Comprehensive Technical Analysis of EUVD-2026-4231 (CVE-2025-66719)

Free5GC NRF Access Token Scope Bypass Vulnerability

1. Vulnerability Assessment and Severity Evaluation

Vulnerability Overview

EUVD-2026-4231 (CVE-2025-66719) is a critical authentication bypass vulnerability in Free5GC’s Network Repository Function (NRF) v1.4.0, specifically within the AccessTokenScopeCheck() function in internal/sbi/processor/access_token.go. The flaw allows an attacker to bypass scope validation when submitting a crafted targetNF (Network Function) value, enabling the generation of access tokens with arbitrary scopes.

CVSS v3.1 Severity Analysis

Metric	Value	Explanation
Attack Vector (AV)	Network (N)	Exploitable remotely over the network.
Attack Complexity (AC)	Low (L)	No specialized conditions required.
Privileges Required (PR)	None (N)	No authentication needed.
User Interaction (UI)	None (N)	No user action required.
Scope (S)	Unchanged (U)	Impact confined to the vulnerable component.
Confidentiality (C)	High (H)	Unauthorized access to sensitive NF data.
Integrity (I)	High (H)	Ability to manipulate NF communications.
Availability (A)	None (N)	No direct impact on system availability.

Base Score: 9.1 (Critical) The vulnerability is highly exploitable due to its low attack complexity and no authentication requirement, making it a severe risk for 5G core networks.

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2. Potential Attack Vectors and Exploitation Methods

Exploitation Mechanism

1. Token Request Manipulation

   • An attacker sends a maliciously crafted OAuth 2.0 token request to the NRF, specifying a fake targetNF (e.g., a non-existent or spoofed NF).
   • The AccessTokenScopeCheck() function fails to validate the scope when the targetNF is manipulated, allowing the attacker to request any scope (e.g., nrf:register, nrf:discover, udm:subscriber-data).

2. Token Abuse for Unauthorized Access

   • The attacker obtains a valid access token with elevated privileges, which can be used to:
     • Register rogue NFs in the 5G core.
     • Discover and enumerate sensitive NFs (e.g., UDM, AUSF, PCF).
     • Access subscriber data (if UDM scope is granted).
     • Manipulate network policies (if PCF scope is granted).

3. Lateral Movement & Persistence

   • Once an attacker gains a privileged token, they can:
     • Impersonate legitimate NFs (e.g., SMF, AMF).
     • Exfiltrate sensitive data (e.g., IMSI, SUPI, authentication vectors).
     • Disrupt network services by deregistering critical NFs.

Proof-of-Concept (PoC) Exploitation

A sample attack flow could involve:

POST /oauth2/token HTTP/1.1
Host: nrf.free5gc.example
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&
client_id=malicious_client&
client_secret=any_value&
scope=nrf:register%20udm:subscriber-data&
targetNF=FAKE_NF_123

• The NRF bypasses scope validation due to the crafted targetNF.
• The attacker receives a token with nrf:register and udm:subscriber-data scopes, enabling full control over NF registration and subscriber data access.

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3. Affected Systems and Software Versions

Vulnerable Software

• Free5GC NRF (Network Repository Function) v1.4.0
• Potential Impact on Other Versions
  • Earlier versions (pre-1.4.0) may also be affected if they share the same AccessTokenScopeCheck() logic.
  • Later versions (post-1.4.0) may remain vulnerable if the patch is not applied.

Affected Deployments

• 5G Core Networks using Free5GC (common in private 5G networks, research labs, and telecom testbeds).
• Cloud-native 5G deployments where NRF is exposed to untrusted networks.
• Multi-vendor 5G environments where Free5GC NRF interacts with other NFs (e.g., Open5GS, Ericsson, Nokia).

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4. Recommended Mitigation Strategies

Immediate Actions

1. Apply the Official Patch

   • Free5GC has released a fix in PR #73 (GitHub PR).
   • Upgrade to the latest NRF version (post-1.4.0) or apply the patch manually.

2. Temporary Workarounds (if patching is delayed)

   • Restrict NRF Access via network segmentation (e.g., VLANs, firewalls).
   • Enforce strict OAuth 2.0 client validation (e.g., IP whitelisting, mutual TLS).
   • Disable unused scopes in NRF configuration to limit exposure.

3. Enhanced Monitoring & Detection

   • Log and alert on suspicious token requests (e.g., unusual targetNF values).
   • Implement rate limiting on /oauth2/token endpoints to prevent brute-force attacks.
   • Deploy an API security gateway (e.g., Kong, Apigee) to filter malicious requests.

Long-Term Security Hardening

1. Implement Zero Trust for 5G Core

   • Enforce mutual TLS (mTLS) for all NF communications.
   • Use OAuth 2.0 with Proof-of-Possession (PoP) to prevent token replay attacks.

2. Regular Security Audits

   • Conduct penetration testing on NRF and other NFs.
   • Review OAuth 2.0 scope policies to ensure least-privilege access.

3. Vendor & Supply Chain Security

   • Monitor Free5GC GitHub for updates and apply patches promptly.
   • Verify third-party NFs for similar vulnerabilities.

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5. Impact on European Cybersecurity Landscape

Strategic & Operational Risks

1. Critical Infrastructure Threats

   • 5G networks are foundational for smart cities, industrial IoT, and emergency services in the EU.
   • A compromise of NRF could lead to cascading failures in telecom, energy, and transportation sectors.

2. Regulatory & Compliance Implications

   • GDPR Violations: Unauthorized access to subscriber data (e.g., IMSI, SUPI) could result in heavy fines (up to 4% of global revenue).
   • NIS2 Directive: EU operators must report critical vulnerabilities and implement risk mitigation measures.
   • ENISA Guidelines: Failure to patch could lead to non-compliance with EU 5G security toolbox.

3. Geopolitical & Espionage Risks

   • State-sponsored actors could exploit this flaw for surveillance or disruption of EU 5G networks.
   • Supply chain attacks on Free5GC could be used to compromise multiple operators.

4. Economic & Reputational Damage

   • Downtime in 5G services could cost millions per hour for telecom providers.
   • Loss of customer trust due to data breaches or service disruptions.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Vulnerable Function: AccessTokenScopeCheck() in internal/sbi/processor/access_token.go
• Flaw: The function skips scope validation if the targetNF is not found in the NRF’s registry, allowing attackers to bypass OAuth 2.0 scope restrictions.
• Code Snippet (Vulnerable Logic):

  func AccessTokenScopeCheck(scope, targetNF string) bool {
      if !nrf.NFExists(targetNF) {  // Bypass if targetNF doesn't exist
          return true
      }
      // ... (rest of scope validation logic)
  }
  

• Patch Fix: The updated version enforces scope validation regardless of targetNF existence.

Exploitation Requirements

• Network Access: Attacker must be able to send HTTP requests to the NRF (e.g., via exposed API endpoints).
• No Authentication Needed: The vulnerability is pre-authentication, making it highly exploitable.
• Minimal Tools Required: A simple HTTP client (e.g., curl, Postman) is sufficient for exploitation.

Detection & Forensics

1. Indicators of Compromise (IoCs)

   • Unusual targetNF values in NRF logs (e.g., FAKE_NF_123).
   • Tokens with excessive scopes (e.g., nrf:register + udm:subscriber-data).
   • Unexpected NF registrations in the NRF database.

2. Log Analysis

   • NRF access logs (/var/log/free5gc/nrf.log) should be monitored for:
     • High-frequency token requests from a single IP.
     • Requests with malformed targetNF values.

3. Network Traffic Analysis

   • Wireshark/Zeek can detect:
     • OAuth 2.0 token requests with unusual scope parameters.
     • Unusual NF registration attempts.

Advanced Mitigation Techniques

1. Runtime Application Self-Protection (RASP)

   • Deploy RASP solutions (e.g., OpenRASP) to block malicious token requests at runtime.

2. OAuth 2.0 Token Binding

   • Implement token binding to prevent token theft and replay attacks.

3. AI-Based Anomaly Detection

   • Use machine learning models to detect abnormal token request patterns.

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Conclusion & Recommendations

EUVD-2026-4231 (CVE-2025-66719) is a critical authentication bypass vulnerability in Free5GC NRF that poses severe risks to 5G core networks. Given its CVSS 9.1 score, low exploitation complexity, and high impact on confidentiality and integrity, immediate patching is mandatory.

Key Recommendations for Security Teams

✅ Patch Free5GC NRF immediately (PR #73). ✅ Isolate NRF from untrusted networks (firewalls, VLANs). ✅ Monitor for suspicious token requests (SIEM, IDS/IPS). ✅ Enforce least-privilege OAuth 2.0 scopes. ✅ Conduct a 5G security audit to identify similar vulnerabilities.

Long-Term Strategic Actions

🔹 Adopt Zero Trust Architecture for 5G core networks. 🔹 Engage with ENISA & national CSIRTs for threat intelligence sharing. 🔹 Participate in EU 5G security initiatives (e.g., EU 5G Toolbox, NIS2 compliance).

Failure to mitigate this vulnerability could lead to catastrophic breaches in European 5G infrastructure, with far-reaching consequences for national security, economic stability, and citizen privacy.