CVE-2026-7781: Open5GS DoS Vulnerability in UDM Handler

CVE-2026-7781 Overview

CVE-2026-7781 is a denial of service vulnerability affecting Open5GS through version 2.7.7. The flaw resides in the udm_nudm_uecm_handle_amf_registration_update function within /src/udm/nudm-handler.c, part of the Unified Data Management (UDM) component handling the amf-3gpp-access endpoint. A remote authenticated attacker can manipulate input to the function and trigger improper resource shutdown or release [CWE-404]. The exploit has been disclosed publicly. The Open5GS project was notified through a public issue report but had not responded at the time of disclosure.

Critical Impact
Remote attackers with low privileges can trigger denial of service against the UDM network function, disrupting 5G core subscriber registration and authentication operations.

Affected Products

Open5GS versions up to and including 2.7.7
Open5GS UDM (Unified Data Management) network function
Open5GS amf-3gpp-access endpoint handler

Discovery Timeline

2026-05-04 - CVE-2026-7781 published to NVD
2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-7781

Vulnerability Analysis

The vulnerability exists in Open5GS, an open-source implementation of 5G Core and EPC. The defective code path is the udm_nudm_uecm_handle_amf_registration_update function in /src/udm/nudm-handler.c. This handler processes UE Context Management (UECM) registration updates from the Access and Mobility Management Function (AMF) over the 3GPP-defined Nudm service-based interface.

The weakness is classified as Improper Resource Shutdown or Release [CWE-404]. When a malformed or unexpected request reaches the handler, the UDM fails to properly release allocated resources or terminate the request flow cleanly. This produces a denial of service condition affecting the UDM service.

Because the UDM is responsible for subscriber data, authentication credentials, and access authorization in 5G core deployments, a sustained outage interrupts UE registration, session establishment, and mobility procedures across the network.

Root Cause

The root cause is improper handling of resource lifecycle within the AMF registration update path. The function does not adequately validate or release state when processing crafted Nudm_UECM requests, leaving the UDM process in a degraded or unresponsive state.

Attack Vector

The attack is network-reachable and requires low privileges, consistent with an actor able to send messages on the service-based interface (SBI) toward the UDM. No user interaction is needed. The attacker submits a crafted amf-3gpp-access registration update, which routes to the vulnerable handler and triggers the resource release flaw.

No verified exploit code is included in this advisory. Refer to the GitHub Issue Tracker and the VulDB Vulnerability #360978 entry for technical disclosure details.

Detection Methods for CVE-2026-7781

Indicators of Compromise

Unexpected restarts or crashes of the Open5GS open5gs-udmd process
Anomalous Nudm_UECM AMF registration update requests targeting /nudm-uecm/v1/{supi}/registrations/amf-3gpp-access
Spikes in 5xx HTTP/2 responses from the UDM SBI endpoint
Subscriber registration failures correlated with UDM service degradation

Detection Strategies

Monitor UDM SBI traffic for malformed JSON payloads or abnormal request patterns to the amf-3gpp-access endpoint
Alert on repeated registration update requests from a single NF consumer within short time windows
Track UDM process health metrics including memory growth, file descriptor counts, and request handler latency
Correlate AMF-side registration failures with UDM service availability events

Monitoring Recommendations

Enable verbose logging on the UDM nudm-handler component to capture request context preceding crashes
Deploy SBI traffic inspection between AMF and UDM to baseline normal Nudm_UECM message structure
Forward 5G core network function logs to a centralized analytics platform for cross-NF correlation

How to Mitigate CVE-2026-7781

Immediate Actions Required

Restrict network access to the UDM SBI so only authorized AMF instances can reach the amf-3gpp-access endpoint
Enforce mutual TLS and OAuth2 token validation on all SBI traffic to the UDM
Monitor the Open5GS GitHub repository for an upstream fix and rebuild from source when available
Implement rate limiting on Nudm_UECM registration update requests at the service mesh or API gateway layer

Patch Information

No official patch was available from the Open5GS project at the time of disclosure. The maintainers had not responded to the public issue report. Track the GitHub Issue Tracker for remediation status and review pull requests touching src/udm/nudm-handler.c for fix candidates.

Workarounds

Segment the 5G core control plane network to isolate UDM instances from untrusted networks and tenants
Deploy a service mesh proxy in front of the UDM to filter malformed SBI requests before they reach the handler
Run multiple UDM instances behind a load balancer with health checks to maintain availability if a single instance fails
Apply strict NF-to-NF authorization policies so only AMF function profiles can invoke the amf-3gpp-access registration endpoint

bash

# Example iptables rule restricting UDM SBI access to known AMF subnets
iptables -A INPUT -p tcp --dport 7777 -s 10.10.20.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 7777 -j DROP