free5GC SMF Unauthenticated State-Mutating Panic-DoS Vulnerability

free5GC’s SMF (Session Management Function) contains a vulnerability where the UPI (User Plane Interface) management route group lacks proper authentication, exposing it to unauthenticated attacks. Specifically, the DELETE /upi/v1/upNodesLinks/{upNodeRef} handler attempts to dereference a potentially nil UPF (User Plane Function) pointer, leading to a nil-pointer dereference and panic. This occurs because AN-typed (Access Node) nodes are constructed without a UPF object. An attacker can exploit this by sending an unauthenticated DELETE request, such as DELETE /upi/v1/upNodesLinks/gNB1, crashing the handler and, critically, mutating the in-memory user-plane topology before the panic occurs. This allows an off-path network attacker to trigger a state-mutating panic-DoS against any AN entry by name. This vulnerability affects free5GC version 4.2.1.

Attack Chain

1. An attacker identifies a vulnerable free5GC SMF instance with the exposed UPI endpoint.
2. The attacker crafts a DELETE request targeting the /upi/v1/upNodesLinks/{upNodeRef} endpoint, specifying an AN node name (e.g., gNB1) without any authentication credentials.
3. The attacker sends the unauthenticated DELETE request to the SMF instance.
4. The SMF receives the request and proceeds to process it within the DeleteUpNodeLink handler.
5. The handler identifies the target node as an AN type and executes UpNodeDelete(upNodeRef), which mutates the in-memory user-plane topology, deleting the specified AN entry.
6. The handler then attempts to dereference the UPF field of the AN node, which is nil for AN nodes by design.
7. This dereference results in a nil-pointer dereference, causing a panic in the SMF process.
8. The SMF returns a 500 Internal Server Error, but the topology has already been mutated, denying SMF’s ability to consider that AN in subsequent UPF selection.

Impact

The vulnerability allows an unauthenticated attacker to perform a denial-of-service attack against the free5GC SMF. By sending a single, unauthenticated DELETE request, an attacker can delete arbitrary named entries from SMF’s in-memory user-plane topology and trigger a panic. This impacts SMF’s ability to select UPFs and establish PFCP paths for legitimate UE sessions. The attacker can repeat this process against any AN entry, sustaining the topology denial without needing to authenticate. This can lead to service disruption and impact the availability of the 5G network.

Recommendation

• Apply the upstream fix available at https://github.com/free5gc/smf/pull/199 to patch the nil-pointer dereference in the DeleteUpNodeLink handler.
• Implement authentication and authorization middleware on the UPI route group to prevent unauthenticated access, as demonstrated in the nsmf-oam route group; monitor webserver logs for DELETE requests to the /upi/v1/upNodesLinks/ endpoint without valid authentication headers.
• Deploy the Sigma rule Detect Unauthenticated SMF UPI DELETE Request to identify unauthenticated DELETE requests to the vulnerable endpoint, monitoring webserver logs.