CVE-2026-40343 Overview
CVE-2026-40343 is a fail-open request handling vulnerability affecting the free5GC User Data Repository (UDR) service, a critical component of the free5GC open-source 5G mobile core network implementation. The vulnerability exists in versions up to and including 1.4.2, where the /nudr-dr/v2/policy-data/subs-to-notify POST handler continues processing requests even after encountering request body retrieval or deserialization errors. This improper check for exceptional conditions (CWE-754) may allow attackers to create unintended Policy Data notification subscriptions with invalid, empty, or partially processed input data.
Critical Impact
Attackers can exploit this fail-open flaw to create malformed Policy Data notification subscriptions in 5G core network infrastructure, potentially disrupting subscriber data management and policy enforcement mechanisms.
Affected Products
- free5GC free5gc versions up to and including 1.4.2
- free5GC UDR (User Data Repository) component
- Go-based free5GC deployments
Discovery Timeline
- 2026-04-22 - CVE-2026-40343 published to NVD
- 2026-04-23 - Last updated in NVD database
Technical Details for CVE-2026-40343
Vulnerability Analysis
The vulnerability resides in the UDR service's handling of POST requests to the /nudr-dr/v2/policy-data/subs-to-notify endpoint. This endpoint is responsible for managing Policy Data notification subscriptions within the 5G core network architecture. The UDR component serves as the centralized repository for subscriber data and policy information, making it a critical component in 5G deployments.
The flaw is classified under CWE-754 (Improper Check for Unusual or Exceptional Conditions), indicating that the application fails to properly validate and halt processing when error conditions occur during request handling. In 5G networks, the UDR interfaces with multiple network functions including the Policy Control Function (PCF) and Unified Data Management (UDM), making this vulnerability particularly concerning for telecommunications infrastructure.
Root Cause
The root cause of this vulnerability is a fail-open design pattern in the request handling logic. When the UDR service encounters errors during request body retrieval or JSON deserialization, it does not properly terminate the request processing pipeline. Instead, execution continues with potentially malformed, incomplete, or empty data structures. This violates the principle of secure defaults and allows invalid subscriptions to be created in the system.
Attack Vector
The attack vector is network-based with low complexity requirements. An unauthenticated attacker with network access to the UDR service can send malformed HTTP POST requests to the vulnerable endpoint. By crafting requests that trigger deserialization errors while still allowing partial processing, attackers can potentially:
- Create policy notification subscriptions with invalid callback URLs
- Register subscriptions with missing or malformed subscriber identifiers
- Inject partially processed data that could affect downstream policy enforcement
- Cause inconsistent state in the policy data store
The vulnerability does not require authentication or user interaction, making it exploitable by any attacker with network access to the 5G core network's internal interfaces. The attack mechanism involves sending specially crafted POST requests designed to fail during body parsing while exploiting the fail-open behavior to continue subscription creation.
Detection Methods for CVE-2026-40343
Indicators of Compromise
- Unusual volume of POST requests to /nudr-dr/v2/policy-data/subs-to-notify endpoint with malformed bodies
- Policy Data subscriptions in the UDR database with empty or invalid callback URLs
- Application logs showing repeated deserialization errors followed by successful subscription creation
- Subscriptions created with incomplete subscriber identification data
Detection Strategies
- Monitor UDR service logs for patterns of request body parsing errors that do not result in HTTP 400 responses
- Implement application-level logging to track subscription creation events and correlate with prior error conditions
- Deploy network monitoring to detect anomalous traffic patterns targeting UDR API endpoints
- Audit the subscription database for entries with invalid or missing required fields
Monitoring Recommendations
- Enable verbose logging on the UDR service to capture request processing flow and error handling
- Implement alerting on HTTP 200 responses that follow internal parsing errors in the same request context
- Monitor for unauthorized network access attempts to 5G core network interfaces
- Track subscription creation rates and flag anomalous spikes in Policy Data notification registrations
How to Mitigate CVE-2026-40343
Immediate Actions Required
- Review network segmentation to ensure UDR services are not directly accessible from untrusted networks
- Implement additional input validation at network boundaries (API gateway, reverse proxy)
- Audit existing Policy Data subscriptions for invalid or suspicious entries and remove malformed records
- Enable enhanced logging to detect exploitation attempts
Patch Information
As of the publication date, a patched version is not available from the free5GC project. Organizations should monitor the free5GC GitHub Security Advisory for updates regarding a security fix. Until a patch is released, implementing the workarounds below is strongly recommended.
Workarounds
- Deploy a reverse proxy or API gateway in front of the UDR service to validate request bodies before forwarding
- Implement strict JSON schema validation at the network boundary for all requests to vulnerable endpoints
- Restrict network access to the UDR service using firewall rules to allow only authorized network functions
- Consider implementing application-layer request filtering to reject malformed or incomplete subscription requests
# Example: Network restriction using iptables to limit UDR access
# Replace UDR_PORT and TRUSTED_NF_IPS with your deployment values
iptables -A INPUT -p tcp --dport ${UDR_PORT} -s ${TRUSTED_NF_IPS} -j ACCEPT
iptables -A INPUT -p tcp --dport ${UDR_PORT} -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
