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5G Troubleshooting

5G Security, Authentication, and Identity Issues

Security, authentication, and identity failures in 5G can happen at several different layers. A registration attempt may fail because the network cannot derive UE identity, authentication cannot complete, NAS security cannot be activated, Security Mode Control fails on the radio side, or the UE reaches the wrong context because stale identity information is reused.

This page is the main troubleshooting hub for that problem family. It helps you identify the first hard failure stage, then move into the right detailed page or reference instead of mixing identity, authentication, NAS security, and radio security problems together. For the identity terms themselves, use 5GS UE and Network Identities as the reference map for SUPI, SUCI, 5G-GUTI, GUAMI, and related identifiers.

What This Section Covers

Authentication failures

AKA, EAP, synch failure, and credential-side continuation problems.

NAS security failures

Protected NAS signalling, security context creation, and stale-context mismatch cases.

AS security and Security Mode failures

SecurityModeCommand, SecurityModeFailure, and radio-side security continuation.

Stale context and continuity issues

Wrong AMF path, old 5G-GUTI, stale GUAMI, and identity-context reuse problems.

Which page to use next

This hub is for triage. Use it to decide the right branch before diving into details.

Where These Problems Sit in the 5G Procedure Chain

The order matters, because many issues that look like generic registration failure are actually identity or security-stage failures.

1. Identity presentation and context reuse

  • SUCI, SUPI, or 5G-GUTI is presented or reused
  • AMF continuity and identity context are checked

2. Authentication and authorization

  • authentication starts
  • AKA, EAP, or related authorization checks run

3. NAS security activation

  • NAS security context is created
  • protected NAS signalling must continue correctly

4. AS security activation

  • SecurityModeCommand becomes the key breakpoint
  • radio-side security activation must succeed

5. Post-security continuation

  • registration or service resumes normal progression
  • later reject or timeout can still reveal the first real break

Fast Triage by First Hard Failure

This is the main jump table for the security branch. Start with the first hard failure, not the broad symptom.

Authentication issue

  • Authentication Request appears
  • AKA or EAP cannot complete
  • synch failure or repeated auth failure

Go to: Authentication Procedure

NAS security issue

  • authentication may have succeeded
  • protected NAS exchange does not continue
  • security context mismatch or replay-style problem

Go to: NAS Security Reference

AS security issue

  • SecurityModeCommand appears
  • SecurityModeFailure is returned
  • radio-side security continuation stops

Go to: Security Mode Command

Later continuation failed

  • security looked successful
  • later registration path still failed
  • explicit reject or area issue appears after security

Go to: Registration Reject Cause Analysis

Identity Issues

Identity problems usually show up before authentication completes cleanly, even if the visible user symptom is just “registration failed.” If you need a quick refresher on how SUPI, SUCI, 5G-GUTI, and GUAMI relate, use 5GS UE and Network Identities.

  • SUPI and SUCI handling
  • 5G-GUTI reuse and stale context
  • GUAMI-related continuity problems
  • identity derivation failure
  • AMF selection side effects

Observed pattern

Observed pattern
- UE reaches access and sends Registration Request
- network cannot continue with the expected UE identity context
- later authentication or registration fails

Likely family
- stale 5G-GUTI
- wrong GUAMI continuity
- identity derivation problem

AMF Selection and GUAMI Issues is the best current detailed troubleshooting page for this branch.

Authentication Issues

Authentication failure is not the same as Security Mode failure. Keep the AKA and EAP branch separate from later security activation.

  • 5G AKA and EAP failure patterns
  • synch failure branch
  • authorization failure branch
  • authentication versus security mismatch

Observed pattern

Observed pattern
- Registration starts
- Authentication Request is seen
- UE or network cannot complete authentication successfully

Likely family
- AKA failure
- synch failure
- authorization or credential-side issue

Use Authentication Procedure as the current best branch reference until the dedicated troubleshooting child page is published.

NAS Security Issues

This branch sits between authentication and AS security. Use it when authentication may be fine, but protected NAS signalling cannot continue correctly.

  • NAS security context creation
  • protected NAS signalling mismatch
  • replay or stale context problems
  • registration path blocked by NAS security handling

Authentication, Security Mode, and Initial NAS Protection is the best current reference for this branch.

AS Security and Security Mode Issues

Keep Security Mode problems distinct from pure NAS authentication problems, because the first hard break is already later in the chain.

  • SecurityModeCommand checkpoint
  • SecurityModeComplete versus SecurityModeFailure
  • capability mismatch versus radio-side continuation failure
  • hand-off to the Security Mode branch

Observed pattern

Observed pattern
- NAS-side registration reaches security activation stage
- gNB sends SecurityModeCommand
- UE returns SecurityModeFailure or continuation stops

Likely family
- AS security activation failure
- capability mismatch
- radio-side security continuation issue

Use Security Mode Command and AS Security Activation Procedure as the current references for this branch.

Worked Cross-Layer Call Flow

Use this umbrella ladder to decide whether the first hard break is identity, authentication, NAS security, or Security Mode.

Call Flow: Identity to Security Decision Ladder

Identity to security decision ladder Access succeeds, Registration Request reaches the AMF side, identity and authentication checks run, NAS security is established, SecurityModeCommand is sent, and registration continues if the security chain succeeds. UE gNB / NG-RAN AMF / AUSF / UDM 1. Access succeeds 2. Registration Request 3. Identity and context checks 4. Authentication 5. NAS security 6. SecurityModeCommand 7. SecurityModeComplete 8. Registration continues

If the first hard break is before authentication, stay on the identity branch. If it is during authentication, use the authentication branch. If it is after authentication but before protected NAS continuation, use the NAS security branch. If it is at SecurityModeCommand, use the Security Mode branch.

Child Troubleshooting Pages

These are the currently published detailed pages and stable references that best match the security, authentication, and identity branches.

Authentication Procedure

Use this call flow when authentication starts and you need to place the failure in the AKA or EAP branch.

Security Mode Command

Use this call flow when the failure appears at SecurityModeCommand, SecurityModeComplete, or SecurityModeFailure.

5GMM Cause Values

Use this when the failure includes mobility-side cause values and you need to separate identity, policy, and area meanings.

Practical Troubleshooting Workflow

1. Identify the first hard failure stage

  • first explicit reject
  • first timeout
  • first mismatch
  • first security message failure

2. Decide what failed first

  • identity
  • authentication
  • NAS security
  • AS security

3. Correlate the logs

  • UE-side NAS and RRC view
  • gNB or NG-RAN signaling view
  • AMF, AUSF, and UDM view when relevant

4. Move to the correct child page

  • do not keep identity, authentication, and Security Mode in one bucket
  • use the branch that matches the first hard break, not the last visible symptom

Evidence Checklist

Minimum UE-side evidence

  • SUPI, SUCI, or 5G-GUTI if available
  • exact failing NAS or RRC message
  • security capability snapshot
  • timer context if relevant

Minimum RAN-side evidence

  • RRC progression up to the failure point
  • SecurityModeCommand or related continuation if present
  • AMF selection clues if identity continuity looks wrong

Minimum core-side evidence

  • AMF decision path
  • AUSF or UDM result when authentication-related
  • NAS security context creation or failure point

Minimum identity context

  • GUAMI if relevant
  • current PLMN, TAC, or TAI when area context matters
  • any stale-context or replay-style clue

Specification Map

  • TS 24.501: NAS mobility management, identity, and NAS security procedures
  • TS 33.501: 5GS security architecture and procedures
  • TS 23.501: 5GS identity and core architecture context
  • TS 23.502: end-to-end procedure context for registration and service continuation
  • TS 38.331: SecurityModeCommand and radio-side security continuation

FAQ

What is the difference between authentication failure and Security Mode failure?

Authentication failure happens earlier, when the UE and network cannot complete identity validation or AKA and EAP handling. Security Mode failure happens later, after the procedure reaches radio-side security activation and SecurityModeCommand becomes the first hard break.

When is an issue really an identity problem instead of a registration problem?

It is mainly an identity problem when the network cannot derive or trust the expected UE context before authentication can continue normally. In that case the visible symptom may be registration failure, but the root cause sits in identity reuse, stale 5G-GUTI, GUAMI continuity, or AMF targeting.

What should I check first if the UE reaches SecurityModeCommand and then fails?

Check whether the failure is a radio-side security activation problem, capability mismatch, or a continuation issue after NAS security succeeded. That is the point where the AS security branch becomes more relevant than the earlier identity and authentication branches.

Which page should I use if the AMF selection looks wrong?

Use the AMF Selection and GUAMI Issues page first, because stale identity continuity and wrong AMF targeting can make later authentication or registration failures look misleading.

When should I analyze Registration Reject separately from security troubleshooting?

Analyze Registration Reject separately when a real explicit reject is present and decoded. At that point the cause-family analysis can be more useful than staying at the generic security hub level.

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