Ubuntu 18.04 LTS – apt update shows NO_PUBKEY for repository signatures

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

apt rejects repository metadata due to missing trusted signing keys.

Environment & Reproduction

Triggered after adding third-party repositories without proper keyring setup.

Root Cause Analysis

Repository key is missing, expired, or incorrectly associated with the source entry.

Quick Triage

Identify failing repository and key fingerprint from apt update output.

Step-by-Step Diagnosis

Review source list entries and installed trusted key material.

Solution – Primary Fix

Install the correct vendor key and update repository signing configuration.

Solution – Alternative Approaches

Disable untrusted repository temporarily until valid signed metadata is available.

Verification & Acceptance Criteria

apt update completes without NO_PUBKEY or signature verification errors.

Rollback Plan

Remove incorrect key entries and restore previous repository configuration backups.

Prevention & Hardening

Use repository onboarding runbooks that include key rotation and expiry checks.

Related Errors & Cross-Refs

The repository is not signed and signature verification failed messages are related.

Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.

View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Ubuntu repository signing and apt-secure documentation for trusted key handling.