CVE-2026-40248: free5GC Auth Bypass Vulnerability

CVE-2026-40248 Overview

CVE-2026-40248 is an Improper Authorization vulnerability affecting the free5GC open-source 5G core network implementation. In versions 4.2.1 and below of the UDR (Unified Data Repository) service, the handler responsible for creating or updating Traffic Influence Subscriptions performs an incomplete validation check. Specifically, the handler checks whether the influenceId path segment equals subs-to-notify, but critically fails to return after sending an HTTP 404 response when this validation fails. As a result, execution continues and the subscription is created or overwritten regardless of the validation outcome.

Critical Impact

An unauthenticated attacker with access to the 5G Service Based Interface can create or overwrite arbitrary Traffic Influence Subscriptions, including injecting attacker-controlled notificationUri values and arbitrary SUPIs (Subscription Permanent Identifiers), by supplying any value for the influenceId path segment.

Affected Products

• free5GC UDR Service version 4.2.1 and below
• free5GC 5G Core Network implementations using vulnerable UDR components

Discovery Timeline

• April 16, 2026 - CVE CVE-2026-40248 published to NVD
• April 16, 2026 - Last updated in NVD database

Technical Details for CVE-2026-40248

Vulnerability Analysis

This vulnerability stems from a classic "missing return statement" programming error in the UDR service's Traffic Influence Subscription handler. When the handler validates the influenceId path segment, it correctly identifies invalid input and sends an HTTP 404 response to the client. However, the code path does not terminate after sending this error response, allowing the subsequent subscription creation or modification logic to execute unconditionally.

The improper authorization classification (CWE-285) is particularly significant in 5G network contexts. Traffic Influence Subscriptions are critical components that control how network traffic is routed and processed. By manipulating these subscriptions, an attacker can inject malicious notificationUri values that redirect sensitive network notifications to attacker-controlled endpoints, potentially exposing subscriber information and enabling further attacks on the 5G infrastructure.

Root Cause

The root cause is a missing return statement or improper control flow in the UDR service handler after validation failure. When checking if the influenceId equals subs-to-notify, the code sends an HTTP 404 error but fails to halt execution. This allows the subscription creation logic to proceed regardless of whether the validation passed or failed, bypassing the intended authorization check entirely.

Attack Vector

The attack is exploitable over the network without authentication by any attacker with access to the 5G Service Based Interface (SBI). The attacker can craft HTTP requests to the UDR service's Traffic Influence Subscription endpoint using arbitrary values for the influenceId path parameter. Despite receiving a 404 error response, the malicious subscription data is processed and stored. This allows attackers to:

1. Create new arbitrary Traffic Influence Subscriptions
2. Overwrite existing legitimate subscriptions
3. Inject attacker-controlled notificationUri values for traffic redirection
4. Specify arbitrary SUPIs to target specific subscribers

The vulnerability mechanism involves the handler's improper validation flow. When a request is made to create or update a Traffic Influence Subscription, the handler checks the influenceId path segment but continues execution after sending an error response, ultimately processing the malicious request. For detailed technical information, see the GitHub Security Advisory.

Detection Methods for CVE-2026-40248

Indicators of Compromise

• HTTP 404 responses from the UDR service followed by successful subscription creation events in logs
• Unexpected or unauthorized Traffic Influence Subscriptions in the UDR database
• Subscriptions containing suspicious notificationUri values pointing to external or unknown endpoints
• Traffic Influence Subscriptions associated with SUPIs that do not match legitimate subscriber records

Detection Strategies

• Monitor UDR service logs for patterns where HTTP 404 responses are followed by successful database write operations
• Implement alerting on Traffic Influence Subscription modifications from untrusted network segments
• Audit all notificationUri values in existing subscriptions against known-good endpoints
• Deploy network traffic analysis to detect unusual patterns on the 5G Service Based Interface

Monitoring Recommendations

• Enable detailed logging on the UDR service to capture all subscription creation and modification events
• Configure SIEM rules to correlate HTTP 404 responses with subsequent subscription database changes
• Implement integrity monitoring on Traffic Influence Subscription tables
• Establish baseline behavior for SBI traffic patterns to identify anomalous access attempts

How to Mitigate CVE-2026-40248

Immediate Actions Required

• Restrict network access to the 5G Service Based Interface to trusted network segments only
• Implement network-level authentication and access controls for all SBI endpoints
• Audit existing Traffic Influence Subscriptions for unauthorized entries or suspicious notificationUri values
• Consider temporarily disabling the Traffic Influence Subscription endpoint if operationally feasible

Patch Information

A patched version was not available at the time of publication. Organizations should monitor the GitHub Security Advisory for updates on patch availability and apply security updates as soon as they are released.

Workarounds

• Implement a network firewall or API gateway in front of the UDR service to validate and sanitize all incoming requests
• Deploy a reverse proxy that enforces proper authorization checks before forwarding requests to the UDR service
• Use network segmentation to isolate the SBI from untrusted network access
• Implement custom middleware or patching at the application layer to add a return statement after the validation failure

# Example: Network segmentation using iptables to restrict SBI access
# Allow only trusted management network (10.0.1.0/24) to access UDR service port
iptables -A INPUT -p tcp --dport 8000 -s 10.0.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8000 -j DROP