attacker with the ability to create a PFD subscription can specify an arbitrary notifyUri. When a PFD change event occurs, the NEF attempts to send an HTTP POST request to the configured notifyUri. If this notification delivery fails (e.g., due to connection refused, DNS resolution failure, or timeout), the NEF process terminates due to an unhandled error condition. This behavior, present in version 4.2.1, allows an unauthenticated attacker to remotely trigger a complete NEF service outage. The vulnerability is triggered by posting to /3gpp-traffic-influence/v1/afdos/subscriptions, /nnef-pfdmanagement/v1/subscriptions, and /3gpp-pfd-management/v1/afdos/transactions, reachable without authentication due to misconfiguration of SBI route groups.

Attack Chain

1. The attacker sends an HTTP POST request to /3gpp-traffic-influence/v1/afdos/subscriptions to create an AF context with afAppId set to “app-nef-dos” and anyUeInd to true.
2. The NEF creates a new AF context subscription and returns a 201 Created response with the Location header indicating the new subscription URI.
3. The attacker sends an HTTP POST request to /nnef-pfdmanagement/v1/subscriptions to create a PFD subscription, including a malicious notifyUri such as http://127.0.0.1:1/notify.
4. The NEF stores the PFD subscription with the attacker-controlled notifyUri.
5. The attacker sends an HTTP POST request to /3gpp-pfd-management/v1/afdos/transactions to trigger a PFD change.
6. The NEF processes the PFD change request and returns a 201 Created response.
7. The NEF attempts to deliver an asynchronous notification to the attacker-specified notifyUri via an HTTP POST request.
8. Because the notifyUri is unreachable (e.g., port 1 is closed), the outbound HTTP POST fails, triggering the logger.Fatal(err) call and terminating the NEF process with exit code 1.

Impact

The vulnerability leads to a complete denial-of-service condition for the free5GC NEF. Successful exploitation results in the NEF process terminating abruptly, causing loss of service and requiring a restart. Since the trigger chain is unauthenticated in v4.2.1, any attacker capable of reaching the NEF’s SBI interface can remotely trigger the process termination. This can be repeated to sustain the outage indefinitely, severely impacting the availability of the 5G network services reliant on the NEF. The vulnerability affects free5GC v4.2.1.

Recommendation

• Apply the upstream fix available at https://github.com/free5gc/nef/pull/25 to prevent the logger.Fatal call on notification delivery failure.
• Deploy the Sigma rule “Detect NEF PFD Subscription with Unreachable notifyUri” to detect attempts to create subscriptions with suspicious callback URLs.
• Monitor NEF container logs for [FATA][NEF][PFDMng] messages, which indicate that the NEF process has terminated due to the vulnerability, as shown in the container log example.
• Review and harden the authentication configuration for NEF SBI route groups to prevent unauthenticated access, as discussed in free5gc/free5gc#858, free5gc/free5gc#859, and free5gc/free5gc#862.
• Audit all code paths that use logger.Fatal and replace them with recoverable error handling.