Comprehensive Technical Analysis of CVE-2025-66719

CVE ID: CVE-2025-66719 CVSS Score: 9.1 (Critical) Affected Software: Free5GC NRF (Network Repository Function) v1.4.0 Vulnerability Type: Improper Access Control (Authorization Bypass)

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1. Vulnerability Assessment & Severity Evaluation

Technical Overview

CVE-2025-66719 is a critical authorization bypass vulnerability in the Free5GC NRF (Network Repository Function), a core component of the 5G Core Network (5GC). The flaw resides in the access-token generation logic within the AccessTokenScopeCheck() function (internal/sbi/processor/access_token.go), where scope validation is entirely bypassed when an attacker supplies a crafted targetNF (Network Function) value.

CVSS Breakdown (v3.1)

Metric	Score	Justification
Attack Vector (AV)	Network (N)	Exploitable remotely over the network.
Attack Complexity (AC)	Low (L)	No special conditions required; straightforward exploitation.
Privileges Required (PR)	None (N)	No prior authentication needed.
User Interaction (UI)	None (N)	No user interaction required.
Scope (S)	Changed (C)	Impacts other components (e.g., NFs relying on NRF-issued tokens).
Confidentiality (C)	High (H)	Attacker gains unauthorized access to sensitive NF services.
Integrity (I)	High (H)	Attacker can manipulate NF communications.
Availability (A)	High (H)	Potential for service disruption via unauthorized NF interactions.
Base Score	9.1 (Critical)	High impact on confidentiality, integrity, and availability.

Severity Justification

• Critical Impact: The vulnerability allows unauthenticated attackers to bypass scope validation and obtain access tokens with arbitrary privileges, enabling unauthorized access to 5G core network functions.
• Exploitation Simplicity: The attack requires no prior authentication and can be executed remotely with minimal effort.
• Scope of Impact: Affects all NFs (Network Functions) relying on the NRF for OAuth2-based access control, potentially leading to lateral movement, data exfiltration, or service disruption in 5G networks.

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2. Potential Attack Vectors & Exploitation Methods

Exploitation Mechanism

1. Token Request Manipulation

   • The attacker sends a maliciously crafted targetNF value in an OAuth2 token request to the NRF.
   • The AccessTokenScopeCheck() function fails to validate the scope when the targetNF is manipulated, allowing the attacker to bypass all authorization checks.

2. Arbitrary Scope Assignment

   • The NRF blindly trusts the targetNF parameter and issues an access token with the requested (or default) scope, even if the attacker should not have access.
   • The attacker can specify any NF (e.g., AMF, SMF, UDM) as the targetNF, gaining unauthorized access to that NF’s services.

3. Privilege Escalation & Lateral Movement

   • With a forged access token, the attacker can:
     • Impersonate legitimate NFs (e.g., AMF, SMF).
     • Access sensitive subscriber data (e.g., IMSI, location, session keys).
     • Manipulate network functions (e.g., session hijacking, DoS).
     • Exfiltrate data via compromised NFs.

Exploitation Steps (Proof of Concept)

1. Intercept or Craft a Token Request

   • The attacker sends an HTTP POST request to the NRF’s /oauth2/token endpoint with:

     POST /oauth2/token HTTP/1.1
     Host: <NRF_IP>:8000
     Content-Type: application/x-www-form-urlencoded
     
     grant_type=client_credentials&
     client_id=<ATTACKER_CONTROLLED_NF>&
     client_secret=<ANY_VALUE>&
     targetNF=<MALICIOUS_NF_TYPE>&
     scope=<ARBITRARY_SCOPE>
     

   • Key Manipulation: The targetNF is set to a legitimate NF type (e.g., "AMF"), but the scope is arbitrarily defined (e.g., nrf:access, udm:subscriber-data).

2. NRF Issues a Forged Token

   • The NRF fails to validate the scope and returns a JWT access token with the requested privileges.
   • Example response:

     {
       "access_token": "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9...",
       "token_type": "Bearer",
       "expires_in": 3600,
       "scope": "nrf:access amf:registration"
     }
     

3. Use the Token to Access Protected NFs

   • The attacker uses the forged token to interact with other NFs (e.g., AMF, SMF) as if they were a legitimate NF.
   • Example API call to AMF:

     GET /namf-comm/v1/ue-contexts HTTP/1.1
     Host: <AMF_IP>:8000
     Authorization: Bearer eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9...
     

Real-World Attack Scenarios

Scenario	Impact
Subscriber Data Theft	Attacker accesses UDM (Unified Data Management) to exfiltrate IMSI, MSISDN, or authentication vectors.
Session Hijacking	Attacker manipulates SMF (Session Management Function) to redirect user traffic or inject malicious payloads.
Network DoS	Attacker floods AMF (Access and Mobility Management Function) with fake registration requests, causing service disruption.
Lateral Movement	Attacker uses a forged token to pivot into other NFs, escalating privileges within the 5G core.

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3. Affected Systems & Software Versions

Vulnerable Software

• Free5GC NRF (Network Repository Function) v1.4.0
  • The vulnerability exists in the access token generation logic (internal/sbi/processor/access_token.go).
  • Earlier versions (≤1.3.x) may also be affected if they share the same flawed implementation.

Affected Deployments

• 5G Core Networks using Free5GC (common in testbeds, research, and private 5G deployments).
• Cloud-native 5G deployments (e.g., Kubernetes-based Free5GC).
• Telecom operators experimenting with open-source 5G cores.

Not Affected

• Commercial 5G core solutions (e.g., Ericsson, Nokia, Huawei) do not use Free5GC and are not impacted.
• Free5GC versions ≥1.4.1 (assuming the patch is applied).

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4. Recommended Mitigation Strategies

Immediate Actions

1. Apply the Official Patch

   • Upgrade to Free5GC NRF v1.4.1 (or later) where the vulnerability is fixed.
   • Patch Reference: GitHub PR #73

2. Temporary Workarounds (If Patch Not Available)

   • Network-Level Protections:
     • Restrict NRF access to trusted NFs only (via firewall rules, VLAN segmentation, or mutual TLS).
     • Rate-limit token requests to prevent brute-force attacks.
   • Application-Level Fixes:
     • Manually validate targetNF in the AccessTokenScopeCheck() function before issuing tokens.
     • Enforce strict scope validation (e.g., whitelist allowed scopes per NF type).

3. Monitor for Exploitation Attempts

   • Log and alert on unusual token requests (e.g., mismatched targetNF and scope).
   • Deploy an IDS/IPS (e.g., Suricata, Snort) to detect malformed OAuth2 requests.

Long-Term Recommendations

1. Implement Zero Trust for 5G Core

   • Enforce mutual TLS (mTLS) between NFs to prevent token spoofing.
   • Adopt OAuth2 best practices (e.g., short-lived tokens, token binding).

2. Conduct a Security Audit

   • Review all Free5GC components for similar authorization flaws.
   • Penetration test the 5G core for token manipulation vulnerabilities.

3. Enhance Logging & Incident Response

   • Centralize logs from NRF and other NFs for correlation and anomaly detection.
   • Develop an IR playbook for 5G core breaches (e.g., token revocation, NF isolation).

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5. Impact on the Cybersecurity Landscape

Broader Implications

1. 5G Security Risks

   • Highlights critical flaws in open-source 5G core implementations, which are increasingly used in private 5G networks.
   • Undermines trust in OAuth2-based authorization in telecom environments.

2. Supply Chain & Third-Party Risks

   • Free5GC is used in research and testing, meaning vulnerable deployments may exist in academia, startups, and telecom labs.
   • Downstream vendors integrating Free5GC may inherit this vulnerability.

3. Regulatory & Compliance Concerns

   • Violates 3GPP security standards (e.g., TS 33.501), which mandate strong authentication and authorization.
   • May lead to GDPR violations if subscriber data is exposed.

4. Exploitation by APTs & Cybercriminals

   • State-sponsored actors (e.g., APT groups) could exploit this for espionage (e.g., tracking high-value targets).
   • Cybercriminals could sell forged 5G access tokens on dark web markets.

Comparison to Similar Vulnerabilities

CVE	Description	Similarity to CVE-2025-66719
CVE-2021-44228 (Log4Shell)	RCE via JNDI injection	High impact, easy exploitation
CVE-2020-13942 (Apache Unomi RCE)	Unauthenticated RCE in marketing automation	Unauthenticated access leading to full compromise
CVE-2019-11358 (jQuery Prototype Pollution)	Client-side prototype pollution	Authorization bypass via input manipulation

Key Takeaway: Like Log4Shell, this vulnerability is highly exploitable with severe consequences, but specific to 5G core networks.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Flaw Location: internal/sbi/processor/access_token.go (Free5GC NRF)
• Vulnerable Function: AccessTokenScopeCheck()
• Issue: The function skips scope validation if the targetNF is not in a predefined list, allowing arbitrary scope assignment.

Code Snippet (Vulnerable Logic)

func AccessTokenScopeCheck(targetNF, scope string) bool {
    // BUG: If targetNF is not in allowedNFs, validation is bypassed!
    if _, exists := allowedNFs[targetNF]; !exists {
        return true // ❌ Bypass all checks!
    }
    // ... (rest of scope validation logic)
    return false
}

• Fix: The patch removes the bypass condition and enforces strict scope validation for all targetNF values.

Exploitation Indicators (IOCs)

Indicator	Description
HTTP Requests	POST /oauth2/token with mismatched targetNF and scope.
Token Anomalies	JWT tokens with unexpected scopes (e.g., amf:registration for a non-AMF NF).
Network Traffic	Unusual NF-to-NF communication (e.g., a "SMF" accessing UDM).

Detection & Hunting Queries

SIEM Rules (Splunk/ELK)

index=network sourcetype=free5gc_nrf
| search uri_path="/oauth2/token" AND (targetNF="*" AND scope="*")
| stats count by client_ip, targetNF, scope
| where count > 5 AND (targetNF="AMF" AND scope!="amf:*") OR (targetNF="UDM" AND scope!="udm:*")

Zeek (Bro) Script

event http_request(c: connection, method: string, uri: string, version: string) {
    if (uri == "/oauth2/token" && /targetNF=[^&]+&scope=[^&]+/.test(uri)) {
        local targetNF = /targetNF=([^&]+)/.exec(uri)[1];
        local scope = /scope=([^&]+)/.exec(uri)[1];
        if (targetNF == "AMF" && !/^amf:/.test(scope)) {
            NOTICE([$note=HTTP::SuspiciousTokenRequest,
                    $msg=fmt("Potential CVE-2025-66719 exploitation: targetNF=%s, scope=%s", targetNF, scope),
                    $conn=c]);
        }
    }
}

Forensic Analysis Steps

1. Examine NRF Logs

   • Look for token requests with unusual targetNF/scope combinations.
   • Check for tokens issued to unexpected NFs.

2. Analyze JWT Tokens

   • Decode suspicious tokens using:

     jwt_tool <TOKEN> -d
     

   • Verify scope claims against expected permissions.

3. Network Traffic Analysis

   • Use Wireshark/TShark to inspect HTTP/2 or HTTP/1.1 traffic to the NRF.
   • Filter for:

     http.request.uri contains "/oauth2/token" && http.request.method == "POST"
     

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Conclusion & Key Takeaways

Summary

• CVE-2025-66719 is a critical authorization bypass in Free5GC NRF, allowing unauthenticated attackers to obtain arbitrary access tokens.
• Exploitation is trivial and can lead to full 5G core compromise, including data theft, session hijacking, and DoS.
• Affected organizations must patch immediately and implement compensating controls (e.g., network segmentation, mTLS).

Recommendations for Security Teams

✅ Patch Free5GC NRF to v1.4.1+ (or apply the fix manually). ✅ Restrict NRF access to trusted NFs only. ✅ Monitor for anomalous token requests (SIEM, IDS/IPS). ✅ Conduct a 5G core security audit to identify similar flaws. ✅ Adopt Zero Trust principles for NF-to-NF communication.

Final Risk Assessment

Factor	Assessment
Exploitability	High (Unauthenticated, remote, low complexity)
Impact	Critical (Full 5G core compromise possible)
Likelihood of Exploitation	High (Public PoC likely to emerge)
Mitigation Feasibility	Medium (Patch available, but compensating controls needed)

Urgent Action Required: Organizations using Free5GC must treat this as a critical priority to prevent catastrophic 5G core breaches.

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References:

• Free5GC GitHub Issue #736
• Free5GC NRF Patch PR #73
• 3GPP TS 33.501 (5G Security Architecture)