Ubuntu 20.04 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 refuses metadata from one or more repositories due to trust failures.

Environment & Reproduction

Common after adding external repositories without proper keyring configuration.

Root Cause Analysis

Repository signing key is missing, expired, or not linked with signed-by.

Quick Triage

Identify failing repo and fingerprint from apt update output.

Step-by-Step Diagnosis

Inspect source entries and keyring files used by apt.

Solution – Primary Fix

Install valid vendor key in keyrings and update repository definition.

Solution – Alternative Approaches

Disable untrusted repository until a valid signed metadata chain is available.

Verification & Acceptance Criteria

apt update completes without NO_PUBKEY or signature validation errors.

Rollback Plan

Remove incorrect keyring entries and restore previous source configuration.

Prevention & Hardening

Require signed-by for non-default repositories and track key expiry dates.

Related Errors & Cross-Refs

InRelease signature mismatch and EXPKEYSIG authentication failures.

Related tutorial: View the step-by-step tutorial for Ubuntu 20.04 LTS.

View all Ubuntu 20.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Ubuntu apt-secure and trusted keyring recommendations.