Ubuntu 22.04 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 repository metadata due to missing trusted signing key.

Environment & Reproduction

Common after adding third-party repositories without keyring setup.

Root Cause Analysis

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

Quick Triage

Identify failing repository and key fingerprint from update output.

Step-by-Step Diagnosis

Inspect repository entries and keyring files used by apt.

Solution – Primary Fix

Install vendor key into /etc/apt/keyrings and rerun metadata refresh.

Solution – Alternative Approaches

Disable untrusted repository until valid signed metadata is available.

Verification & Acceptance Criteria

apt update completes without NO_PUBKEY or signature verification failures.

Rollback Plan

Remove incorrect keyring and restore prior repository list backup.

Prevention & Hardening

Enforce signed-by usage for all non-default repositories.

Related Errors & Cross-Refs

Related to InRelease signature mismatch and expired repository keys.

Related tutorial: View the step-by-step tutorial for Ubuntu 22.04 LTS.

View all Ubuntu 22.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Ubuntu 22.04 apt-secure and repository keyring guidance.